A Gaussian-2 ab initio study of [C2H5S](+) ions: III. H-2 and CH4 eliminations from CH3CHSH+ and CH3SCH2+

Gaussian-2 ab initio calculations were performed to examine the six modes o f unimolecular dissociation of cis-CH3CHSH+ (1(+)), trans-CH3CHSH+ (2(+)), and CH3SCH2+ (3(+)): 1+ --> CH3+ + trans-HCSH (1); 1+ --> CH3 + trans-HCSH (2); 1+ --> CH4 + HCS+ (3); 1(+) --> H-2 + c-CH2CHS+ (4); 2(+) --> H-2 + C H3CS+ (5); and 3(+) --> H-2 + c-CH2CHS+ (6). Reactions (1) and (2) have end othermicities of 584 and 496 kJ mol(-1), respectively. Loss of CH4 from 1() (reaction (3)) proceeds through proton transfer from the S atom to the me thyl group, followed by cleavage of the C-C bond. The reaction pathway has an energy barrier of 292 kJ mol(-1) and a transition state with a wide spec trum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol(-1) and it also proceeds through the same proton transfer step as re action (3), followed by elimination of H-2. Formation of CH3CS+ from 2(+) ( reaction (5)) by loss of H-2 proceeds through protonation of the methine (C alphaH) group, followed by dissociation of the H-2 moiety. Its energy barri er is 276 kJ mol(-1). On both the MP2/6-31G* and QCISD/6-31G* potential-ene rgy surfaces, the H-2 1,1-elimination from 3(+) (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH3SCH2+ and has a critical energ y of 269 kJ mol(-1). (C) 1999 Elsevier Science B.V. All rights reserved.