Gaussian-2 theoretical and direct ab initio molecular dynamics study of the reaction of O(P-3) with thiirane, O(P-3)+C2H4S((1)A(1))-> SO((3)Sigma(-))+C2H4((1)A(g))

Gaussian-2 theoretical and direct density-functional molecular dynamics cal culations have been carried out to characterize the reaction mechanism on t he triplet potential energy surface of the title reaction. The Gaussian-2 p rocedure is employed to determine accurately the energies of the reactants, collision complexes, transition states and products on the O(P-3)+C2H4S((1 )A(1)) potential energy surface. The minimum energy pathway on the triplet potential energy surface reveals two structural isomers as collision comple xes which lie approximately 49 kcal mol(-1) below the reactants. Both of th ese complexes show cleavage of one of the S-C bonds in the three-membered r ing. The transition states leading to formation of SO((3)Sigma(-))+H-2=C2CH (2)((1)A(g)) lie only 2.8 and 3.3 kcal mol(-1), respectively, above the two diradical complexes, indicating that they are short-lived. Direct density- functional molecular dynamics study indicates that, as O(P-3) approaches th e S atom of C2H4S((1)A(1)), an S-C bond in the strained thiirane ring ruptu res, giving rise to one of the diradical complexes predicted by Gaussian-2 theory, followed within 20-80 fs by rupture of the second S-C bond. The dir adical complex is so short-lived that the desulfurization reaction may be v iewed as a direct-mode abstraction reaction. The results of the combined Ga ussian-2 theory and direct dynamics study are consistent with the recent ex perimental findings that angular distributions are highly anisotropic (Gao et al., J. Phys. Chem. A, 1997, 101, 187) and that nascent SO((3)Sigma(-)) product vibrational state distribution is inverted due to short-lived colli sion complexes (Ravichandran et al., Chem. Phys. Lett., 1996, 252</BO>, 348 ).