A G2 ab initio study of C2H5S isomers: Structures, energetics, and unimolecular reaction pathways

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: E-a(1 --> 3) = 147.0 kJ mol(-1), E-a(1 --> 5) = 140.0 kJ mol(-1), E-a(2 --> 5) = 187.2 kJ mol(-1), E-a(3 --> 6) = 191.7 kJ mol(-1), and E-a(4 --> 4) = 176.9 kJ mol(-1). The TS structures for the unimolecular decompositions: 5 --> C2H4 + SH (6), 1 --> CH3 + H2CS (7), acid 4 --> CH3 + H2CS (8) are loose and product-like. Their UHF determinant functions have considerable spin contamination ([S-2] approximate to 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 [S-2] less than or equal to 0.96. Correction with spin projection is required for systems with [S-2] > 1.0. Among the reactions of C2H5S studied in this work, the pathway with the lowest energy is 6. The G2 E-a = 41.7 kJ mol(-1) for 6 at 700 K is in good agreement with Shum and Benson's (Int. J. Chem. Kinet., 17 (1985) 277, 749) estimated value (46 kJ mol(-1)). At the same temperature, the calculated E-a'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 E-a's (145.5 kJ mol(-1) for 7 and 101.3 kJ mol(-1) for 8). The addition reaction CH3 + H2CS --> 4 ((8) under bar) is predicted to be negative temperature dependent with an E-a of -3.0 kJ mol(-1) at 700 K. This is in line with the lower bound of the estimated E-a (ca. 11.7 kJ mol(-1) +/- 12.6 kJ mol(-1)) for (8) under bar at 644 K as reported by Shum and Benson. (C) 1997 Elsevier Science B.V.