Yu-San Cheung

A Gaussian-2 ab initio study of isomeric CH3S2, CH3S2+ and CH3S2−

Abstract The energetics and structures for isomers of the CH 3 S 2 , CH 3 S 2 + , and CH 3 S 2 − systems have been studied with the ab initio Gaussian-2 (G2) method. At the MP2(full)/6-31G(d) level, there are five minima on the potential energy surface of CH 3 S 2 , corresponding to equilibrium structures CH 3 SS, CH 2 SHS, CH 2 SSH, HSCH 2 S and HSCHSH. At the same level of theory, CH 3 S 2 + may exist as six isomers: CH 3 SS + , CH 2 SHS + , CH 2 SSH + , c-HSCH 2 S + , HSCHSH + and CHSHSH + ; CH 3 S 2 − may exist as four isomers: CH 3 SS − , CH 2 SHS − , HSCH 2 S − and HSCHSH − . The most stable CH 3 S 2 , CH 3 S + 2 and CH 3 S 2 − species are CH 3 SS, HSCHSH + and CH 3 SS − , respectively. Transition structures for the rearrangements of the neutral radicals and the cations are also located. The structures and energetics of all the species located are discussed. Also, the G2 standard molar enthalpies of formation and ionization energies of various species are in good agreement with the available experimental data.

A G2 AB INITIO STUDY OF C2H5S ISOMERS : STRUCTURES, ENERGETICS, AND UNIMOLECULAR REACTION PATHWAYS

Abstract The potential energy surface of C2H5S isomers has been studied with the G2 ab initio method. The structures and G2 heats of formation of the six stable isomers/conformers, CH3CH2S (1), syn-CH3CHSH (2), anti-CH3CHSH (3), CH3SCH2 (4), gauche-HSCH2CH2 (5), and anti-HSCH2CH2 (6), have been reported previously (Chem. Phys. Lett., 213 (1993) 250; J. Chem. Phys., 104 (1996) 130). Their structural properties upon change of conformation can be rationalized by the perturbative molecular orbital model. The C2H5S radicals are very flexible due to low-energy barriers for rotations and inversions. Unimolecular rearrangements are exclusively 1,2- and 1,3-H shifts with large G2 activation energies at 0 K: Ea(1 → 3) = 147.0 kJ mol−1, Ea(1 → 5) = 140.0 kJ mol−1, Ea(2 → 5) = 187.2 kJ mol−1, Ea(3 → 6) = 191.7 kJ mol−1, and Ea(4 → 4) = 176.9 kJ mol−1. The TS structures for the unimolecular decompositions: 5 → C2H4 + SH (6), 1 → CH3 + H2CS (7), and 4 → CH3 + H2CS (8) are loose and product-like. Their UHF determinant functions have considerable spin contamination (〈S2〉 ≈ 0.96–1.08). Energy barriers for the analogous reactions of 6 and 7 for the oxygen analogs were also computed and compared to the observed rate parameters. In general, it is found that the G2 method is still reliable to estimate the reaction barriers for systems with 〈S2〉 ≤ 0.96. Correction with spin projection is required for systems with 〈S2〉 > 1.0. Among the reactions of C2H5S studied in this work, the pathway with the lowest energy is 6. The G2 Ea = 41.7 kJ mol−1 for 6 at 700 K is in good agreement with Shum and Bensons (Int. J. Chem. Kinet., 17 (1985) 277, 749) estimated value (46 kJ mol−1). At the same temperature, the calculated Ea′s for 7 and 8, after spin-projection corrections, are 158.9 kJ mol−1 and 109.0 kJ mol−1, respectively, in agreement with the estimated Eas (145.5 kJ mol−1 for 7 and 101.3 kJ mol−1 for 8). The addition reaction CH 3 + H 2 CS → 4 ( 8 ) is predicted to be negative temperature dependent with an Eaof −3.0 kJ mol−1 at 700 K. This is in line with the lower bound of the estimated Ea(ca. 11.7 kJ mol−1 ± 12.6 kJ mol−1) for 8 at 644 K as reported by Shum and Benson.

An ab initio molecular orbital study of the CH3O2 and CH3O2+ potential energy surfaces

Abstract By carrying out Gaussian-2 calculations, which is an approximation to the ab initio level of QCISD(T)/6-311+G(3df,2p), the potential energy surfaces of the CH 3 O 2 and CH 3 O 2 + systems have been constructed. At the MP2(full)/6-31G(d) level, CH 3 O 2 may exist as six stable isomers: HOCHOH ( C 1 ) ( 1 ), OCH 2 OH ( C 1 ) ( 2 ), CH 3 OO ( C s ) ( 3 ), CH 2 OOH ( C 1 ) ( 4 ), CH 2 OHO ( C 1 ) ( 5 ) and CH 2 O ··· HO ( C s ) ( 6 ), while CH 3 O 2 + may exist as eight isomers: HOCHOH + ( C s ) ( 1 + ), cyclic-OCH 2 OH + ( C 1 ) ( 2 + ), CH 3  OO + ( C s ) ( 3 + ), CH 2 OOH + ( C s ) ( 4 + ), CH 2  OHO + ( C 1 ( 5 + ), cyclic-HOCHOH + ( C 1 ) ( 6 + ), H 2 O ··· CHO + ( C s ) ( 7 + ) and OH 3 ··· CO ( C s ) ( 8 + ). The most stable CH 3 O 2 and CH 3 O 2 + species are HOCHOH and HOCHOH + , respectively. Transition structures for the rearrangements of the radicals and cations were also located. The structures and energetics of all the species located are discussed. The Gaussian-2 results are in excellent agreement with the available experimental data.

An ab initio molecular orbital study of the GaH4 potential energy surface

Abstract The potential energy surface for the GaH4 radical was investigated using ab initio MO theory. Three minima corresponding to equilibrium structures with D2d, C3v and Cs, symmetry were found. The D2d and C3v structures are 144 and 95kJmol− above the Cs structure, respectively. From both structural and energetic points of view, the Cs structure is likely to be a weak van der Waals molecular complex between GaH2 and H2, which may easily dissociate into these two fragments.

An ab initio study of the FCP potential energy surface

Abstract The potential energy surface of FCP was studied with high-level single-reference electron correlated optimization. It was found that linear FPC is a potential energy minimum at the RMP2 and RMP4 levels of theory but a maximum at UMP2, RPM3, UMP3 and UMP4. Moreover, various numbers of minima (Min) and saddle points (TSs) were found on the restricted and unrestricted MPn (n = 2–4) surfaces: 3 Min and 2 TSs on RMP2, 3 Min and 3 TSs on UMP2, 2 Min and 2 TSs on RMP3, 3 Min and 3 TSs on UMP3, 3 Min and 2 TSs on RMP4, and 3 Min and 3 TSs on UMP4. However, at the more reliable quadratic CI and Brueckner doubles levels of theory, only two minima, corresponding to linear FCP and bent FPC (bond angle about 110 °), and two saddle points were found on the potential energy surface.