A G2 AB-INITIO STUDY OF C2H5S- I - STRUCTURES, ENERGETICS, AND UNIMOLECULAR ISOMERIZATIONS OF NON-CARBENOID ISOMERS( IONS )

The potential energy surface of [C2H5S+] ions was studied with the G2 method. The structures and G2 heats of formation (Delta H-f0) of CH3CH 2S+ (1(+)), syn-CH3CHSH+ (2(+)), anti-CH3CHSH+ (3(+)), and CH3SCH2+ (4 (+)) were reported previously (1993, Chem. Phys. Lett., 213, 250). The G2 Delta H-f298 = 802 kJ mol(-1) of c-CH2CH2SH+ (7(+)) is in good agr eement with the experimental Delta H-f298 (7(+)) = 798 kT mol(-1) (198 8, J. Phys. Chem. Ref. Data, Suppl. 1). The symmetric conformer of pro tonated vinyl thiol CH2CHSH2+ (Delta H-f0 (9(+)) = 857 W mol(-1)) is s lightly more stable than the non-symmetric one (8(+)) by 8 kJ mol(-1). The C-C protonated thiirane 11(+) with Delta H-f0 = 989 W mol(-1) is an intermediate for the 1,3-H shift 4(+) --> 4(+). The isomerization r eactions 8(+)/9(+) --> 2(+)/3(+) --> 7(+) --> 4(+) are energetically f acile compared to the unimolecular dissociation processes. The Delta H -f0 of the transition state structures for these isomerizations lies w ithin a narrow range of 960-1000 kJ mol(-1). The calculated energetics of [C2H5S+] are in line with the observation (1979, Org. Mass Spectro m. 14, 543) that both 2(+)/3(+) and 4(+) isomerize to a common structu re 7+ and decompose through the same reaction channels. (C) 1998 Publi shed by Elsevier Science B.V. All rights reserved.