A Gaussian-2 ab initio study of [C2H5S]+ ions: II. Fragmentation pathways of CH3SCH+/2 and CH2CHSH+/2 revisited

Abstract

Gaussian-2 ab initio method was performed to examine the five modes of unimolecular dissociation of CH3SCH+/2 (1+) and CH2CHSH2/+ (5+): 1+ → CH3 + H2CS+ (1); 1+ → CH3/+ + H2CS (2); 1+ → CH4 + HCS+ (3); 5+ → H2S + [C2H3]+(4); and 5+ → H3S+ + C2H2 (5). Both reactions 1 and 2, with endothermicities 359 and 395 kJ mol-1 respectively, proceed without a reverse barrier. Loss of CH4 from 1+ (reaction 3) proceeds via a transition state resembling an ion-radical complex CH3 H2CS+. The elimination reaction has an energy barrier of 332 kJ mol-1 Loss of H2S from 5+ (reaction 4) initially proceeds through either cleavage of the C-S bond or formation of the ion-molecule complex H2S/[C2H3]+ (8+/9+), followed by dissociation of the complex. The endothermicity of reaction 4 is calculated to be 245-261 kJ mol-1. Reaction 5, the lowest energy process among the dissociation pathways studied in this work, proceeds via intermediates 9+ and H3S+/C2H2 (10+). Its rate-determining step 5+ → 9+ has an energy barrier of 220 kJ mol-1.