A Gaussian-2 ab initio study of [C2H5S]+ ions: III. H2 and CH4 eliminations from CH3CHSH+ and CH3SCH2+

Abstract Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis -CH 3 CHSH + ( 1 + ), trans -CH 3 CHSH + ( 2 + ), and CH 3 SCH 2 + ( 3 + ): 1 + →CH 3 + + trans -HCSH (1); 1 + →CH 3 + trans -HCSH + (2); 1 + →CH 4 +HCS + (3); 1 + →H 2 + c -CH 2 CHS + (4); 2 + →H 2 +CH 3 CS + (5); and 3 + →H 2 + c -CH 2 CHS + (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol −1 , respectively. Loss of CH 4 from 1 + (reaction (3)) proceeds through proton transfer from the S atom to the methyl 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 spectrum 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 reaction (3), followed by elimination of H 2. Formation of CH 3 CS + from 2 + (reaction (5)) by loss of H 2 proceeds through protonation of the methine (C α H) group, followed by dissociation of the H 2 moiety. Its energy barrier is 276 kJ mol −1 . On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H 2 1,1-elimination from 3 + (reaction (6)) proceeds via a nonclassical intermediate resembling c -CH 3 SCH 2 + and has a critical energy of 269 kJ mol −1 .