Gaussian-2 theoretical and direct ab initio molecular dynamics study of the reaction of O(3P) with thiirane, O(3P) + C2H4S(1A1)→SO(3Σ−) + C2H4(1Ag)

Abstract

Gaussian-2 theoretical and direct density-functional molecular dynamics calculations have been carried out to characterize the reaction mechanism on the triplet potential energy surface of the title reaction. The Gaussian-2 procedure is employed to determine accurately the energies of the reactants, collision complexes, transition states and products on the O(³P) + C₂H₄S(¹A₁) potential energy surface. The minimum energy pathway on the triplet potential energy surface reveals two structural isomers as collision complexes which lie approximately 49 kcal mol⁻¹ below the reactants. Both of these complexes show cleavage of one of the S–C bonds in the three-membered ring. The transition states leading to formation of SO(³Σ⁻) + H₂CCH₂(¹A_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(³P) approaches the S atom of C₂H₄S(¹A₁), an S–C bond in the strained thiirane ring ruptures, 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 diradical complex is so short-lived that the desulfurization reaction may be viewed as a direct-mode abstraction reaction. The results of the combined Gaussian-2 theory and direct dynamics study are consistent with the recent experimental findings that angular distributions are highly anisotropic (Gao et al., J. Phys. Chem. A, 1997, 101, 187) and that nascent SO(³Σ⁻) product vibrational state distribution is inverted due to short-lived collision complexes (Ravichandran et al., Chem. Phys. Lett., 1996, 252, 348).