1) Wolfe, S.; Schlegel, H. B.; Whangbo, M.-H.; Bernardi, F. "On the origin of the Bohlmann bands." Can. J. Chem. 1974, 52, 3787-3792.

2) Bernardi, F.; Csizmadia, I. G.; Schlegel, H. B.; Tiecco, M.; Whangbo, M.-H.; Wolfe, S. "The irrelevance of d-orbital conjugation. II. Non-empirical molecular orbital calculations on the CH2SH radical." Gazz. Chim. Ital. 1974, 104, 1101-1108.

3) Wolfe, S.; Schlegel, H. B.; Csizmadia, I. G.; Bernardi, F. "The chemical dynamics of symmetric and asymmetric reaction coordinates." J. Am. Chem. Soc. 1975, 97, 2020-2024.

4) Bernardi, F.; Csizmadia, I. G.; Mangini, A.; Schlegel, H. B.; Whangbo, M.-H.; Wolfe, S. "The irrelevance of d-orbital conjugation. I. The alpha-thiocarbanion. A comparative quantum chemical study of the static and dynamic properties and proton affinities of carbanions adjacent to oxygen and to sulfur." J. Am. Chem. Soc. 1975, 97, 2209-2218.

5) Bernardi, F.; Csizmadia, I. G.; Schlegel, H. B.; Wolfe, S. "On the pi-donating abilities of sulfur and oxygen. A comparative quantum chemical investigation of the static and dynamic properties and gas phase acidities of +CH2OH and +CH2SH." Can. J. Chem. 1975, 53, 1144-1153.

6) Schlegel, H. B.; Wolfe, S.; Mislow, K. "Ab initio molecular orbital calculations on silaethylene H2Si=CH2. The theoretical infrared spectrum." J. Chem. Soc. Chem. Comm. 1975, 246-247.

7) Wolfe, S.; Schlegel, H. B.; Csizmadia, I. G.; Bernardi, F. "The structure of acetaldehyde enolate anion." Can J. Chem. 1975, 53, 3365-3370.

8) Schlegel, H. B.; Wolfe, S.; Bernardi, F. "Ab initio computation of force constants. I. The second and third period hydrides." J. Chem. Phys. 1975, 63, 3632-3638.

9) Schlegel, H. B.; Wolfe, S.; Bernardi, F. "Ab initio computation of force constants. II. The estimation of dissociation energies from ab-initio SCF calculations." Can. J. Chem. 1975, 3599-3601.

10) Bernardi, F.; Pedulli, G. F.; Guerra, M.; Schlegel, H. B. "Ab initio studies on ion pairs. The sodium-formaldehyde ion pair." Gazz. Chim. Ital. 1975, 105, 711-722.

11) Schlegel, H. B. Some Theoretical Aspects of Internal Molecular Motion: Energy Surfaces and Molecular Vibrations; Queen's University: Kingston, Ont. Canada, 1975.

12) Bernardi, F.; Epiotis, N. D.; Cherry, W.; Schlegel, H. B.; Whangbo, M.-H.; Wolfe, S. "A molecular orbital interpretation of the static, dynamic and chemical properties of CH2X radicals." J. Am. Chem. Soc. 1976, 98, 469-478.

13) Colpa, J. P.; Schlegel, H. B.; Wolfe, S. "An inequality formation of potential energy differences." Can. J. Chem. 1976, 54, 526-530.

14) Wolfe, S.; Schlegel, H. B.; Whangbo, M.-H. "On the rigidity of planar tricoordinate carbon." Can. J. Chem. 1976, 54, 795-799.

15) Bernardi, F.; Epiotis, N. D.; Yates, R.; Schlegel, H. B. "Non-bonded attractions in methyl vinyl ether." J. Am. Chem. Soc. 1976, 98, 2385-2390.

16) Bernardi, F.; Schlegel, H. B.; Wolfe, S. "Ab initio computation of force constants. IV. A simple procedure for the evaluation of X-H bond dissociation energies." J. Mol. Structure 1976, 35, 149-153.

17) Schlegel, H. B.; King, F. W. "An ab initio calculation of the cyclopropylcarbinyl radical." J. Mol. Structure 1976, 35, 155-158.

18) King, F. W.; Schlegel, H. B. "An ab initio study of bicyclobutyl radical. An anti W long-range coupled system." J. Mag. Resonance 1976, 22, 389-391.

19) Dougherty, D. A.; Hounshell, W. D.; Schlegel, H. B.; Bell, R. A.; Mislow, K. "Long bonds and through-bond coupling." Tetrahedron Lett. 1976, 39, 3479-3482.

20) Schlegel, H. B.; Mislow, K.; Bernardi, F.; Bottoni, A. "An ab initio investigation into the SN2 reaction: Frontside versus backside attack in the reaction of F- with CH3F." Theor. Chim. Acta 1977, 44, 245-256.

21) Whangbo, M.-H.; Schlegel, H. B.; Wolfe, S. "Molecular orbitals from group orbitals. III. Quantitative perturbation molecular orbital analysis of ab initio SCF-MO wavefunctions." J. Am. Chem. Soc. 1977, 99, 1296-1304.

22) Schlegel, H. B.; Wolfe, S.; Bernardi, F. "Ab initio computation of force constants. IV. The anharmonic force field and vibrational frequencies of methylamine, methanol and methanethiol." J. Chem. Phys. 1977, 67, 4181-4193.

23) Schlegel, H. B.; Wolfe, S.; Bernardi, F. "Ab initio computation of force constants. V. The anharmonic force field and vibrational frequencies of methyl fluoride and methyl chloride." J. Chem. Phys. 1977, 67, 4194-4198.

24) Bernardi, F.; Schlegel, H. B.; Whangbo, M.-H.; Wolfe, S. "A comparative quantum chemical investigation of the bonding in first and second row ylides." J. Am. Chem. Soc. 1977, 99, 5633-5636.

25) Csizmadia, I. G.; Theodorakopoulos, G.; Schlegel, H. B.; Whangbo, M.-H.; Wolfe, S. "The balance between nuclear and electronic energy in conformational change." Can. J. Chem. 1977, 55, 986-991.

26) Schlegel, H. B.; Coleman, B.; Jones, J., M. "An ab initio molecular orbital calculation of the structure of silabenzene." J. Am. Chem. Soc. 1978, 100, 6499-6501.

27) Goetz, D. W.; Schlegel, H. B.; Allen, L. C. "Ab initio electronic structure calculations for classical and nonclassical structures of the 2-norbornyl cation." J. Am. Chem. Soc. 1977, 99, 8118-8120.

28) Dougherty, D. A.; Schlegel, H. B.; Mislow, K. "Bond lengthening and through-bond interactions in p,p'-dibenzene and related molecules." Tetrahedron 1978, 34, 1441-1447.

29) Bernardi, F.; Schlegel, H. B.; Tonachini, G. "Ab initio computation of force constants. VI. Applications of the force relaxation method for geometry optimization." J. Mol. Structure 1978, 48, 243-248.

30) Chandrasekhar, J.; Schleyer, P. v. R.; Schlegel, H. B. "The 1-bicyclo [1.1.1] pentyl cation as a CH+.trimethylmethane complex." Tetrahedron Lett. 1978, 36, 3393-3396.

31) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. "Electron correlation theories and their application to the study of simple reaction potential surfaces." Int. J. Quantum Chem. 1978, 14, 545-560.

32) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. "Derivative studies in Hartree-Fock and Møller-Plesset theories." Int. J. Quantum. Chem. Quantum Chem. Symp. 1979, 13, 225-241.

33) Kost, D.; Schlegel, H. B.; Mitchell, D. J.; Wolfe, S. "Molecular orbitals from group orbitals. IX. The problem of hybrid lone pairs." Can. J. Chem. 1979, 57, 729-732.

34) Krishnan, R.; Schlegel, H. B.; Pople, J. A. "Derivative studies in configuration-interaction theory." J. Chem. Phys. 1980, 72, 4654-4655.

35) Harding, L. B.; Schlegel, H. B.; Krishnan, R.; Pople, J. A. "A Møller-Plesset study of the H4CO potential energy surface." J. Phys. Chem. 1980, 84, 3394-3401.

36) Binkley, J. S.; Whiteside, R. A.; Krishnan, R.; Seeger, R.; DeFrees, D. J.; Schlegel, H. B.; Topiol, S.; Kahn, L. R.; Pople, J. A. "GAUSSIAN 80, an ab initio molecular orbital program. Quantum Chemistry Program Exchange, Indiana University, Bloomington, Ind." QCPE 1980, 13, 406.

37) Whiteside, R. A.; Binkley, J. S.; Krishnan, R.; DeFrees, D. J.; Schlegel, H. B.; Pople, J. A. Carnegie-Mellon Quantum Chemistry Archive; Carnegie-Mellon University, : Pittsburgh, PA, 1980.

38) Gund, P.; Schlegel, H. B. "Theoretical calculations and drug design." Ann. N. Y. Acad. Sci. 1981, 367, 510-517.

39) DeFrees, D. J.; Krishnan, R.; Schlegel, H. B.; Pople, J. A. "A theoretical study of the fluorohydroxyl boranes BFn(OH)(3-n)." Inorg. Chim. Acta 1981, 47, 19-23.

40) Harding, L. B.; Schlegel, H. B.; Krishnan, R.; Pople, J. A. "Theoretical studies on the unimolecular decomposition of methanol" in Potential Energy Surfaces and Dynamics Calculations; Truhlar, D. G., Ed.; Plenum,: New York, 1981, , pp 169-183.

41) Schlegel, H. B.; Poe, M.; Hoogsteen, K. "Models for the binding of methotrexate to Escherichia coli dihydrofolate reductase. Direct effect of carboxylate of ASP 27 upon UV spectrum of methotrexate." Mol. Pharmacol. 1981, 20, 154-158.

42) Bock, M.; Smith, G. M.; Schlegel, H. B. "Theoretical estimation of pKa values of pyrazinylquanidine derivatives." J. Org. Chem. 1981, 46, 1925-1927.

43) Schlegel, H. B. "FORCE/DRVEXP, analytical energy derivatives and gradients package. Quantum Chemistry Program Exchange, Indiana University, Bloomington, Ind." QCPE 1981, 13, 427.

44) Hartman, G. D.; Schlegel, H. B. "The relation of the carcinogenic/mutagenic potential of arylamines to their singlet-triplet nitrenium ion energies." Chemico-Biological Interactions 1981, 36, 319-330.

45) Schlegel, H. B. "Ab initio energy derivatives calculated analytically" in Computational Theoretical Organic Chemistry; Csizmadia, I. G. and Daudel, R., Ed.; D. Reidel: Holland, 1981,, pp 129-159.

46) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. "Molecular orbital studies of vibrational frequencies." Int. J. Quantum. Chem., Quantum Chem. Symp. 1981, 15, 269-278.

47) Mitchell, D. J.; Wolfe, S.; Schlegel, H. B. "A theoretical study of the CSH4 and CPH4 hypersurfaces, geometries, tautomerization and dissociation of sulfonium and phosphonium ylides." Can. J. Chem. 1981, 59, 3280-3292.

48) Wolfe, S.; Mitchell, D. J.; Schlegel, H. B. "Theoretical studies of SN2 transition states. 1. Geometries." J. Am. Chem. Soc. 1981, 103, 7692-7694.

49) Wolfe, S.; Mitchell, D. J.; Schlegel, H. B. "Theoretical studies of SN2 transition states. 2. Intrinsic barriers, rate-equilibrium relationships and the Marcus equation." J. Am. Chem. Soc. 1981, 103, 7694-7696.

50) Schlegel, H. B. "Optimization of equilibrium geometries and transition structures." J. Comput. Chem. 1982, 3, 214-218.

51) Amato, J. S.; Karady, S.; Reamer, R. A.; Schlegel, H. B.; Springer, J. P.; Weinstock, L. M. "1,2,5-thiadiazole- 1-oxides III. An experimental and theoretical investigation of the inversion barrier." J. Am. Chem. Soc. 1982, 104, 1375-1380.

52) Wolfe, S.; Mitchell, D. J.; Schlegel, H. B.; Minot, C.; Eisenstein, O. "Theoretical studies of SN2 transition states. The alpha effect." Tetrahedron Lett. 1982, 23, 615-618.

53) Eisenstein, O.; Schlegel, H. B.; Kayser, M. M. "A theoretical study of borohydride addition to formaldehyde: A one step, non-synchronous transition state." J. Org Chem. 1982, 47, 2886-2891.

54) Schlegel, H. B.; Gund, P.; Fluder, E. M. "Tautomerization of formamide, 2-pyridone and 4-pyridone. An ab initio study." J. Am. Chem. Soc. 1982, 104, 5347-5351.

55) DeFrees, D. J.; K., R.; Schlegel, H. B.; Pople, J. A. "Effect of electron correlation on theoretical equilibrium geometries. 2. Comparison of third order perturbation and configuration interaction results with experiment." J. Am. Chem. Soc. 1982, 104, 5576-5580.

56) Duchovic, R. J.; Hase, W. L.; Schlegel, H. B.; Frisch, M. J.; K., R. "Ab initio potential energy curve for CH bond dissociation in methane." Chem. Phys. Lett. 1982, 89, 120-125.

57) Schlegel, H. B. "An efficient algorithm for calculating ab initio energy gradients using s,p Cartesian gaussians." J. Chem. Phys. 1982, 77, 3676-3681.

58) Wolfe, S.; Mitchell, D. J.; Schlegel, H. B. "Theoretical studies of SN2 transition states. Substituent effects." Can. J. Chem. 1982, 60, 1291-1294.

59) Schlegel, H. B. "An ab initio molecular orbital study of the tautomerism of 4-hydroxy-2-pyridinone." Int. J. Quantum. Chem. 1982, 22, 1041-1047.

60) Hase, W. L.; Schlegel, H. B. "Resolution of a paradox concerning the forward and reverse rate constants for C2H5 ? H + C2H4." J. Phys. Chem. 1982, 86, 3901-3904.

61) Schlegel, H. B. "Ab initio molecular orbital studies of H + C2H4 and F + C2H4. I. Comparison of the equilibrium geometries, transition structures and vibration frequencies." J. Phys. Chem. 1982, 86, 4878-4882.

62) Schlegel, H. B.; Bhalla, K. C.; Hase, W. L. "Ab initio molecular orbital studies of H + C2H4 and F + C2H4. II. Comparison of the energetics." J. Phys. Chem. 1982, 86, 4883-4888.

63) Schlegel, H. B.; Robb, M. A. "MCSCF gradient optimization of the H2CO ? H2 + CO transition structure." Chem. Phys. Lett. 1982, 93, 43-46.

64) Binkley, J. S.; Frisch, J. M.; DeFrees, D. J.; Krishnan, R.; Whiteside, R. A.; Schlegel, H. B.; Fulder, E. M.; Pople, J. A. GAUSSIAN 82; Carnegie-Mellon Chemistry Publishing Unit, : Pittsburgh, PA, 1982.

65) Hiberty, P. C.; Ohnanessian, G.; Schlegel, H. B. "Theoretical ab initio study of 1,3 dipolar cycloaddition of fulminic acid to acetylene. Support for Firestone's mechanism." J. Am. Chem. Soc. 1983, 105, 719-723.

66) Rohlfing, C. M.; Allen, L. C.; Cook, C. M.; Schlegel, H. B. "The structure of (H3O2)-." J. Chem. Phys. 1983, 78, 2498-2503.

67) Schlegel, H. B.; Sosa, C. "Ab initio molecular orbital studies of Cl + C2H4 and H + C2H3Cl." J. Phys. Chem. 1984, 88, 1141-1145.

68) Bernardi, F.; Robb, M. A.; Schlegel, H. B.; Tonachini, G. "An MCSCF study of [1,3] and [1,2] sigmatropic shifts in propene." J. Am. Chem. Soc. 1984, 106, 1198-1202.

69) Duchovic, R. J.; Hase, W. L.; Schlegel, H. B. "An analytic function for the H + CH3 ? CH4 potential energy surface." J. Phys. Chem. 1984, 88, 1339-1347.

70) Lohr, L. L.; Schlegel, H. B.; Morokuma, K. "Theoretical studies of the gas-phase proton affinities of molecules containing phosphorus-carbon multiple bonds." J. Phys. Chem. 1984, 88, 1981-1987.

71) Schlegel, H. B.; Binkley, J. S.; Pople, J. A. "First and second derivatives of two electron integrals over Cartesian Gaussians using Rys polynomials." J. Chem. Phys. 1984, 80, 1976-1981.

72) Sosa, C.; Schlegel, H. B. "Carbene and silylene insertion reactions. Ab initio calculations on the effect of fluorine substitution." J. Am. Chem. Soc. 1984, 106, 5847-5852.

73) Schlegel, H. B. "Estimating the Hessian for gradient-type geometry optimizations." Theor. Chim. Acta 1984, 66, 333-340.

74) Schlegel, H. B. "Heats of formation of fluorine substituted silylenes, silyl radicals and silanes." J. Phys. Chem. 1984, 88, 6254-6258.

75) Bernardi, F.; Bottoni, A.; Robb, M. A.; Schlegel, H. B.; Tonachini, G. "The structure of the tetramethylene diradical intermediate." Chem. Phys. Lett. 1984, 108, 599-601.

76) Lumma, J., W. C.; Baldwin, J. J.; Bicking, J. B.; Bolhofer, W. A.; Hoffman, J. M.; Phillips, B. T.; Robb, C. M.; Torchiana, M. L.; Schlegel, H. B.; Smith, G. M.; Hirshfield, J. M.; Snyder, J. P.; Springer, J. P. "Structure-Activity, theoretical and x-ray studies on intramolecular interactions in a series of novel histamine H2 receptor antagonists." J. Med. Chem. 1984, 27, 1047-1052.

77) Bernardi, F.; Bottoni, A.; McDouall, J. J.; Robb, M. A.; Schlegel, H. B. "MC-SCF gradient calculations of transition structures in organic reactions." Faraday Symp. Chem. Soc. 1984, 19, 137-147.

78) Schlegel, H. B.; Sosa, C. "SiH2 + SiH3F ? Si2H5F. An ab initio study of silylene insertion into a silicon fluorine bond." J. Phys. Chem. 1985, 89, 537-541.

79) Mitchell, D. J.; Schlegel, H. B.; Shaik, S. S.; Wolfe, S. "Relationships between geometries and energies of identity SN2 transition states: the dominant role of the distortion energy and its origin." Can. J. Chem. 1985, 63, 1642-1648.

80) Bach, R. D.; Wolber, G. J.; Schlegel, H. B. "The origin of barriers to thermally allowed six-electron pericylcic reactions. The effects of HOMO-LUMO interactions on the trimerization of acetylene." J. Am. Chem. Soc. 1985, 107, 2837-2841.

81) Bernardi, F.; Bottoni, A.; Robb, M. A.; Schlegel, H. B.; Tonachini, G. "An MC-SCF Study of the thermal cycloaddition of two ethylenes." J. Am. Chem. Soc. 1985, 107, 2260-2265.

82) Schlegel, H. B. "Potential energy curves using unrestricted Møller-Plesset perturbation theory with spin annihilation." J. Chem. Phys. 1986, 84, 4530-4534.

83) Sosa, C.; Schlegel, H. B. "Ab initio calculations on the barrier heights for hydrogen atom addition to ethylene and formaldehyde. The importance of spin projection." Int. J. Quantum Chem. 1986, 29, 1001-1015.

84) Sosa, C.; Schlegel, H. B. "Ab initio calculations on the barrier heights for hydrogen atom addition to ethylene and formaldehyde. The importance of spin projection. Erratum." Int. J. Quantum Chem. 1986, 30, 155-156.

85) Tonachini, G.; Schlegel, H. B.; Bernardi, F.; Robb, M. A. "The addition of 1?g oxygen molecule and ethene to give dioxetane: an MCSCF study and characterization of some previously proposed pathways." Theochem. 1986, 138, 221-227.

86) Ignacio, E. W.; Schlegel, H. B.; Bicerano, J. "Ab initio calculations on (SiH3)2F+: stability in the gas phase and a model for bridging fluorine atom in ion-implanted amorphous silicon." Chem. Phys. Lett. 1986, 127, 367-373.

87) Bicerano, J.; Keem, J. E.; Schlegel, H. B. "Theoretical studies of hydrogen storage in binary Ti-Ni, Ti-Cu and Ti-Fe alloys." Theor. Chim. Acta 1986, 70, 265-296.

88) Frisch, J. M.; Binkley, J. S.; DeFrees, D. J.; Raghavachari, K.; Schlegel, H. B.; Whiteside, R. A.; Fox, D. J.; Martin, R. L.; Fulder, E. M.; Melius, C. F.; Kahn, L. R.; Stewart, J. J. P.; Bobrowicz, F. W.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon Chemistry Publishing Unit,: Pittsburgh, PA, 1986.

89) Pinto, B. M.; Schlegel, H. B.; Wolfe, S. "Bond angle variations in XCY fragments and their relation to the anomeric effect." Can. J. Chem. 1987, 65, 1658-1662.

90) Schlegel, H. B. "Optimization of equilibrium geometries and transition structures." Adv. Chem. Phys. 1987, 67, 249-286.

91) Tonachini, G.; Schlegel, H. B. "Hartree-Fock derivatives with respect to basis set exponents. Integral derivatives using Rys polynomials." J. Chem. Phys. 1987, 87, 514-519.

92) DeFrees, D. J.; Raghavachari, K.; Schlegel, H. B.; Pople, J. A.; Schleyer, P. v. R. "Binary association complexes of LiH, BeH2, and BH3. Relative isomer stabilities and barrier heights for their interconversion; energy barriers in the dimerization reactions." J. Phys. Chem. 1987, 91, 1857-1864.

93) Sosa, C.; Schlegel, H. B. "Calculated barrier heights for OH + C2H2 and OH + C2H4 using unrestricted Møller-Plesset perturbation theory with spin annihilation." J. Am. Chem. Soc. 1987, 109, 4193-4198.

94) Sosa, C.; Schlegel, H. B. "An ab initio study of the reaction pathways for OH + C2H4 ? HOCH2CH2 ? products." J. Am. Chem. Soc. 1987, 109, 7007-7015.

95) Illies, A. J.; McKee, M. L.; Schlegel, H. B. "Ab initio study of the CO2 dimer and the CO2 ion complexes, (CO2)2+ and (CO2)3+." J. Phys. Chem. 1987, 91, 3489-3494.

96) Maccagnani, G.; Schlegel, H. B.; Tonachini, G. "An ab initio theoretical study of the ring opening of ethene episulphoxide under base attack." J. Org. Chem. 1987, 52, 4961-4966.

97) Sosa, C.; Schlegel, H. B. "A theoretical study of the infrared vibrational intensities of CH3F." J. Chem. Phys. 1987, 86, 6937-6945.

98) McDouall, J. J. W.; Robb, M. A.; Niazi, U.; Bernardi, F.; Schlegel, H. B. "An MCSCF study of the mechanisms for 1,3 dipolar cycloadditions." J. Am. Chem. Soc. 1987, 109, 4642-4648.

99) Sosa, C.; Schlegel, H. B. "Ab initio calculations on H + C2H2 ? C2H3 using unrestricted Møller-Plesset perturbation theory with spin projection." Int. J. Quantum Chem. Quantum Chem. Symp. 1987, 21, 267-282.

100) Shaik, S. S.; Schlegel, H. B.; Wolfe, S. "Transition state geometries and the magnitudes of SN2 barriers: A theoretical study." J. Chem. Soc. Chem. Comm. 1988, 1322-1323.

101) Schlegel, H. B. "Moller-Plesset Perturbation-Theory With Spin Projection." Journal of Physical Chemistry 1988, 92, 3075-3078.

102) Schlegel, H. B.; Sosa, C. "Ab Initio Molecular-Orbital Calculations On F+H₂->HF+H and OH+H₂->H₂O+H Using Unrestricted Moller-Plesset Perturbation- Theory With Spin Projection." Chemical Physics Letters 1988, 145, 329-333.

103) Francisco, J. S.; Schlegel, H. B. "Photodissociation Dynamics of Ethylsilane - Ab Initio and RRKM Study." Journal of Chemical Physics 1988, 88, 3736-3746.

104) Tonachini, G.; Bernardi, F.; Schlegel, H. B.; Stirling, C. J. M. "An Ab Initio Theoretical-Study of the Eliminative Ring Fission in Cyclopropylmethanide and Cyclobutylmethanide." Journal of the Chemical Society-Perkin Transactions 2 1988, 705-709.

105) Bernardi, F.; Bottoni, A.; Olivucci, M.; Robb, M. A.; Schlegel, H. B.; Tonachini, G. "Do Supra Antara Paths Really Exist For 2 + 2 Cyclo-Addition Reactions - Analytical Computation of the MC-SCF Hessians For Transition-States of C₂H₄ With C₂H₄, Singlet O₂ and Ketene." Journal of the American Chemical Society 1988, 110, 5993-5995.

106) Frisch, J. M.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R.; Kahn, L. R.; Stewart, J. J. P.; Fulder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN 88; Gaussian, Inc: Pittsburgh, PA, 1988.

107) Gonzalez, C.; Schlegel, H. B. "An Improved Algorithm For Reaction-Path Following." Journal of Chemical Physics 1989, 90, 2154-2161.

108) Gonzalez, C.; Sosa, C.; Schlegel, H. B. "Ab Initio Study of the Addition-Reaction of the Methyl Radical to Ethylene and Formaldehyde." Journal of Physical Chemistry 1989, 93, 2435-2440.

109) Bach, R. D.; McDouall, J. J. W.; Schlegel, H. B.; Wolber, G. J. "Electronic Factors Influencing the Activation Barrier of the Diels-Alder Reaction - an Ab Initio Study." Journal of Organic Chemistry 1989, 54, 2931-2935.

110) McDouall, J. J. W.; Schlegel, H. B.; Francisco, J. S. "A Theoretical Investigation of the Primary Dissociation Paths of Ethynylsilane." Journal of the American Chemical Society 1989, 111, 4622-4627.

111) Gonzales, C.; Schlegel, H. B.; Francisco, J. S. "Multiple Pathways For the Direct Formation of SiH₂ From the Photodissociation of Ethylsilane." Molecular Physics 1989, 66, 859-862.

112) McDouall, J. J. W.; Schlegel, H. B. "Analytical Gradients For Unrestricted Hartree-Fock and 2nd Order Moller-Plesset Perturbation-Theory With Single Spin Annihilation." Journal of Chemical Physics 1989, 90, 2363-2369.

113) Schlegel, H. B. "Analytical 2nd Derivatives of 2 Electron Integrals Over s and p Cartesian Gaussians." Journal of Chemical Physics 1989, 90, 5630-5634.

114) Schlegel, H. B. "Some practical suggestions for optimizing geometries and locating transition states" in New Theoretical Concepts for Understanding Organic Reactions; Bertrán, J., Ed.; Kluwer Academic, the Netherlands, 1989; Vol. 267, pp 33-53.

115) Tonachini, G.; Schlegel, H. B.; Bernardi, F.; Robb, M. A. "MC-SCF Study of the Addition-Reaction of the 1-Delta-G Oxygen Molecule to Ethene." Journal of the American Chemical Society 1990, 112, 483-491.

116) Wolfe, S.; Schlegel, H. B. "On the Constitution of Dimethyl-Sulfoxide." Gazzetta Chimica Italiana 1990, 120, 285-290.

117) Ignacio, E. W.; Schlegel, H. B. "Heats of Formation of SiHmFn Calculated By Ab Initio Molecular- Orbital Methods." Journal of Chemical Physics 1990, 92, 5404-5416.

118) Bach, R. D.; Coddens, B. A.; McDouall, J. J. W.; Schlegel, H. B.; Davis, F. A. "The Mechanism of Oxygen-Transfer From an Oxaziridine to a Sulfide and a Sulfoxide - a Theoretical-Study." Journal of Organic Chemistry 1990, 55, 3325-3330.

119) Fox, G. L.; Schlegel, H. B. "An Ab Initio Study of the Vibrational Frequencies and Infrared Intensities of CH₂F₂." Journal of Chemical Physics 1990, 92, 4351-4356.

120) Gonzalez, C.; Schlegel, H. B. "Reaction-Path Following in Mass-Weighted Internal Coordinates." Journal of Physical Chemistry 1990, 94, 5523-5527.

121) Sola, M.; Gonzalez, C.; Tonachini, G.; Schlegel, H. B. "Gradient Optimization of Polarization Exponents in Ab Initio Mo Calculations On H₂SO->HSOH and CH₃SH->CH₂SH₂." Theoretica Chimica Acta 1990, 77, 281-287.

122) Bach, R. D.; McDouall, J. J. W.; Owensby, A. L.; Schlegel, H. B. "Potential For Water Catalysis in Flavin-Mediated Hydroxylation - a Theoretical-Study." Journal of the American Chemical Society 1990, 112, 7064-7065.

123) Bach, R. D.; McDouall, J. J. W.; Owensby, A. L.; Schlegel, H. B. "Theoretical-Study of Oxygen Atom Transfer - the Role of Electron Correlation." Journal of the American Chemical Society 1990, 112, 7065-7067.

124) Ignacio, E. W.; Schlegel, H. B. "An Ab Initio Study of the Structures and Heats of Formation of SiH_mF_n+ (m+n=1-4)." Journal of Physical Chemistry 1990, 94, 7439-7445.

125) Gonzalez, C.; McDouall, J. J. W.; Schlegel, H. B. "Ab Initio Study of the Reactions Between Methane and OH, H,and ³O." Journal of Physical Chemistry 1990, 94, 7467-7471.

126) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; A., R. M.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R.; Kahn, L. R.; Stewart, J. J. P.; Fulder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN 90; Gaussian, Inc.: Pittsburgh, PA, 1990.

127) Schelgel, H. B. "Do you have SCF stability and convergence problems?" in Computational Advances in Organic Chemistry; Ögretir, C. and Csizmadia, I. G., Ed.; Kluwer Academic, the Netherlands, 1991; Vol. 330, pp 167-185.

128) Bach, R. D.; McDouall, J. J. W.; Owensby, A. L.; Schlegel, H. B.; Holubka, J. W.; Ball, J. C. "Structure and Dynamics of Dicyandiamide - a Theoretical-Study." Journal of Physical Organic Chemistry 1991, 4, 125-134.

129) Schlegel, H. B.; Frisch, M. J. "Computational Bottlenecks in Molecular Orbital Calculations" in Theoretical and Computational Models for Organic Chemistry; Formosinho, J. S. C., I. G.; Arnaut, L. G., Ed.; Kluwer Academic, the Netherlands, 1991; Vol. 339, pp 5-33.

130) Manzanares, C.; Walla, D.; Seburg, R.; Wedlock, M. R.; Gonzalez, C.; Schlegel, H. B. "Overtone Spectra of C-H Bonds and Vibrational Ab Initio Study of Methoxy Boranes." Journal of Chemical Physics 1991, 95, 3031-3039.

131) Ignacio, E. W.; Schlegel, H. B. "On the Additivity of Basis Set Effects in Some Simple Fluorine Containing Systems." Journal of Computational Chemistry 1991, 12, 751-760.

132) Bach, R. D.; Owensby, A. L.; Gonzalez, C.; Schlegel, H. B.; McDouall, J. J. W. "Nature of the Transition Structure For Oxygen Atom Transfer From a Hydroperoxide - Theoretical Comparison Between Water Oxide and Ammonia Oxide." Journal of the American Chemical Society 1991, 113, 6001-6011.

133) Bach, R. D.; Owensby, A. L.; Gonzalez, C.; Schlegel, H. B.; McDouall, J. J. W. "Transition Structure For the Epoxidation of Alkenes With Peroxy Acids - a Theoretical-Study." Journal of the American Chemical Society 1991, 113, 2338-2339.

134) Bach, R. D.; Owensby, A. L.; Andres, J. L.; Schlegel, H. B. "Relative Oxygen Donor Potential of Dioxirane and Carbonyl Oxide - a Theoretical-Study." Journal of the American Chemical Society 1991, 113, 7031-7033.

135) Gonzalez, C.; Theisen, J.; Zhu, L.; Schlegel, H. B.; Hase, W. L.; Kaiser, E. W. "Kinetics of the Reaction Between OH and HO₂ On the Singlet Potential-Energy Surface." Journal of Physical Chemistry 1991, 95, 6784-6792.

136) Gonzalez, C.; Schlegel, H. B. "Improved Algorithms For Reaction-Path Following - Higher-Order Implicit Algorithms." Journal of Chemical Physics 1991, 95, 5853-5860.

Eight new algorithms for reaction path following are presented, ranging from third order to sixth order. Like the second-order algorithm [J. Chem. Phys. 90, 2154 (1989)] these are implicit methods, i.e., they rely on the tangent (and in some cases the curvature) at the endpoint of the step. The tangent (and the curvature, if needed) are obtained by a constrained optimization using only the gradient. At most, only one Hessian calculation is needed per step along the path. The various methods are applied to the Muller-Brown surface and to a new surface whose reaction path is known analytically to test their ability to follow the reaction path and to reproduce the curvature along the path.

137) Schlegel, H. B. "Following Gradient Extremal Paths." Theoretica Chimica Acta 1992, 83, 15-20.

For any point on a gradient extremal path, the gradient is an eigenvector of the hessian. Two new methods for following the gradient extremal path are presented. The first greatly reduces the number of second derivative calculations needed by using a modified updating scheme for the hessian. The second method follows the gradient extremal using only the gradient, avoiding the hessian evaluation entirely. The latter algorithm makes it possible to use gradient extremals to explore energy surfaces at higher levels of theory for which analytical hessians are not available.

138) Fox, G. L.; Schlegel, H. B. "An Ab Initio Study of Hydrogen-Atom Abstractions From Substituted Methanes By Substituted Methyl Radicals." Journal of Physical Chemistry 1992, 96, 298-302.

The reactions CH3X + .CH2Y --> .CH2X + CH3Y (X, Y = H, F, Cl, OH, NH2, CN) have been studied using Ab Initio molecular orbital theory at the UHF/3-21G and UHF/6-31G** levels. The Marcus relation can be used to predict the barrier heights of the cross reactions based on the information from the identity reactions and the changes in energy for the cross reactions. This method predicts the barrier heights accurately with two exceptions (X = CN and Y = NH2 or OH); the average absolute error is 0.98 kcal/mol when compared to the Ab Initio barrier heights. Charge-transfer states have been found to play a significant role in the reactions where X = CN and Y = NH2 or OH and a minor role in a few other reactions.

139) Ignacio, E. W.; Schlegel, H. B. "An Ab Initio Molecular-Orbital Study of the Thermal- Decomposition of Fluorinated Monosilanes, SiH_4-nF_n (n=0-4)." Journal of Physical Chemistry 1992, 96, 1620-1625.

The reactants, clusters, transition structures, and products for the various channels for the thermal decomposition of SiH4- nFn were optimized at the HF/6-31G* level. The electron correlation contributions were calculated at MP4/6-31G* and MP4/6-31G** levels. In the decomposition of SiH4, SiH2 + H-2 is favored over SiH3 + H. For SiH3F, the SiHF + H-2 channel is preferred over SiH2F + H, SiH2 + HF, and SiH3 + F. In the decomposition of SiH2F2, the barrier for the formation of SiF2 + H-2 is slightly lower than those of SiHF2 + H and SiHF + HF, while those of the SiH2F + F and SiH2 + F2 channels are considerably higher. For SiHF3, decomposition into SiF2 + HF is favored with SiF3 + H lying slightly higher. The other two channels SiHF2 + F and SiHF + F2 are much higher in energy. In SiF4, the decomposition into SiF3 + F is favored over the reaction SiF4 --> SiF2 + F2.

140) Ignacio, E. W.; Schlegel, H. B. "Ab Initio Mo Study of the Thermal-Decomposition of Fluorinated Disilanes, Si₂H_6-nF_n (n = 0, 1, 2)." Journal of Physical Chemistry 1992, 96, 1758-1764.

The reactants, transition structures, and products for the various channels for the thermal decomposition of Si2H6-nF(n) were optimized at the HF/3-21G and HF/6-31G* levels. The electron correlation contributions were calculated at the MP4/6-31G* level with the zero-point energies at the HF/3-21G level. Isodesmic reactions have been used to estimate the heats of formation of SiH3SiH2F, SiH2FSiH2F, SiH3SiHF2, SiH2FSiH, SiHF2SiH, SiH3SiF, and SiH2FSiF. In the decomposition of disilane, the SiH2 + SiH4 and SiH3SiH + H-2 paths are favored over other modes of decomposition. For monofluorodisilane, the SiH3F + SiH2, SiH2FSiH + H-2, and SiH3SiF + H-2 channels are nearly equal in energy and are preferred over SiH4 + SiHF. In the decomposition of the 1,1- difluorodisilane, SiH3SiHF2, the barriers for decomposition into SiH2F2 + SiH2, SiHF2SiH + H-2, and SiH3F + SiHF are very close in energy, while the SiH4 + SiF2 channel is considerably higher. In the decomposition of the 1,2-difluorodisilane, SiH2FSiH2F, the barriers for decomposition into SiH2F2 + SiH2, SiH2FSiF + H-2, and SiH3F + SiHF are similar in energy. For all the fluorodisilanes considered, the barriers for the elimination of HF and the homolytic cleavage of the Si-Si bond are considerably higher than those of the elimination of H-2 or the extrusion of a silylene.

141) Chen, W.; Hase, W. L.; Schlegel, H. B. "Ab initio MO calculations of the thermochemistry of BX, AlX, OBX and OAlX (X = O, F, Cl)" in Gas-Phase Metal Reactions; Fontijn, A., Ed.; Elsevier, the Netherlands, 1992, pp 179-187.

142) Shaik, S. S.; Schlegel, H. B.; Wolfe, S. Theoretical aspects of physical organic chemistry : the S*N2 mechanism; Wiley: New York, 1992.

143) Bach, R. D.; Andres, J. L.; Owensby, A. L.; Schlegel, H. B.; McDouall, J. J. W. "Electronic-Structure and Reactivity of Dioxirane and Carbonyl Oxide." Journal of the American Chemical Society 1992, 114, 7207-7217.

Dioxirane, the diradical dioxymethane, and carbonyl oxide have been studied at a variety of levels including MP2, MP4, CASSCF, QCISD, and QCISD(T). MP2 optimization is sufficient for dioxirane, the various dioxymethane biradicals, and substituted carbonyl oxide, but optimization at the QCISD or QCISD(T) level is needed to obtain the correct structure for carbonyl oxide. Significantly, both levels of theory give essentially the same C-O/O-O bond length ratio for this controversial molecule (1.281/1.369 angstrom and 1.287/1.356 angstrom for QCISD/6-31G* and QCISD(T)/6-31G**, respectively). The unsubstituted carbonyl oxide is probably anomalously sensitive to the level of calculation, since MP2/6-31G* calculations on dimethylcarbonyl oxide afford a C-O/O-O bond ratio of 1.283/1.345 angstrom, consistent with the higher level calculations on the parent carbonyl oxide. Transition structures have been obtained at the MP2/6-31G* level for the isomerization of carbonyl oxide to dioxirane and for the donation of oxygen from carbonyl oxide and dioxirane to ethylene. A scan of the potential energy curves for O-O bond dissociation in hydrogen peroxide and water oxide shows that the MP4SDTQ and QCISD(T) curves are nearly superimposable for bond elongation up to 2.5 angstrom. At the QCISD(T)/6-31G*//MP2/6-31G* level, carbonyl oxide is 28.6 kcal/mol higher than dioxirane and separated from it by a barrier of 19.1 kcal/mol; the barriers for donation of an oxygen atom from dioxirane and carbonyl oxide to ethylene are 16.7 and 11.9 kcal/mol, respectively. The MP4SDTQ level is adequate for determining the barriers for the first two reactions, but QCISD(T) is required for a proper treatment of the barrier for dioxirane plus ethylene (MP4 overestimates the triples contribution). Theoretical evidence is presented that suggests that both the reactants and the transition structures involved in these epoxidation reactions are principally closed shell in nature.

144) Frisch, M.; Ragazos, I. N.; Robb, M. A.; Schlegel, H. B. "An Evaluation of 3 Direct MC-SCF Procedures." Chemical Physics Letters 1992, 189, 524-528.

The direct MC-SCF approach for modest active spaces is evaluated for (a) extended basis sets on small molecules and (b) modest sets on extended systems. An algorithm that scales as n(a)2N4 (n(a) = number of active orbitals, N = number of AO) using Fock matrices appears to be quite efficient until n(a) gets to 8 where an algorithm that scales as n(a)N4 using 3/4 transformed integrals begins to become competitive. The storage requirement for the J and K operators in the n(a)N4 algorithm has been removed with the cost of an additional direct Fock matrix construction.

145) Ignacio, E. W.; Schlegel, H. B. "Ab Initio Study of the Structures, Properties, and Heats of Formation of Fluorochloromethanes and Fluorochlorosilanes, CH_4-m-nF_mCl_n and SiH_4-m-nF_mCl_n(m+n=0-4)." Journal of Physical Chemistry 1992, 96, 5830-5837.

Equilibrium geometries and vibrational frequencies for CH4-m- nFmCln and SiH4-m-nFmCln (m + n - 0-4) were computed at the HF/6-31G* levels; electron correlation contributions were calculated at the MP4/6-31G** level. Heats of formation were obtained from isodesmic reactions using the experimental DELTA- H(f)-degrees for AH4, AF4, and ACl4 (A = C, Si). Good agreement between theory and experiment is found for CH4-m-nFmCln. For the mixed fluorochlorosilanes, the calculations predict the following heats of formation (in kcal/mol at 298 K and 1 atm): SiH2FCl, -129.3 +/- 3; SiHF2Cl,-230.3 +/- 3; SiHFCl2,-171.6 +/- 3; SiF3Cl, -329.5 +/- 3; SiF2Cl2,-272.9 +/- 3; SiFCl3, -215.8. Trends in the bond lengths, AH vibrational frequencies, and stabilization energies can be explained as a balance between electrostatic effects due to the electronegative substituents and negative hyperconjugation from the halogen lone pairs to adjacent sigma* orbitals.

146) Gonzalez, C.; Schlegel, H. B. "Atmospheric Chemistry of Titan - Ab Initio Study of the Reaction Between Nitrogen-Atoms and Methyl Radicals." Journal of the American Chemical Society 1992, 114, 9118-9122.

Ab Initio calculations were performed on the reaction N + CH3 - -> products. Optimized geometries have been calculated for all reactants, transition states, and products at the MP2/6-31G** level. Barriers and heats of reaction have been estimated by fourth-order Moller-Plesset perturbation theory with spin projection (PMP4(SDTQ)). Harmonic vibrational frequencies and zero-point energy corrections were calculated at the MP2/6- 31G** level. The two-step process N(4S) + CH3 --> 3[CH3N] --> H2CN + H appears to be the most important channel in this reaction.

147) Schlegel, H. B. "A Comparison of Geometry Optimization With Internal, Cartesian, and Mixed Coordinates." International Journal of Quantum Chemistry 1992, 243-252.

Improvements in mo programs and computer speeds have dramatically increased the size of molecules that can be optimized by MO methods. This has put a considerable strain on existing geometry optimization techniques. For a given molecular system, the performance of a geometry optimization method can vary significantly depending on the coordinates used. Nonredundant internal coordinates (e.g., Z-matrix coordinates) are easily constructed and behave well for acyclic molecules but can be quite troublesome for cyclic systems. Cartesian coordinates behave well for rigid, cyclic molecules but can he quite poor for flexible, acyclic systems. Mixed cartesian and internal coordinates have been suggested as a means of combining the best of both approaches and are particularly convenient for cyclic systems with flexible substituents. The efficiency of these different approaches is compared for a number of molecules.

148) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andrés, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; Gaussian, Inc.: Pittsburgh, PA, 1992.

149) Darling, C. L.; Schlegel, H. B. "Ab Initio Study of the Reaction Pathways For Si⁺ + SiX₄ (X = F, Cl)." Journal of Physical Chemistry 1993, 97, 1368-1373.

The reaction pathways of Si+ + SiF4 and Si+ + SiCl4 have beer studied by Ab Initio molecular orbital methods. Equilibrium geometries and transition structures were fully optimized at the Hartree-Fock level with the 3-21 G and 6-31G* basis sets. Barrier heights have been computed with fourth-order Moller- Plesset perturbation theory, with and without annihilation of spin contamination. Direct attack of Si+ on X in SiX4 forms a cluster X3Si-X-Si+, which has a substantial barrier for rearrangement to X3Si-SiX+. Both clusters can dissociate into SiX3 + SiX+ or SiX3+ + SiX; the second cluster can rearrange further to yield X2Si-SiX2+, which can dissociate to SiX2 and SiX2+. The calculated thresholds agree well with the observed values for SiF+ + SiF3, SiCl+ + SiCl3, SiCl3+ + SiCl and SiCl2+ + SiCl2; however, the observed thresholds for SiF3+ + SiF and SiF2+ + SiF2 are considerable higher than the calculated values, suggesting an activation barrier or a dynamic bottleneck.

150) Fox, G. L.; Schlegel, H. B. "A Curious Artifact in the Potential-Energy Surface For the F+HF Abstraction Reaction." Journal of the American Chemical Society 1993, 115, 6870-6876.

The reaction F + HF --> HF + F has been studied using Ab Initio molecular orbital theory at UHF, MP2, MP3, MP4, CCD, CCSD, CID, CISD, QCISD, QCISD(T), and CASSCF levels of theory with the 3- 21G, 6-31G**, 6-311++G**, and D95++(3df,2p) basis sets. The UHF, MP4, CIS, CISD, CCSD, QCISD, QCISD(T), and CASSCF levels of theory predict the transition state to be a highly bent C2v symmetry structure, whereas MP2, MP3, and CCD levels predict the C2v symmetry structure to be a shallow minimum bound by up to 0.4 kcal/mol. The MP2/6-31G** C2v structure is bracketed by two C(s) symmetry transition states with bond lengths of 1.1411 and 1.0541 angstrom and a bond angle of 127.0-degrees. The C2v minimum on the MP2, MP3, and CCD surfaces is caused by inadequately treated avoided crossings between the bond making/breaking state and two broken symmetry F+HF hydrogen bonding states. Dynamic correlation methods that include single excitations in an interative or self consistent manner, such as CASSCF, CISD, CCSD, and QCISD, appear to treat the avoided crossings properly and predict a suitable transition state. At the QCISD(T)/D95++(3df,2p) level, the C2v transition state geometry is R(H-F) = 1.102 angstrom and angle F-H-F = 134.6- degrees and the reaction barrier is 17.5 kcal/mol.

151) Schlegel, H. B.; Skancke, P. N. "Theoretical-Study of Reaction Pathways For F(-)+H₃SiCHO." Journal of the American Chemical Society 1993, 115, 10916-10924.

The potential surface for the system F- + H3SiCHO has been studied using a 6-31++G** basis. Complete geometry optimizations using analytical gradients have been carried through for all species involved. All stationary points have been characterized by analytically calculated force constant matrices using the same basis. Energies were calculated at the MP4(SDTQ)/6-31++G**//RHF/6-31++G** level. Relative energies have been corrected for differences in ZPE. The initially formed adduct (FH3Si-CHO)- is found to have an energy of -50.5 kcal/mol relative to the reactants and is formed without a barrier from F- and formylsilane. The axial form of the adduct is predicted to be energetically favored by 9.8 kcal/mol relative to the equatorial isomers. The reaction energy for the displacement reaction with CHO- as the leaving group is found to be +6.3 kcal/mol, i.e. an endothermic reaction. Among the many subsequent reaction products obtained after rearrangement of the initially formed adduct we mention the following low- energy species: the ether (HFSi-O-CH3)- having a relative energy of -71.7 kcal/mol, and two forms of the cyclic anion (FH2Si-CH2-O)- having the relative energies -60.3 and -61.3 kcal/mol. The highest reaction barriers predicted are associated with H-migration from Si to C in the initially formed adduct (23.9 kcal/mol) and the H-migration from Si to O in the alkoxide leading to the alcohol (H2FSi-CH2-OH)- (23.3 kcal/mol). It has been found that the association of CH2O and SiFH2- yielding the alkoxide (H2FSi-CH2-O)- probably occurs without an energy barrier. The Brook rearrangement, implying a migration of a Si moiety from carbon to oxygen, has been proposed for solution reactions. The present calculations are unable to confirm such an arrangement in the gas phase.

152) Schlegel, H. B.; Skancke, A. "Thermochemistry, Energy Comparisons, and Conformational- Analysis of Hydrazine, Triazane, and Triaminoammonia." Journal of the American Chemical Society 1993, 115, 7465-7471.

The series NH3, H2NNH2, HN(NH2)2 (triazane), and N(NH2)3 (triaminoammonia) has been studied by computational methods (HF/6-31G* and MP2/6-31G* optimizations for all species, G2 (see paragraph on computations) energies for the most stable form of each molecule). Thermodynamic functions have been computed. Structural aspects have been discussed in terms of stabilizing and destabilizing factors. The importance of hydrogen bonding and hyperconjugation has been discussed on the basis of potential surface differences and vibrational spectra. Relative stabilities have been discussed from isodesmic reaction schemes, from which also the following estimates of N- N bond energy are obtained: hydrazine, 64 kcal/mol; triazane, 66 kcal/mol; triaminoammonia, 59 kcal/mol. The kinetic stabilities of triazane and triaminoammonia have been considered from various modes of decomposition.

153) Foresman, J. B.; Schlegel, H. B. "Application of the CI-singles method in predicting the energy, properties and reactivity of molecules in their excited states" in Recent experimental and computational advances in molecular spectroscopy:; Fausto, R. and Hollas, J. M., Ed.; Kluwer Academic, the Netherlands, 1993; Vol. 406, pp 11-26.

154) Darling, C. L.; Schlegel, H. B. "Heats of Formation of Sihno and SiH_nO₂ Calculated By Ab-Initio Molecular-Orbital Methods At the G-2 Level of Theory." Journal of Physical Chemistry 1993, 97, 8207-8211.

To help facilitate the study of the energetics and the mechanism of silane combustion and the oxidation of related silicon species, H(m)Si-OH(n), H(m)O-SiH(n)-OH(p), and H(m)Si- O-OH(n) have been examined by Ab Initio molecular orbital methods. Geometries have been optimized at the MP2/6-31G(d) level of theory, and vibrational frequencies have been computed at HF/6-31G(d). Heats of formation have been calculated at the G-2 level of theory (estimated mean absolute error of +/-2 kcal/mol or less). The following theoretical heats of formation (at 298 K) have been obtained: SiO, -22.6; HSiO, 8.5; SiOH, 0.1; H2SiO, -23.5; HSiOH, -23.7; H3SiO, 1.0; H2SiOH, -25.6; H3SiOH, -67.5; OSiO, -66.2; OSiHO, -37.7; OSiOH, -72.9; OSiH2OBAR, -28.1; OSiHOH, -110.6; HOSiOH, -117.5; OSiH2OH, - 79.8; HOSiHOH, -104.2; HOSiH2OH, -149.0; HSiOO, 47.7; SiOOH, 23.7; H2SiOO, 35.5; HSiOOH, 0.2; H3SiOO, -2.7; H2SiOOH, 1.1; H3SiOOH, -41.4 kcal/mol.

155) Su, M. D.; Schlegel, H. B. "Heats of Formation of Sihmcln Calculated By Ab-Initio Molecular-Orbital Methods." Journal of Physical Chemistry 1993, 97, 8732-8735.

Optimized geometries and vibrational frequencies for SiH(m)Cl(n) were calculated at the MP2/6-31G(d,p) level. Energy differences were computed at the MP4/6-31+G(2df,p) level (all structures) and the G-2 level (structures containing no more than two chlorines). The heats of formation of SiH(m)Cl(n) were estimated using the following isodesmic and isogyric reactions: [(4-n)/4]SiH4 + (n/4)SiCl4 --> SiH4-nCln, SiHm + SiH4-nCln-- >SiHCln + SiH4, 3/4Si + 1/4H-2 + SiH3Cl --> SiCl + 1/2H + 3/4SiH4,1/2Si + 1/2H-2 + SiH4-nCln - SiH2-nCl + H + 1/2SiH4, and 1/4Si + 1/2H + SiH4-nCln --> SiH3-nCln + 1/4H-2 + 1/4SiH4. The calculated heats of formation (kcal/mol; 298 K, 1 atm) are as follows: SiCl, 36.5 +/- 1.5; SiHCl, 15.0 +/- 1.5; SiCl2, - 38.6 +/- 1.5; SiH2Cl, 8.0 +/- 1.5; SiHCl2, -34.0 +/- 1.5; SiCl3, -75.8 +/- 1.5; SiH3Cl, -32.0 +/- 1.5; SiH2Cl2, -74.2 +/- 1.5; SiHCl3, -116.8 +/- 1.5 (based on experimental values for SiH(n) and SiCl4). The present study uses a significantly higher level of theory than that used in earlier work and confirms the heats of formation predicted by the best previous theoretical studies to within 4-0.8 kcal/mol. The theoretical heats of formation are within the error bars of the most recent experimental values except for SiCl (46 +/- 5 kcal/mol), suggesting that a new experimental value would be desirable for SiCl.

156) Su, M. D.; Schlegel, H. B. "An Ab-Initio Mo Study of the Thermal-Decomposition of Chlorinated Monosilanes, SiH_4-nCl_n (n = 0-4)." Journal of Physical Chemistry 1993, 97, 9981-9985.

The reactants, clusters, transition structures, and products for the various channels for the thermal decomposition of SiH4- nCln were optimized at the HF/6-31G(d) and MP2/6-31G(d,p) levels. The electron correlation contributions were calculated at the MP4/6-31 G(d,p), MP4/6-31+G(2df,p), MP2/6- 31++G(3df,3pd), and G-2 levels of theory. In the decomposition of SiH4, SiH2 + H-2 is favored over SiH3 + H. For SiH3Cl, the SiHCl + H-2 channel is preferred over the SiH2 + HCI channel, with fragmentation into SiH2Cl + H and SiH3 + Cl lying significantly higher. The barriers for SiH2Cl2 decomposition are SiCl2 + H-2 almost-equal-to SiHCl + HCI < SiHCl2 + H < SiH2Cl + Cl < SiH2 + Cl2. For SiHCl3, decomposition into SiCl2 + HCI is favored over SiCl3 + H, with the SiHCl2 + Cl and SiHCl + Cl2 channels lying substantially higher. In tetrachlorosilane, SiCl4 --> SiCl3 + Cl is favored over SiCl4 - -> SiCl2 + Cl2.

157) Bach, R. D.; Mintcheva, I.; Kronenberg, W. J.; Schlegel, H. B. "Torsional Barriers in Alpha-Keto Amides - Model Studies Related to the Binding-Site of Fk506." Journal of Organic Chemistry 1993, 58, 6135-6138.

158) Bach, R. D.; Su, M. D.; Aldabbagh, E.; Andres, J. L.; Schlegel, H. B. "A Theoretical-Model For the Orientation of Carbene Insertion Into Saturated-Hydrocarbons and the Origin of the Activation Barrier." Journal of the American Chemical Society 1993, 115, 10237-10246.

Transition states for carbene insertion reactions into C-H bonds can be classified as sigma or pi approaches. In the pi approach, the empty orbital of the carbene is aligned with the carbon p orbital of the pi(CH2) fragment orbital; in the sigma approach, the empty carbene p orbital is aligned with the sigma(CH2) fragment orbital. Concerted hydrogen migration to the larger lobe of the carbene lone pair is energetically favorable. The transition state for CH2 insertion into methane has been calculated at the HF/6-31G*, MP2/6-31G*, and QCISD/6- 31G* levels. At all levels, the sigma approach is slightly favored over the pi approach. The barrier at HF is too high, and the C-C bond in the transition state is too short. A small barrier (0.4 kcal/mol) relative to a long-range complex was found at the QCISD/6-31G* level, but none is found at QCISD(T)/6-31G* and QCISD/6-311G**. Transition states have also been optimized at the HF/6-31G* and MP2/6-31G* levels for HCCH3, C(CH3)2, CHF, and CF2 inserting into methane. The sigma approach is again slightly favored over pi. For vinylidene (C=CH2) there is a slight preference for the pi approach for insertion into both methane and ethane. Transition states for insertion of CH2, HCCH3, C(CH3)2, CF2, vinylidene, and silylene into ethane have been optimized at HF/6-31G* and MP2/6-31G*. The sigma(CH2) pathways are favored over pi(CH2) and pi(CHCH3) With sigma(CHCH3) lying significantly higher. For the carbenes considered, there is a wide variation in the barrier heights. The transition state with the carbene in an inverted orientation and the hydrogen migrating to the wrong side of the carbene lone pair is typically 5-10 kcal/mol higher. Contrary to expectations, the barrier heights do not correlate with the HOMO or LUMO energies or the HOMO-LUMO gap. Instead, the trend correlates with the singlet-triplet energy differences in the carbenes. The valence bond state correlation method has been used to develop an explanation for the barrier heights.

159) Bach, R. D.; Su, M. D.; Andres, J. L.; Schlegel, H. B. "Structure and Reactivity of Diamidoiron(III) Hydroperoxide - the Mechanism of Oxygen-Atom Transfer to Ammonia." Journal of the American Chemical Society 1993, 115, 8763-8769.

Diamidoiron(III) bydroperoxide has been examined as a highly simplified model for the hydroperoxide intermediate in hemoproteins such as peroxidases and cytochrome P-450. Equilibrium structures and transition states were optimized at the UHF and UMP2 levels using the 3-21 G basis and the Wachters-Hay basis set (augmented with an f function on iron). The MP2 level was found to be essential for a proper description of the geometry and bonding. Diamidoiron(III) hydroperoxide is a peroxo-bridged structure with Fe-O bond lengths of 1.98 and 2.15 angstrom. A CASSCF/3-21G geometry optimization also afforded a cyclic Fe-OOH structure with an Fe-O bond angle of 70.4-degrees. Similar calculations on (HO)3TiOOH yielded a bridged structure in excellent agreement with X-ray data. The barrier for oxygen transfer from diamidoiron(III) hydroperoxide to ammonia is only 10.2 kcal/mol (MP4 energies using the Wachters-Hay basis), a barrier lower than that for LiOOH (20.0 kcal/mol) and peroxyformic acid (17.8 kcal/mol). Protonated diamidoiron(III) hydroperoxide is no longer bridged (R(Fe-O) = 2.01 and 2.85 angstrom) and has a negative barrier for oxygen donation to NH3. Although these highly abbreviated models have many shortcomings, the present calculations suggest that in certain cases oxygen atom transfer from iron hemoproteins could precede the formation of the ferryl complex.

160) Frisch, M. J. T., G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andrés, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Ba GAUSSIAN 92/DFT; Gaussian, Inc.,: Pittsburgh, PA, 1993.

161) Gordon, M. S. F., J. S.; Schlegel, H. B. "Theoretical investigations of the thermochemistry and thermal decomposition of silanes, halosilanes and alkylsilanes." Advances in Silicon Chemistry 1993, 2, 137-185.

162) Peng, C. Y.; Schlegel, H. B. "Combining Synchronous Transit and Quasi-Newton Methods to Find Transition-States." Israel Journal of Chemistry 1993, 33, 449-454.

A linear synchronous transit or quadratic synchronous transit approach is used to get closer to the quadratic region of the transition state and then quasi-newton or eigenvector following methods are used to complete the optimization. With an empirical estimate of the hessian, these methods converge efficiently for a variety of transition states from a range of starting structures.

163) Bach, R. D.; Schlegel, H. B.; Andres, J. L.; Sosa, C. "A Model For the Free-Radical and Electrophilic Hydroxylation of Bicyclo[2.1.0]Pentane." Journal of the American Chemical Society 1994, 116, 3475-3482.

Ab Initio molecular orbital calculations have been used to model the stereochemistry of bicyclo[2.1.0]pentane hydroxylation. Equilibrium geometries and transition states were fully optimized at the MP2 level of theory using the 6- 31G* and 6-31G** basis sets; all transition states were confirmed to be first-order saddle points by MP2 frequency calculations; energy differences and barrier heights were computed at the MP4 level with and without spin projection. Both the endo- and exo-bicyclo[2.1.0]pent-2-yl radicals are significantly pyramidal, but are nearly equal in energy (Delta E < 0.3 kcal/mol) and are separated by a very low (<0.4 kcal/mol) barrier. The barrier for trapping the bicyclopentyl radical by H2S is 1.5 kcal/mol lower for the endo radical. Even though the endo and exo bond strengths are nearly identical in bicyclo[2.1.0]pentane, abstraction of the endo hydrogen via the OH radical is favored over the exo hydrogen by 1.4 kcal/mol. Concerted oxygen insertion was modeled by reaction of bicyclo[2.1.0]pentane with water oxide, H2OO; the insertion transition state yielding the endo alcohol is 1.3 kcal/mol lower in energy. The endo preference of all the reactions in the present study can be attributed to cyclopropylcarbinyl stabilization of the transition states. The relevance of these calculations to cytochrome P-450 hydroxylation is discussed.

164) Schlegel, H. B. "Some Thoughts On Reaction-Path Following." Journal of the Chemical Society-Faraday Transactions 1994, 90, 1569-1574.

Errors in reaction-path following have been examined with the aid of a simple model surface and a local correction scheme has been proposed. Two new fourth-order explicit methods for reaction-path following have been developed, and two potential- energy surfaces with analytical reaction paths have been constructed. Some aspects of reaction-path bifurcation are also discussed.

165) Bach, R. D.; Su, M. D.; Schlegel, H. B. "Oxidation of Amines and Sulfides With Hydrogen-Peroxide and Alkyl Hydrogen-Peroxide - the Nature of the Oxygen-Transfer Step." Journal of the American Chemical Society 1994, 116, 5379-5391.

Ab Initio molecular orbital methods have been used to study the oxidation of ammonia, dihydrogen sulfide, and dimethyl sulfide by hydrogen peroxide and methyl hydroperoxide. Geometries of reactants, complexes, transition states, and products were fully optimized at the MP2/6-31G* level, and relative energies were computed at the MP4/6-31G*//MP2/6-31G* level. Without protic solvent catalysis, a 1,2-hydrogen shift is needed before oxygen transfer from H2O2 takes place and the barriers are much too high (ca. 50 kcal/mol). One or two molecules of protic solvent reduce these barriers by ca. 10 kcal/mol. In the absence of protic solvents, the hydroperoxide itself can act as a general acid catalyst, leading to second-order kinetics in H2O2 (in agreement with experiment). An increase in pK(a) of the general acid catalyst results in a decrease in activation energy for oxygen atom transfer from hydrogen peroxide. The mechanism for oxidation of sulfur is similar to that of oxidation of NH3, and methyl substitution at sulfur results in a modest stabilization of the transition state. For methyl hydroperoxide oxidation of ammonia, an ionic pathway is slightly lower in energy and involves a proton shift occurring after transfer of HO+. In agreement with experiment, protonation by strong acids has a much larger effect on the barrier than catalysis by weak acids such as H2O, H2O2, and NH3. However, the calculations show that under acidic conditions protonation occurs at the nucleophile rather than the peroxide, thereby greatly diminishing the catalytic effect. Nevertheless, all of these oxidation reactions have calculated barriers that are unreasonably high, in the range 35-50 kcal/mol. However, if proton transfer from solvent is combined with protic solvent stabilization of an ionic transition state, the calculated barriers are reduced to 5-15 kcal/mol, in good agreement with experiment. Thus the mechanism for amine or sulfide oxidation involves a protonated solvent molecule transferring a proton to the distal oxygen of the hydroperoxide in concert with a second molecule of solvent stabilizing the transfer of HO+ from the hydroperoxide to the nucleophile.

166) Darling, C. L.; Schlegel, H. B. "Dipole-Moments, Polarizabilities, and Infrared Intensities Calculated With Electric-Field Dependent Functions." Journal of Physical Chemistry 1994, 98, 5855-5861.

Electrical properties of molecules are difficult to compute accurately, and traditional calculations typically require large basis sets with multiple sets of diffuse and polarization functions. Ditchfield, Sadlej, and Almlof have shown that much smaller basis sets can be used if electric dependence is incorporated into the basis set. We have combined the features of these methods and allowed the center of each Gaussian, A, to depend on the field, epsilon, according to A(epsilon) = A(0) - alpha(n) lambda epsilon, where alpha is the Gaussian exponent, n is a fixed integer, and lambda is a parameter chosen to maximize the polarizability in a series of reference molecules. The field dependence is used in both the exponential and the preexponential factors in the Gaussian basis functions. Electric field dependent HF/6-31+G** calculations of dipole moments, polarizabilities, and infrared intensities are approximately the same quality as HF/6-311++G(3d,3p) calculations without field dependent functions (the improvement is most striking for the polarizabilities of linear molecules). Field dependence of the outer valence and diffuse functions appears to be the most important. The results for n = -2 and n = 0 are very similar; the existing codes for geometrical derivatives of the energy can be easily modified to compute improved electrical properties for the n = 0 case.

167) Darling, C. L.; Schlegel, H. B. "Ab-Initio Study of the Initial Reactions in Silane Combustion - SiH₃+O₂->Products." Journal of Physical Chemistry 1994, 98, 8910-8913.

To help understand the mechanism and kinetics of silane combustion, the initial steps of silane oxidation have been studied by Ab Initio molecular orbital theory. Geometries have been optimized at the MP2/6-31G(d) level of theory and vibrational frequencies have been computed at HF/6-31G(d); relative energies and barrier heights have been calculated at the G-2 level of theory. Silyl radical and O-2 react to form H3SiOO, which undergoes a 1,3-hydrogen shift to form H2SiOOH, via a transition state that is 17.5 kcal/mol below that of the reactants. A low barrier of ca. 4 kcal/mol separates H2SiOOH from H2SiO and OH. Hydroxyl radical can abstract a hydrogen from H2SiO to form HSiO + H2O or it can add to H2SiO to form H2Si(O)OH-both processes appear to be barrierless. The latter intermediate can lose hydrogen to form HSi(O)OH or rearrange to form HSi-(OH)(2).

168) Schlegel, H. B.; Harris, S. J. "Thermochemistry of BH_mCl_n Calculated At the G-2 Level of Theory." Journal of Physical Chemistry 1994, 98, 11178-11180.

The heats of formation of BHmCln have been computed at the G-2 level of theory, which is estimated to be accurate to +/-2 kcal/mol. The calculated ideal gas values in kcal/mol are Delta H-f298(0)(BH) = 105.8, Delta H-f298(0)(BCl) = 42.5, Delta H- f298(0)(BH2) = 79.4, Delta H-f298(0)(BHCl) = 34.5, Delta H- f298(0)(BCl2) = -6.8, Delta H-f298(0)(BH3) = 24.9, Delta H- f298(0)(BH2Cl) = -19.0, Delta H-f298(0)(BHC12) = -60.7, Delta H-f298(0)(BCl3) = -98.6.

169) Chen, W.; Schlegel, H. B. "Evaluation of S² For Correlated Wave-Functions and Spin Projection of Unrestricted Moller-Plesset Perturbation-Theory." Journal of Chemical Physics 1994, 101, 5957-5968.

The value of S-2 can be an important diagnostic tool for judging the quality of correlated wave functions. A production code has been developed to evaluate S-2 for unrestricted Moller-Plesset perturbation theory (UMPn), coupled clusters (UCCSD), quadratic configuration interaction (UQCISD), and Brueckner doubles (UBD) methods, and to evaluate UMP3 and UMP4 energies with spin projection. The code has been used to examine the bond. dissociation potentials for HF-->H+F and CH4- ->CH3+H. For both systems, the onset of the RBD-UBD instabitity occurs near S-2 approximate to 0.35 for UCCSD or UQCISD calculations. Maximum errors in the UMP4, UCCSD(T), UQCISD(T), and UBD(T) single bond dissociation curves are near S-2 approximate to 0.5. The behavior of S-2 for UCCSD and UQCISD is closer to BD than MP4. Projected MP4 energies are in good agreement with full CI calculations, but between the onset of the RHF-UHF instability and the RBD-UBD instability, CCSD(T), QCISD(T), and BD(T) are significantly better. If A(Norm)>1.2 for restricted calculations on single bond dissociations, it-is better to use a spin-unrestricted method.

170) Bearpark, M. J.; Robb, M. A.; Schlegel, H. B. "A Direct Method For the Location of the Lowest Energy Point On a Potential Surface Crossing." Chemical Physics Letters 1994, 223, 269-274.

We present a method, which avoids the use of Lagrange multipliers, for the optimisation of the lowest energy point of the intersection of two potential energy surfaces. The efficiency of this unconstrained algorithm is demonstrated for the n - 2 intersection space of a conical intersection and the n - 1 intersection space of the crossing of two states of different spin multiplicity.

171) Ayala, P. Y.; Schlegel, H. B. "An Ab-Initio Molecular-Orbital Study of SiH₂+F₂->SiH₂F₂." Chemical Physics Letters 1994, 225, 410-415.

The insertion of SiH2 into F2 has been studied with MP2, CCD, CISD and QCISD. For fluorine approaching syn to the silicon lone pair, the complex is 2.5 kcal/mol below reactants and the insertion barrier is 0.5 kcal/mol above reactants at QCISD/6- 31G*; for the anti approach, there is no barrier (only a broad plateau) and insertion proceeds with inversion at silicon. With RQCISD, both pathways go directly to SiH2F2, but with RMP2 and RCCD, there are several structures in the SiH2F-F region of the surface that are artifacts of the level of theory.

172) Bach, R. D.; Andres, J. L.; Winter, J. E.; Schlegel, H. B.; Ball, J. C.; Holubka, J. W. "A Model For Adhesion-Producing Interactions of Zinc-Oxide Surfaces With Alcohols, Amines, and Alkenes." Journal of Adhesion Science and Technology 1994, 8, 249-259.

The interactions between paint/adhesive polymers and metal surfaces that are critical for adhesion have been studied theoretically. This study used zinc oxide as a model of a galvanized steel surface, and ammonia, water, and ethylene as models for amino, hydroxy, and unsaturated functionalities in paint/adhesive polymers. Ab Initio molecular orbital calculations were carried out on zinc oxide and zinc oxide dimer. Geometries were optimized at the HF/3-21G level and relative energies were calculated by CASSCF/3-21G and by MP2 with the DZP basis set of Wachters and Hay. Ethylene forms a stable complex with zinc oxide dimer that has a stabilization energy of 24.9 kcal/mol. Insertion of ethylene into zinc oxide dimer to form a stable six-membered ring adduct occurs with a surprisingly low activation energy of 8.8 kcal/mol. The binding energy of ammonia with zinc oxide dimer is 38.5 kcal/mol and the activation energy for insertion of ammonia forming covalent Zn-NH2 and O-H bonds is calculated to be 9.6 kcal/mol. Aminolysis of zinc oxide dimer with two ammonia molecules has a predicted barrier height of 6.7 kcal/mol. The transition structure for Zn-O bond rupture with one NH3 and one H2O molecule is only 1.5 kcal/mol higher in energy than the reactant cluster. The calculations suggest that alkenes, amines, and alcohols could readily form covalent bonds with the ZnO surface, thereby facilitating adhesion of the polymer containing these functional groups to a galvanized surface.

173) Chen, W.; Hase, W. L.; Schlegel, H. B. "Ab-Initio Classical Trajectory Study of H₂CO->H₂+CO Dissociation." Chemical Physics Letters 1994, 228, 436-442.

Classical trajectories for H2CO-->H-2+CO dissociation have been calculated directly from ah initio molecular orbital computations at the HF/3-21G and HF/6-31G** levels of theory, without constructing a global potential energy surface. The classical equations of motion were integrated on local fifth- order polynomial surfaces fitted to the energies, gradients and hessians computed at the beginning and end points of each step along the trajectory. The calculated vibrational and rotational energy distributions and average impact parameter of the products are in very good agreement with experiment. The relative translational energy is higher than experiment because the barrier is overestimated at both levels of theory.

174) Schlegel, H. B.; Frisch, M. J. "Transformation Between Cartesian and Pure Spherical Harmonic Gaussians." International Journal of Quantum Chemistry 1995, 54, 83-87.

Spherical Gaussians can be expressed as linear combinations of the appropriate Cartesian Gaussians. General expressions for the transformation coefficients are given. Values for the transformation coefficients are tabulated up to h-type functions. (C) 1995 John Wiley & Sons, Inc.

175) Bach, R. D.; Gonzalez, C.; Andres, J. L.; Schlegel, H. B. "Kinetic Isotope Effects As a Guide to Transition-State Geometries For the Intramolecular Cope and Ylide Elimination- Reactions - an Ab-Initio Mo Study." Journal of Organic Chemistry 1995, 60, 4653-4656.

176) Schlegel, H. B. "Geometry optimization on potential energy surfaces" in Modern Electronic Structure Theory; Yarkony, D. R., Ed.; World Scientific Publishing: Singapore, 1995, pp 459-500.

177) Frisch, M. J. T., G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Na GAUSSIAN 94; Gaussian, Inc.: Pittsburgh PA, 1995.

178) Chen, W.; Schlegel, H. B. "Ab-Initio Molecular-Orbital Calculations of the Mechanism of Oxidation of Boron and Aluminum Monohalides." Chemical Physics Letters 1995, 241, 516-521.

The mechanisms of BF, BCl, AIF and AlCl reacting with O-3(2) have been investigated by Ab Initio molecular orbital. methods at the PMP4/6-31G* and G2 levels of theory. The BX + O-3(2) reactions proceed via an XBOO intermediate that can dissociate to give XBO + O-3, which can react further to yield BO2 + X. At the G2 level of theory, the initial barrier for BCl + O-3(2) is 8.5 kcal/mol and for BF + O-3(2) is 12.4 kcal/mol. These compare very favorably with the experimental barriers of 6.5 and 12 kcal/mol, obtained by fitting model transition states to the observed rate data. For AlX + O-3(2), the barrier for XA100 --> XAlO + O-3 is much higher than the barrier for decomposition back to reactants, and the reaction may proceed by a different mechanism.

179) Bach, R. D.; Mintcheva, I.; Estevez, C. M.; Schlegel, H. B. "Theoretical-Model For an Alternate Mechanism For the Cytochrome-P-450 Hydroxylation of Quadricyclane." Journal of the American Chemical Society 1995, 117, 10121-10122.

180) Peng, C. Y.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. "Using redundant internal coordinates to optimize equilibrium geometries and transition states." Journal of Computational Chemistry 1996, 17, 49-56.

A redundant internal coordinate system for optimizing molecular geometries is constructed from all bonds, all valence angles between bonded atoms, and all dihedral angles between bonded atoms. Redundancies are removed by using the generalized inverse of the G matrix; constraints can be added by using an appropriate projector. For minimizations, redundant internal coordinates provide substantial improvements in optimization efficiency over Cartesian and nonredundant internal coordinates, especially for flexible and polycyclic systems. Transition structure searches are also improved when redundant coordinates are used and when the initial steps are guided by the quadratic synchronous transit approach. (C) 1996 by John Wiley & Sons, Inc.

181) Yamamoto, N.; Vreven, T.; Robb, M. A.; Frisch, M. J.; Schlegel, H. B. "A direct derivative MC-SCF procedure." Chemical Physics Letters 1996, 250, 373-378.

A direct method for the computation of energy second derivatives, and first derivatives which require the solution of the coupled perturbed MC-SCF equations, is presented. The two-electron derivative integral transformation is formulated in terms of 3/4 transformed integrals. The optimum strategy for the solution of the CP-MCSCF linear equations involves the solution in a Krylov space that involves all the right-hand sides. The feasibility of the method is demonstrated in a computation on the excited states of indene, styrene and octatetraene.

182) Hase, W. L.; Schlegel, H. B.; Balbyshev, V.; Page, M. "An Ab Initio study of the transition state and forward and reverse rate constants for C₂H₅ reversible arrow H+C₂H₄." Journal of Physical Chemistry 1996, 100, 5354-5361.

Different levels of Ab Initio theory are used to calculate geometries, vibrational frequencies, and energies for stationary points on the H + C2H4 reversible arrow C2H5 potential energy surface. Frequencies and geometries calculated for C2H4 at the QCISD/6-311G** and MRCI/cc-pVDZ levels of theory are in very good agreement with experiment. The Ab Initio normal modes for the H---C2H4 transition state are similar to those for C2H4, and ratios between the C2H4 experimental anharmonic frequencies and Ab Initio harmonic frequencies are used to estimate anharmonic frequencies for the transition state. Nine of the 10 frequencies assigned for C2H5 from experiment are consistent with the Ab Initio calculations. The CC stretch was apparently misassigned and reassigned here. The Ab Initio calculations do not give definitive values for the H + C2H4 --> C2H5 barrier height and heat of reaction. Using these two energy terms as adjustable parameters in concert with the above stationary point geometries and frequencies, transition state theory (TST) rate constants are calculated for H + C2H4 --> C2H5 recombination and C2H5 --> H + C2H4 dissociation in the high-pressure limit. A H + C2H4 --> C2H5 barrier height of 3.0-3.1 kcal/mol gives TST recombination rate constants in excellent agreement with experiment. Because of the uncertainty in the experimental C2H5 --> H + C2H4 rate constant, a definitive value could not be deduced for the dissociation barrier from the TST fits to the experimental dissociation rate constants. Some of the experimentally determined dissociation A factors are more than an order of magnitude smaller than the TST value.

183) Bach, R. D.; Shobe, D. S.; Schlegel, H. B.; Nagel, C. J. "Thermochemistry of iron chlorides and their positive and negative ions." Journal of Physical Chemistry 1996, 100, 8770-8776.

The iron chlorides FeCl, FeCl2, and FeCl3 and their ions FeCl+, FeCl2+, FeCl-, FeCl2-, and FeCl3- were investigated using MP2 and QCISD(T) calculations with double- and triple-zeta basis sets augmented with multiple sets of diffuse and polarization functions. The dissociation enthalpies for FeCl --> Fe + Cl, FeCl2 --> FeCl + Cl, and FeCl3 --> FeCl2 + Cl are predicted to be 82.5, 109.6, and 59.6 kcal/mol at 298.15 K, respectively. The calculated heats of formation of these species in the gas phase at 298.15 K are +45.3 kcal/mol for FeCl, -35.8 kcal/mol for FeCl2, and -66.8 kcal/mol for FeCl3. The calculated heat of formation of FeCl is 15 kcal/mol lower than the estimated value of +60.0 (+/-20.0) kcal/mol reported in the JANAF tables, but is in reasonably good agreement with a recent experimental determination (49.5 +/- 1.6 kcal/mol). The calculated ionization potential of FeCl is 7.89 eV and that of FeCl2 is 10.10 eV. The electron affinities are 1.54 eV for FeCl, 0.99 eV for FeCl2, and 3.90 eV for FeCl3. Comparison of the bond dissociation enthalpies in FeCln, FeCln+, and FeCln- reveals a preference for iron to exist in the +2 oxidation state (as FeCl2, FeCl+, or FeCl3-); this preference is also seen when comparing IPs and the EAs of the iron chlorides. We also evaluated the dissociation energies, IPs and EAs of the iron chloride species using the B3LYP version of density functional theory. Comparison to the high-level Ab Initio results shows that density functional theory with the targe basis set is accurate to 5-10 kcal/mol for these species.

184) Schlegel, H. B.; Baboul, A. G.; Harris, S. J. "Potential surfaces for unimolecular and bimolecular gas phase reactions of BH_mCl_n calculated at the G2 level of theory." Journal of Physical Chemistry 1996, 100, 9774-9779.

Transition structures and reaction paths for the BHmCln system have been characterized at the MP2(full)/6-31G(d) level of theory; heats of reaction and barriers have been computed at the G2 level of theory. Calculations show that the insertion reactions of BHmCln into H-2 and HCl (m + n = 0, 1, 2) occur by highly distorted, non-least motion transition states, with barriers that increase with chlorine substitution on boron. Hydrogen abstraction from BH and BH3-nCln by chlorine proceeds with little or no barrier by a linear transition state. By contrast, BH3-nCln + H --> BH2-nCln + H-2 occurs by an addition-elimination path via a tetracoordinate intermediate. Likewise, BH2-nCln + X --> BH1-nCln + HX or BH2-nCln-1 + XCl (X = H, Cl) are also addition-elimination processes. Abstraction of Cl from BCl and BH3-nCln by H is endothermic and occurs by a bent transition state.

185) Bach, R. D.; Schilke, I. L.; Schlegel, H. B. "The energetics of valence isomerization in the norbornadiene- quadricyclane system." Journal of Organic Chemistry 1996, 61, 4845-4847.

186) Baboul, A. G.; Schlegel, H. B. "Structures and energetics of some silicon-phosphorus compounds: SiH_mPH_n, SiH_mPH_nSiH_o, and (SiH₃)₃P. An Ab Initio molecular- orbital study." Journal of the American Chemical Society 1996, 118, 8444-8451.

The geometries of 48 isomers of SiHmPHn (m + n = 0-5), SiHmPHnSiHo (m + n + o = 0-7), and (SiH3)(3)P have been optimized at the MP2/6-31G(d) level of theory. Silylenes and cyclic structures dominate the compounds with low numbers of hydrogens; nevertheless, there are several examples of silicon- phosphorus multiple bonding. Relative energies, heats of formation, and bond dissociation energies have been calculated at the G2 level of theory. Two empirical schemes have been constructed to fit the atomization energies. A simple bond additivity approach reproduces the data with a mean absolute deviation of 5.3 kcal/mol. Better results are obtained with a group additivity scheme which gives a mean absolute deviation of 3.4 kcal/mol.

187) Wittbrodt, J. M.; Schlegel, H. B. "Some reasons not to use spin projected density functional theory." Journal of Chemical Physics 1996, 105, 6574-6577.

Spin unrestricted calculations using density functional theory can yield wave functions with spin contamination. In conventional post Hartree-Fock calculations (such as Moller- Plesset perturbation theory), spin projection can ameliorate some of the problems caused by spin contamination. However, spin projection can seriously degrade the quality of potential energy surfaces calculated by density functional methods, just as spin projection can yield poor results for Hartree-Fock potential energy surfaces. (C) 1996 American Institute of Physics.

188) Bach, R. D.; Ayala, P. Y.; Schlegel, H. B. "A reassessment of the bond dissociation energies of peroxides. An Ab Initio study." Journal of the American Chemical Society 1996, 118, 12758-12765.

The strength of the O-O bond is of fundamental importance in a variety of chemical processes. Traditionally, a value of 34 kcal/mol has been ascribed to a generic O-O bond dissociation energy. The present, high-level Ab Initio calculations indicate that the average O-O bond energy is significantly higher, ca. 45 kcal/mol, and that the bond energy is sensitive to the bonding environment. Calculations at the G2 level of theory give bend dissociation enthalpies at 298 K of 50 kcal/mol for HOOH, 45 kcal/mol for CH3OOH, 39 kcal/mol for CH3OOCH3, and 48 kcal/mol for HC(O)OOH and CH3C(O)OOH. The G2(MP2) results are similar and, additionally, give bond dissociation enthalpies of 38 kcal/mol for diacetyl peroxide, 49 kcal/mol for trifluoroperoxyacetic acid, 23 kcal/mol for isopropenyl hydroperoxide, and 22 kcal/mol for peroxynitrous acid.

189) Glukhovtsev, M. N.; Pross, A.; Schlegel, H. B.; Bach, R. D.; Radom, L. "Gas-phase identity S_N2 reactions of halide anions and methyl halides with retention of configuration." Journal of the American Chemical Society 1996, 118, 11258-11264.

High-level Ab Initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X(-)+CH(3)X, for X=F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X=F), 193.8 (X=Cl), 178.9(X=Br), and 171.4 kJ mol(-1) (X=I), substantially higher than the corresponding barriers for back-side attack (- 8.0 for X=F, 11.5 for X=Cl, 5.8 for X=Br, and 6.5 kJ mol(-1) for X=I). The difference between the overall barrier for back- side attack and front-side attack is smallest for X=I (164.9 kJ mol(-1)). Central barrier heights for front-side attack decrease in the following order: 241.0 (X=F), 237.8 (X=Cl), 220.0 (X=Br), and 207.4 kJ mol(-1) (X=I). The minimum energy pathways for both back-side and front-side S(N)2 reactions are found to involve the same ion-molecule complex (X(-).. H(3)CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH3Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side S(N)2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side S(N)2 reaction with retention of configuration in going from X=F to X=I. Ion-molecule complexes CH3-X .. X(-), which may be pre- reaction complexes in direct collinear halophilic attack, were found for X=Br and I but not for X=F and Cl. The calculated complexation energies (Delta H-comp) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol(-1) for X=Br and I, respectively) than those for the corresponding pre-reaction complexes for S(N)2 attack at carbon (41.1 and 36.0 kJ mol(-1) for X=Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH(3)X (X=F-I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.

190) Martinez, F. N.; Schlegel, H. B.; Newcomb, M. "Ab Initio molecular orbital calculations of ring opening of cyclopropylcarbinyl radicals." Journal of Organic Chemistry 1996, 61, 8547-8550.

Ab Initio molecular orbital calculations have been performed on the ring-opening reactions of the cyclopropylcarbinyl radical and analogs containing methyl substitution on the ring. The barrier height and heat of reaction for the cyclopropylcarbinyl radical ring opening calculated at the G2 level of theory are in good agreement with experiment. Barrier heights for the ring opening of substituted cyclopropylcarbinyl radicals and relative rate constants were computed at HF, UMP2, and PMP2 levels of theory using the 6-31G* basis set. The calculated relative rates are in good agreement with the experimental data available, and the trends in the kinetics can be explained primarily by steric interactions.

191) Wittbrodt, J. M.; Schlegel, H. B. "An Ab Initio study of the thermal decomposition of dichlorosilane." Chemical Physics Letters 1997, 265, 527-531.

The reactants, products and transition states for the two lowest channels of the thermal decomposition of dichlorosilane were optimized at the MP2/6-311+G(2df,2p) and QCISD/6- 311+G(2df,2p) levels of theory. Corrections to the energy were calculated at the QCISD(T)/6-311G(2df,2p) and MP2/6- 311++G(3df,3pd) levels of theory and were used to estimated the QCISD(T)/6-311++G(3df,3pd) energies. The barrier for the SiH2Cl2-->SiHCl+HCl decomposition channel is found to be 1.9 kcal/mol lower than for SiH2Cl2-->SiCl2+H-2.

192) Wittbrodt, J. M.; Schlegel, H. B. "An Ab Initio study of the thermal decomposition of dichlorosilane (Erratum, vol 256, pg 527, 1997)." Chemical Physics Letters 1997, 269, 391-391.

193) Wittbrodt, J. M.; Schlegel, H. B. "Estimating stretching force constants for geometry optimization." Theochem-Journal of Molecular Structure 1997, 398, 55-61.

The rate of convergence of geometry optimizations depends upon the accuracy of the initial estimate of the Hessian. A set of parameters dependent on the rows of the periodic table are developed for use in Badger's rule to approximate bond stretching force constants. (C) 1997 Elsevier Science B.V.

194) Guzei, I. A.; Baboul, A. G.; Yap, G. P. A.; Rheingold, A. L.; Schlegel, H. B.; Winter, C. H. "Surprising titanium complexes bearing eta(2)-pyrazolato ligands: Synthesis, structure, and molecular orbital studies." Journal of the American Chemical Society 1997, 119, 3387-3388.

195) Ayala, P. Y.; Schlegel, H. B. "A combined method for determining reaction paths, minima, and transition state geometries." Journal of Chemical Physics 1997, 107, 375-384.

Mapping out a reaction mechanism involves optimizing the reactants and products, finding the transition state and following the reaction path connecting them. Transition states can be difficult to locate and reaction paths can be expensive to follow. We describe an efficient algorithm for determining the transition state minima and reaction path in a single procedure. Starting with an approximate path represented by N points, the path is iteratively relaxed until one of the N points reached the transition state, the end points optimize to minima and the remaining points converged to a second order approximation of the steepest descent path. The method appears to be more reliable than conventional transition state optimization algorithms, and requires only energies and gradients, but not second derivative calculations. The procedure is illustrated by application to a number of model reactions. In most cases, the reaction mechanism can be described well using 5 to 7 points to represent the transition state, the:minima and the path. The computational cost of relaxing the path is less than or comparable to the cost of standard techniques for finding the transition State and the minima, determining the transition vector and following the reaction path on both sides of the transition state. (C) 1997 American Institute of Physics.

196) Bach, R. D.; Glukhovtsev, M. N.; Gonzalez, C.; Marquez, M.; Estevez, C. M.; Baboul, A. G.; Schlegel, H. B. "Nature of the transition structure for alkene epoxidation by peroxyformic acid, dioxirane, and dimethyldioxirane: A comparison of B3LYP density functional theory with higher computational levels." Journal of Physical Chemistry a 1997, 101, 6092-6100.

The performance of the B3LYP density functional theory calculations has been studied for the epoxidation reactions of ethylene, propene, and cis-and trans-2-butene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane. The transition structures for the epoxidation of ethylene and propene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane calculated at the B3LYP level as well as at the QCISD and CCSD levels are symmetrical with nearly identical C-O bond distances, whereas the MP2 calculations favor unsymmetrical transition structures. The geometrical parameters of the transition structures calculated using the B3LYP functional are close to those found at the QCISD and CCSD levels. While the activation barriers for the epoxidation reactions calculated at the B3LYP/6-31G* and B3LYP/6-31+G* levels are very close to the MP4SDTQ/6-31G*//MP2/6-31G* and MP2/6-31G*//MP2/6-31G* values, these activation energies are systematically lower (up to 5-6 kcal/mol) than the barrier heights calculated at such higher correlated levels as the QCISD(T)/6-31G*//QCISD/6-31G*, CCSD(T)/6-31G*//CCSD/6-31G*, and BD(T)/6-31G*//QCISD/6-31G*. The calculations on the epoxidation reactions of ethylene and propene with peroxyformic acid using the BH&HLYP functional also lead to symmetrical transition structures, but the calculated barriers are overestimated when compared with the QCISD(T) results. The activation barriers calculated for these epoxidation reactions at the QCISD(T)/6- 31G*//B3LYP/6-31G* level are very close to those computed at the QCISD(T)/6-31C*//QCISD/6-31G* level.

197) Vreven, T.; Bernardi, F.; Garavelli, M.; Olivucci, M.; Robb, M. A.; Schlegel, H. B. "Ab Initio photoisomerization dynamics of a simple retinal chromophore model." Journal of the American Chemical Society 1997, 119, 12687-12688.

198) Baboul, A. G.; Schlegel, H. B. "Improved method for calculating projected frequencies along a reaction path." Journal of Chemical Physics 1997, 107, 9413-9417.

Some of the factors affecting the accuracy of following reaction paths and calculating projected frequencies perpendicular to the reaction path have been examined. The S(N)2 reaction of Cl- with CH3Cl computed at the HF/6-31G* level of theory has been used as a test case. The symmetric C-H stretching mode couples strongly to the reaction path, and the projected frequency of this mode is very sensitive to the numerical accuracy of the path following and frequency projection methods. The transition state geometry must be converged very tightly so that the path steps in the correct direction. For second order implicit algorithms, improved accuracy can be obtained by computing the tangent used for path following and frequency projection from the displacement along the path rather than from the gradient. An even greater increase in accuracy can be achieved by employing the Hessian, used to compute the frequencies, to take a Newton-Raphson step to improve the convergence of the reaction path following. Taken together, these techniques yield a one to three order of magnitude decrease in the errors in the projected frequencies along the reaction path. (C) 1997 American Institute of Physics.

199) Shaik, S.; Danovich, D.; Sastry, G. N.; Ayala, P. Y.; Schlegel, H. B. "Dissociative electron transfer, substitution, and borderline mechanisms in reactions of ketyl radical anions. Differences and difficulties in their reaction paths." Journal of the American Chemical Society 1997, 119, 9237-9245.

Computational studies on ketyl anion radicals with methyl chloride and on omega-chloroalkanal radical anions, Cl(CH2)(n)C(H)O-. (n=2, 3), find competing mechanisms: a dissociative electron transfer (ET) mechanism and a substitution (SUB(C)) mechanism leading to a C-alkylation product. H(CN)C=O-./CK3Cl proceeds unequivocally via the SUB(C) mechanism, and omega-chloroalkanal radical anions proceed by the ET mechanism, but the interpretation of the mechanism for H2C=O-./CH3Cl depends on the coordinate system used to explore the path. The steepest descent path in Z-matrix internal coordinates leads to the ET product at both the ROHF/6-31G* and UHF/6-31G* levels. The mass-weighted path leads to the ET product on the restricted open-shell Hartree-Fock (ROHF) surface but to the SUB(C) product on the unrestricted Hartree- Fock (UHF) surface. A valley-ridge inflection point heading in the direction of ET products was located on the mass-weighted UHF path, indicating that the potential energy surface branches toward ET products. Closer examination of the two-dimensional portion of the surface shows that the potential energy surface for this reaction descends from the transition state to a broad saddle point region and branches into two valleys: one leading to the ET product and the other to the SUB(C) product. The;ridge and saddle point region on the UHF surface are at lower energy and longer C-C and C-Cl bond lengths than on the ROHF surface, allowing the UHF mass-weighted reaction path to traverse the ridge into the SUB(C) valley. On the ROHF surface as the path descends from the transition state, the H2C=O-. moiety continues to approach the methyl chloride while the C-Cl bond lengthens, but then recoils to give the ET products. Cross-sections of the surface calculated at the UQCISD(T)/6- 31G* level resemble the UHF cross-sections, indicating that the branching of the potential surface into two, mechanisms is expected at this level, too. Thus, whereas from inspection of the surface in internal coordinates, the OCH2C-CH3-Cl- transition state connects to the ET product, the mass-weighted path can cross the broad and shallow ridge and bifurcate thereafter to ET and SUB(C) products. Our study reveals a scenario where a group of isostructural transition states define a mechanistic family consisting of substitution, electron transfer, and borderline situations. Molecular dynamics studies may be necessary to explore the borderline situations.

200) Baboul, A. G.; Schlegel, H. B.; Glukhovtsev, M. N.; Bach, R. D. "Computational study on nature of transition structure for oxygen transfer from dioxirane and carbonyloxide." Journal of Computational Chemistry 1998, 19, 1353-1369.

The relative reactivity of a series of nucleophiles that includes ethylene, sulfides, sulfoxides, amines, and phosphines toward dioxirane, dimethyldioxirane, carbonyloxide and dimethylcarbonyloxide has been examined at the MP4/6- 31G*//MP2/6-31G*, QCISD(T)/6-31G*//MP2/6-31G*, and B3-LYP/6- 31G* levels of theory. The barriers for the oxidations with dimethyldioxirane are higher (up to 2.5 kcal/mol for the oxidation of H2S) than those for the oxidations with the parent dioxirane. The oxidation barriers for dioxirane are larger than those for the oxidations with peroxyformic acid, except the barriers for the oxidation of sulfoxides. The reactivity of dimethylsulfide toward dimethyldioxirane was found to be comparable to that of dimethylsulfoxide both in the gas phase and in solution (chloroform). The classical gas phase barrier for the oxidation of trimethylamine to trimethylamine oxide was higher (6.3 kcal/mol at the MP4//MP2/6-31G* level) than that for oxygen atom transfer to trimethylphosphine. When the transition states were examined by self-consistent reaction field (SCRF) methods, the predicted barriers for the oxidation of amines and phosphines were found to be in good agreement with experiment. The general trend in reactivity for oxidation by dioxirane was R2S approximate to R2SO, R3P > R3N in the gas phase, and R2S approximate to R2SO, R3N approximate to R3P (R = Me) in solution. The oxidation barriers calculated using the B3-LYP functional were lower than those computed at the MP4 and QCISD(T) levels. (C) 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1353-1369, 1998.

201) Ayala, P. Y.; Schlegel, H. B. "Identification and treatment of internal rotation in normal mode vibrational analysis." Journal of Chemical Physics 1998, 108, 2314-2325.

A procedure that automatically identifies internal rotation modes and rotating groups during the normal mode vibrational analysis is outlined, and an improved approximation to the corrections for the thermodynamic functions is proposed. The identification and the characterization of the internal rotation modes require no user intervention and make extensive use of the information imbedded in the redundant internal coordinates. Rigid-rotor internal rotation modes are obtained by fixing stretching, bending, and out-of-plane bending motions and solving the vibrational problem for the constrained system. Normal vibrational modes corresponding to internal rotations are identified by comparing them with the constrained modes. The atomic composition of the rotating groups is determined automatically and the kinetic energy matrix for internal rotation is given by either the constrained Wilson-G matrix or the Kilpatrick and Fitter protocol. The potential periodicity, the rotating tops' symmetry numbers, and the well-multiplicity are obtained using simple rules. These parameters can be altered by user input. An improved analytical approximation to the partition function for a one-dimensional hindered internal rotation has been developed that reproduces the accurate values tabulated by Fitter and Gwinn to +/-0.4% with a maximum error of 2.1%. This approximation is shown to behave better than previously available approximations over a wider range of regimes. The one-dimensional rotor treatment is generalized to give useful approximations to the multidimensional rotor thermodynamic functions that can be a good start for more thorough studies. (C) 1998 American Institute of Physics.

202) Schlegel, H. B. "Geometry optimization" in Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Kollman, P. A., Clark, T., Schaefer III, H. F., Gasteiger, J. and Schreiner, P. R., Ed.; Wiley: Chichester, 1998; Vol. 2, pp 1136-1142.

203) Schlegel, H. B. "Reaction paths" in Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Kollman, P. A., Clark, T., Schaefer III, H. F., Gasteiger, J. and Schreiner, P. R., Ed.; Wiley: Chichester, 1998; Vol. 4, pp 2432-2437.

204) Schlegel, H. B. "Spin contamination" in Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Kollman, P. A., Clark, T., Schaefer III, H. F., Gasteiger, J. and Schreiner, P. R., Ed.; Wiley: Chichester, 1998; Vol. 4, pp 1665-2671.

205) Ayala, P. Y.; Schlegel, H. B. "A nonorthogonal CI treatment of symmetry breaking in sigma formyloxyl radical." Journal of Chemical Physics 1998, 108, 7560-7567.

Spatial symmetry breaking can occur in Hartree-Fock wave functions when there are two or more close lying configurations that can mix strongly, such as in HCO2, NO2, and allyl radical. Like spin contamination, spatial symmetry breaking can cause sizeable errors when perturbation theory is used to estimate the correlation energy. With conventional methodology, very large MCSCF and MRCI calculations are necessary to overcome the spatial symmetry breaking problem. This paper explores an alternative approach in which a 2X2 nonorthogonal CI is used to recombine the two symmetry broken Hartree-Fock determinants. The necessary matrix elements closely resemble those used in the spin projection calculations. Second order perturbation theory is used to include electron correlation energy in this approach. With perturbative corrections for correlation energy, this approach predicts that the B-2(2) structure is a minimum, in agreement with the best available calculations. (C) 1998 American Institute of Physics.

206) Martinez, F. N.; Schlegel, H. B.; Newcomb, M. "Ab Initio molecular orbital calculations of electronic effects on the kinetics of cyclopropylcarbinyl radical ring openings." Journal of Organic Chemistry 1998, 63, 3618-3623.

Ab Initio molecular orbital calculations have been performed on the ring-opening reactions of cyclopropylcarbinyl radical analogues containing vinyl and methoxy substituents on the ring and for vinyl, methoxy, and methyl substituents on the radical center. Barrier heights were calculated at the UHF/6-31G*, UMP2/6-31G*//LTHF/6-31G*, and PMP2/6-31G*//UHF/6-31G* levels of theory. Substituent effects were analyzed using isodesmic reactions. Vinyl substituents on the ring reduce the barrier by 7-8 kcal/mol because of allylic conjugation stabilizing the transition state. Methoxy substituents on the ring reduce the barrier by ca. 2 kcal/mol, primarily as a result of hyperconjugation. Vinyl and methoxy substituents on. the radical center raised the barrier 4.5 and 0.8 kcal/mol, respectively, because the stabilizing effects on the reactants are greater than the effects on the transition states. A methyl substituent decreased the barrier slightly in contrast to intuitive expectations. The computational results were compared to experimental thermochemical and kinetic data when available, and the agreements were good in an absolute sense and excellent in a relative sense. This work demonstrates that predictions of reactivity patterns and relative kinetics for radical reactions that do not involve polarized transition states can be computed without the use of supercomputers.

207) Baboul, A. G.; Schlegel, H. B. "Structures and energetics of some potential intermediates in titanium nitride chemical vapor deposition: TiCl_m(NH₂)_n, TiCl_m(NH₂)_nNH, and TiCl_m(NH₂)_nN. An Ab Initio molecular orbital study." Journal of Physical Chemistry B 1998, 102, 5152-5157.

The various potential intermediates in the chemical vapor deposition production of TiN films have been studied using a variant of the G2 level of theory. The structures of TiClm(NH2)(n), 0 less than or equal to m + n less than or equal to 4, TiClm(NH2)(n)NH, 0 less than or equal to m + n less than or equal to 2, and TiClm(NH2)(n)N, 0 less than or equal to m + n less than or equal to 1, were optimized at B3LYP level of theory with the Wachters-Hay basis set for Ti and the 6-311G(d) basis set for H, N, and Cl. The energies were calculated at the MP4 and QCI levels with these basis sets augmented by multiple sets of polarization and diffuse functions using the B3LYP optimized geometries. Bond dissociation energies, heals of atomization, heats of formation, and entropies have been calculated at this modified G2 level of theory.

208) Bolton, K.; Hase, W. L.; Schlegel, H. B.; Song, K. "A direct dynamics study of the F+C₂H₄ -> C₂H₃F+H product energy distributions." Chemical Physics Letters 1998, 288, 621-627.

Ab Initio direct dynamics was used to calculate product energy distributions for the F + C2H4 --> C2H3F + H reaction. A broad product translational energy distribution, similar to that observed experimentally, is found when the trajectories are initialized with a statistical vibrational energy distribution at the exit channel barrier. The trajectories show that, on average, orbital angular momentum is conserved in going from the exit channel barrier to products, and a model which incorporates this dynamical constraint reproduces the ensemble averaged trajectory results. (C) 1998 Elsevier Science B.V. All rights reserved.

209) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Co GAUSSIAN 98; Gaussian, Inc.: Pittsburgh, PA, 1988.

210) Wittbrodt, J. M.; Hase, W. L.; Schlegel, H. B. "Ab Initio study of the interaction of water with cluster models of the aluminum terminated (0001) alpha-aluminum oxide surface." Journal of Physical Chemistry B 1998, 102, 6539-6548.

Ab Initio calculations were performed on three cluster models to investigate the interaction of water with the (0001) surface of alpha-Al2O3. Surface relaxation effects are found to be similar to those in previous periodic Hartree-Fock and density functional calculations. Two types of dissociative reactions, 1-2 and 1-4 addition of water, were explored. Catalysis of the 1-4 addition reaction by a second water molecule was also examined. Of these three routes to hydroxylation of the aluminum terminated surface, the two water process was found to be the most exothermic. In all cases, little difference is observed between the energies of molecular and dissociative adsorption of one water. Multiple 1-2 dissociation events at a single surface site were also explored and found to be overall exothermic. Transition state theory was used to calculate molecular --> dissociative adsorption unimolecular rate constants. Both the 1-2 and 1-4 dissociations are predicted to be rapid processes at 300 K, occurring within 10(-2) s.

211) Farkas, O.; Schlegel, H. B. "Methods for optimizing large molecules. II. Quadratic search." Journal of Chemical Physics 1999, 111, 10806-10814.

Geometry optimization has become an essential part of quantum- chemical computations, largely because of the availability of analytic first derivatives. Quasi-Newton algorithms use the gradient to update the second derivative matrix (Hessian) and frequently employ corrections to the quadratic approximation such as rational function optimization (RFO) or the trust radius model (TRM). These corrections are typically carried out via diagonalization of the Hessian, which requires O(N-3) operations for N variables. Thus, they can be substantial bottlenecks in the optimization of large molecules with semiempirical, mixed quantum mechanical/molecular mechanical (QM/MM) or linearly scaling electronic structure methods. Our O(N-2) approach for solving the equations for coordinate transformations in optimizations has been extended to evaluate the RFO and TRM steps efficiently in redundant internal coordinates. The regular RFO model has also been modified so that it has the correct size dependence as the molecular systems become larger. Finally, an improved Hessian update for minimizations has been constructed by combining the Broyden- Fletcher-Goldfarb-Shanno (BFGS) and (symmetric rank one) SR1 updates. Together these modifications and new methods form an optimization algorithm for large molecules that scales as O(N- 2) and performs similar to or better than the traditional optimization strategies used in quantum chemistry. (C) 1999 American Institute of Physics. [S0021-9606(99)30648-6].

212) Halls, M. D.; Schlegel, H. B. "Comparison of the performance of local, gradient-corrected, and hybrid density functional models in predicting infrared intensities." Journal of Chemical Physics 1998, 109, 10587-10593.

Density functional theory has been used to calculate infrared (IR) intensities for a series of molecules (KF, CO, H2O, HCN, CO2, C2H2, H2CO, NH3, C2H4, CH2F2, CH2Cl2, and CH2Br2) in an effort to evaluate relative performance of different functionals. The methods employed in this study comprise most of the popular local, gradient-corrected, and hybrid functionals, namely, S-VWN, S-PL, B-LYP, B-P86, B-PW91, B3-LYP, B3-P86, and B3-PW91. Calculations were carried out using various qualities of split valence basis sets augmented with diffuse and polarization functions, both to determine basis set dependence and to evaluate the limit performance. Computed intensities were compared with results from conventional correlated Ab Initio methods (MP2 and QCISD). Hybrid functionals give results in closest agreement with QCISD over the other methods surveyed. Local and gradient-corrected methods performed remarkably alike, both are comparable to MP2, and outperform Hartree-Fock. Hartree-Fock intensities can be dramatically improved by scaling, making them similar to MP2 results. (C) 1998 American Institute of Physics. [S0021- 9606(98)30748-5].

213) Perera, J. R.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. "Ruthenium complexes bearing eta(5)-pyrazolato ligands." Journal of the American Chemical Society 1999, 121, 4536-4537.

214) Bolton, K.; Schlegel, H. B.; Hase, W. L.; Song, K. Y. "An Ab Initio quasi-classical direct dynamics investigation of the F+C₂H₄ -> C₂H₃F+H product energy distributions." Pccp Physical Chemistry Chemical Physics 1999, 1, 999-1011.

A direct dynamics technique, using energies, forces and second derivatives calculated at the UHF/6-31G* level of theory, was used to investigate product energy distributions of the F + C2H4 --> C2H3F + H collision reaction. The shifting and broadening of the product translational energy distribution as the system moves from the exit-channel barrier to the products was studied. Since properties associated with the rupturing C .. H bond are similar for the C(2)H(5)double dagger and C2H4F double dagger exit-channel barriers, and integration of the C(2)H(5)double dagger --> C2H4 + H reaction is approximately 2.5 times faster than the C2H4F double dagger --> C2H3F + H reaction, trajectories of the former reaction were propagated to gain insight into the exit-channel dynamics. Ensemble averaged results for C(2)H(5)double dagger dissociation are well described by a model based on isotropic exit-channel dynamics which assumes that the product relative translational distribution arises from the centrifugal potential and relative translational energy distributions at the exit-channel barrier plus the exit-channel potential release. The width of the product translational energy distribution is sensitive to overall rotational angular momentum and its partitioning between C2H4.. H double dagger orbital angular momentum and C(2)H(4)double dagger rotational angular momentum. The simulated product translational energy distribution for the C2H4F double dagger --> C2H3F + H reaction is broadened by relative translation-vibrational couplings in the exit-channel and is similar to the distribution used to fit crossed molecular beam data. Approximately 50% of the available energy is in product relative translation, which also agrees with experiment. RRKM calculations indicate that a second reaction mechanism, involving 1-2 hydrogen migration prior to C .. H bond fission, does not significantly contribute to C2H3F + H product formation.

215) Millam, J. M.; Bakken, V.; Chen, W.; Hase, W. L.; Schlegel, H. B. "Ab Initio classical trajectories on the Born-Oppenheimer surface: Hessian-based integrators using fifth-order polynomial and rational function fits." Journal of Chemical Physics 1999, 111, 3800-3805.

Classical trajectories can be computed directly from electronic structure calculations without constructing a global potential- energy surface. When the potential energy and its derivatives are needed during the integration of the classical equations of motion, they are calculated by electronic structure methods. In the Born-Oppenheimer approach the wave function is converged rather than propagated to generate a more accurate potential- energy surface. If analytic second derivatives (Hessians) can be computed, steps of moderate size can be taken by integrating the equations of motion on a local quadratic approximation to the surface (a second-order algorithm). A more accurate integration method is described that uses a second-order predictor step on a local quadratic surface, followed by a corrector step on a better local surface fitted to the energies, gradients, and Hessians computed at the beginning and end points of the predictor step. The electronic structure work per step is the same as the second-order Hessian based integrator, since the energy, gradient and Hessian at the end of the step are used for the local quadratic surface for the next predictor step. A fifth-order polynomial fit performs somewhat better than a rational function fit. For both methods the step size can be a factor of 10 larger than for the second order approach without loss of accuracy. (C) 1999 American Institute of Physics. [S0021-9606(99)30131-8].

216) Pfeiffer, D.; Ximba, B. J.; Liable-Sands, L. M.; Rheingold, A. L.; Heeg, M. J.; Coleman, D. M.; Sehlegel, H. B.; Kuech, T. F.; Winter, C. H. "Synthesis, structure, and molecular orbital studies of yttrium, erbium, and lutetium complexes bearing eta(2)-pyrazolato ligands: Development of a new class of precursors for doping semiconductors." Inorganic Chemistry 1999, 38, 4539-4548.

Treatment of yttrium metal with bis(pentafluorophenyl)mercury (1.5 equiv), 3,5-di-tert-butylpyrazole (3 equiv), and pyridine (2 equiv) in toluene at ambient temperature for 120 h afforded tris(3,5-di-tert-butylpyrazolato)bis(pyridine)yttrium(III) (33%). In an analogous procedure, the reaction of erbium metal with 3.5-dialkylpyrazole (alkyl = methyl or tert-butyl), bis(pentafluorophenyl)mercury, and a neutral nitrogen donor (4- tert-butylpyridine, pyridine, n-butylimidazole, or 3,5-di-tert- butylpyrazole) yielded tris(3,5-di-tert-butylpyrazolato)bis(4- tert-butylpyridine)erbium(III) (63%), tris(3,5-di-tert- butylpyrazolato)bis(pyridine)erbium(III) (88%), tris(3,5-di- tert-butylpyrazolato)bis(n-butylimidazole)erbium(III) (48%), tris(3,5-dimethylpyrazolato)bis(4-tert- butylpyridine)erbium(III)(50%), and tris(3,5-di-tert- butylpyrazolato)(3,5-di-tert-butylpyrazole)erbium(III) (59%), respectively. Treatment of tris(cyclopentadienyl)lutetium(III) or tris(cyclopentadienyl)erbium(In) with 3,5-di-tert- butylpyrazole (3 equiv) (2 equiv) in toluene at ambient temperature for 24 h afforded tris(3,5-di-tert- and 4-tert- butylpyridine butylpyrazolato)bis(4-tert- butylpyridine)lutetium(III) (83%) and tris(3,5-di-tert- butylpyrazolato)bis(4-tert-buylpyridine)erbium(III) (41%), respectively. The X-ray crystal structures of all new complexes were determined. The X-ray structure analyses revealed seven- and eight-coordinate lanthanide complexes with all-nitrogen coordination spheres and eta(2)-pyrazolato ligands. Molecular orbital calculations were carried out on dichloro(pyrazolato)diammineyttrium(III). The calculations demonstrate that eta(2)-bonding of the pyrazolato ligand is favored over the eta(1)-bonding mode and give insight into the bonding between yttrium and the pyrazolato ligands. Complexes bearing 3,5-di-tert-butylpyrazolato ligands can be obtained in a high state of purity and sublime without decomposition (150 degrees C, 0.1 mmHg). Application of these complexes as source compounds for chemical vapor deposition processes is discussed.

217) Wittbrodt, J. M.; Schlegel, H. B. "Structures, energetics, and transition states of the silicon- phosphorus compounds Si₂PH_n (n=7, 5, 3, 1). An Ab Initio molecular orbital study." Journal of Physical Chemistry a 1999, 103, 8547-8558.

This study examines a variety of compounds containing silicon- phosphorus multiple bonds as well as a selection of hydrogen- bridged species, including a doubly bridged structure. The structures of 29 minima and 37 transition states of the form Si2PHn (n = 7, 5, 3, 1) have been optimized at the MP2(full)/6- 31G(d) level of theory; a representative subset was also optimized at the QCISD/6-311G(d,p) level. Relative energies were computed using a modification of the G2 level of theory. The stability of these compounds was investigated with respect to both unimolecular rearrangement and H-2 addition/elimination. Barrier heights. for 1-2 and 1-3 H-shifts and for H-2 addition/elimination span wide ranges and are dependent on the nature of the bonding. Structures that contain true silicon-phosphorus double bonds are particularly stable with respect to unimolecular rearrangement.

218) Bakken, V.; Millam, J. M.; Schlegel, H. B. "Ab Initio classical trajectories on the Born-Oppenheimer surface: Updating methods for Hessian-based integrators." Journal of Chemical Physics 1999, 111, 8773-8777.

For the integration of the classical equations of motion in the Born-Oppenheimer approach, each time the energy and gradient of the potential energy surface are needed, a properly converged wave function is calculated. If Hessians (second derivatives) can be calculated, significantly larger steps can be taken in the numerical integration of the equations of motion without loss of accuracy. Even larger steps can be taken with a Hessian-based predictor-corrector algorithm. Since updated Hessians are used successfully in quasi-Newton methods for geometry optimization, it should be possible to improve the performance of trajectory calculations using updated Hessians. The Murtagh-Sargent (MS) update, the Powell-symmetric-Broyden (PSB) update and Bofill's update (a weighted combination of MS and PSB) were tested, and Bofill's update was found to be the best. Slightly smaller step sizes were needed with Hessian updating to maintain good conservation of the energy, but this was more than compensated by the reduction in total computational cost. An overall factor of 3 in speed-up was obtained for trajectories of systems containing 4 to 6 heavy atoms computed at the HF/3-21G level. (C) 1999 American Institute of Physics. [S0021-9606(99)30443-8].

219) Halls, M. D.; Schlegel, H. B. "Comparison study of the prediction of Raman intensities using electronic structure methods." Journal of Chemical Physics 1999, 111, 8819-8824.

Raman intensities have been computed for a series of test molecules (N-2, H2S, H2O, H2CO, CH4, C2H2, C2H4, C2H6, SiO2, NH3, CH2F2, and CH2Cl2) using Hartree-Fock, second-order Moller-Plesset perturbation theory (MP2), and density functional theory, including local, gradient-corrected, and hybrid methods (S-VWN, B-LYP and B3-LYP, and MPW1-PW91) to evaluate their relative performance. Comparisons were made with three different basis sets: 6-31G(d), Sadlej, and aug-cc-pVTZ. The quality of basis set used was found to be the most important factor in achieving quantitative results. The medium sized Sadlej basis provided excellent quantitative Raman intensities, comparable to those obtained with the much larger aug-cc-pVTZ basis set. Harmonic vibrational frequencies computed with the Sadlej basis set were in good agreement with experimental fundamentals. For the quantitative prediction of vibrational Raman spectra, the Sadlej basis set is an excellent compromise between computational cost and quality of results. (C) 1999 American Institute of Physics. [S0021-9606(99)30543- 2].

220) Daniels, A. D.; Scuseria, G. E.; Farkas, O.; Schlegel, H. B. "Geometry optimization of Kringle 1 of plasminogen using the PM3 semiempirical method." International Journal of Quantum Chemistry 2000, 77, 82-89.

The results of a geometry optimization on the 1226 atom Kringle I of plasminogen are presented. The energy and gradients were calculated using a linear-scaling PM3 semiempirical method with a conjugate gradient density matrix search replacing the diagonalization step. The geometry was optimized with the rational function optimization technique combined with a modified version of the direct inversion in the iterative subspace procedure. The optimization required 362 geometry update steps to reach a local minimum. An analysis is given of the optimization and timing results using a single processor on the SGI Origin2000. (C) 2000 John Wiley & Sons, Inc.

221) Schlegel, H. B. "Perspective on "Ab Initio calculation of force constants and equilibrium geometries in polyatomic molecules. I. Theory" - Pulay P (1969) Mol Phys 17 : 197-204." Theoretical Chemistry Accounts 2000, 103, 294-296.

This article provides an outline of the title paper by Peter Pulay and discusses some of the methodology that grew from it, and the impact that it has had on the development of computational chemistry.

222) McGill, A. D.; Zhang, W.; Wittbrodt, J.; Wang, J. Q.; Schlegel, H. B.; Wang, P. G. "para-substituted N-nitroso-n-oxybenzenamine ammonium salts: A new class of redox-sensitive nitric oxide releasing compounds." Bioorganic & Medicinal Chemistry 2000, 8, 405-412.

N-Nitroso-N-oxybenzenamine ammonium salts with -OMe, -Me, -H, - F, -Cl, -CF3, and -SO2Me substituents at the para position of the phenyl ring constitute a new class of-redox sensitive nitric oxide (NO) releasing compounds. These compounds yield nitric oxide and the corresponding nitrosobenzene derivatives by a spontaneous dissociation mechanism after undergoing a one electron oxidation. Oxidation of these compounds can be achieved through chemical, electrochemical and enzymatic methods. It was observed electrochemically that the amount of NO generated was dependent on the substituent effect and the applied oxidation potential. Electron-withdrawing substituents increase the oxidation potential of the compound. A linear correlation was observed when the peak potentials for the oxidation were graphed Versus the Hammett substituent constant. Density functional theory calculations were also performed on this series of compounds. The theoretical oxidation energies of the corresponding anions show a strong linear correlation with the experimental potentials. Furthermore, enzymatic oxidation using horseradish peroxidase showed a similar substituent effect. These results indicate that substitution at the par a position of the phenyl ring has a profound effect on the stability, oxidation potential and enzymatic kinetic properties of the compounds. Thus pala-substituted N-nitroso-N'- oxybenzenamine salts comprise a new class of redox-sensitive nitric oxide releasing agents. (C) 2000 Elsevier Science Ltd. All rights reserved.

223) Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. "Ultrafast radiationless deactivation of organic dyes: Evidence for a two-state two-mode pathway in polymethine cyanines." Journal of the American Chemical Society 2000, 122, 2911-2924.

CASSCF quantum chemical calculations (including dynamics) have been used to investigate the ultrafast photoisomerization of three symmetric cyanine dye models of different chain lengths. For the "model" trimethine cyanine, the photochemical isomerization path can be divided into two phases: initial barrierless skeletal stretching coupled with torsional motion and the decay process that takes place in the region of the twisted intramolecular charge-transfer (TICT) minimum state with an adjacent conical intersection. The path is consistent with both biexponential decay of fluorescence without rise time at short wavelengths and the rise time followed by monoexponential decay at long wavelengths observed in time- resolved experiments. For penta- and heptamethine cyanines, the photoisomerization about different C-C bonds is shown to be an activated process, where the torsional reaction path terminates, again, at a TICT state and the decay takes place at a twisted S-1/S-0 conical intersection. In agreement with the experimental results, the activation energies increase with the length of the polymethine chain. In contrast to the differences in the potential energy surface between short and long cyanines, we demonstrate that the excited state evolution of these systems can be understood in terms of the same two-state two-mode model of the reaction coordinate previously reported for the (isoelectronic) retinal protonated Schiff base models.

224) Sawilowsky, E. F.; Meroueh, O.; Schlegel, H. B.; Hase, W. L. "Structures, energies, and electrostatics for methane complexed with alumina clusters." Journal of Physical Chemistry a 2000, 104, 4920-4927.

Ab initio calculations were used to investigate properties of complexes formed from the association of CH4 with Al2O3, Al4O6, and Al8O12 alumina clusters. Methane attaches to a surface Al atom of the cluster to form a complex with an Al-C separation that varies between 2.2 and 2.5 Angstrom. The rotational motion for methane in these complexes is highly fluxional. Extrapolated G2MP2 well depths fbr the CH4- - -Al2O3, CH4- - - Al4O6. and CH4- - -Al8O12 complexes are 21, 14, and 17 kcal/mol, respectively. These different well depths ale determined by the accessibility of the Al atom to which CH4 binds and the size of the alumina cluster. The electrostatics of the three alumina clusters are very similar, with a charge on the surface Al atom of + 2.2 to 2.3. The potential energy surface for a CH4- - -Al2nO3n cluster is represented semiquantitatively by an analytic Function consisting of two- body potentials. The results of this study suggest that the adsorption energy for alkane molecules binding to alumina materials depends very strongly on the structure of the binding site.

225) Cross, J. B. S., H. B.; "Molecular Orbital Studies of Titanium Nitride Chemical Vapor Deposition: Gas Phase Complex Formation, Ligand Exchange and Elimination Reactions." Chem. Materials , (accepted).

226) Halls, M. D. S., H. B.; Aroca, R.; "Surface-Enhanced Vibrational Spectroscopy. Interpretation of the SERS Spectra of a Chemisorbed Adsorbate on Silver Islands." J. Phys. Chem. , (submitted).

227) Halls, M. D. S., H. B. "Harmonic Frequenciy Scaling Factors for Hartree-Fock, S-VWN, B-LPY, B3-LYP, B3-PW91 and MP2 with the Sadlej pVTZ electric property basis set." Theor. Chem. Acc. , (submitted).

228) Yu, Z. W., J. M.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. "Unusually Stable Pyrazolate-Bridged Dialuminum Complexes Containing Bridging Methyl Groups." J. Am. Chem. Soc. , (submitted).

229) Kotra, L. P. C., J. B.; Fridman, R.; Schlegel, H. B.; Mobashery, S. "Insight into the Complex Process of Activation of Matrix Metalloproteinases." J. Am. Chem. Soc. , (submitted).