A G2 ab initio study of C_2H_5S~+ ions: I. Structures, energetics, and unimolecular isomerizations of non-carbenoid isomers

摘要

The potential energy surface of [C_2H_5S~+] ions was studied with the G2 method. The structures and G2 heats of formation (ΔH_(f0)) of CH_3CH_2S~+ (1~+), syn-CH_3CHSH~+ (2~+), anti-CH_3CHSH~+ (3~+), and CH_3SCH_2~+ (4~+) were reported previously (1993, Chem. Phys. Lett., 213, 250). The G2 ΔH_(f298) = 802 kJ mol~(-1) of c-CH_2CH_2SH~+ (7~+) is in good agreement with the experimental ΔH_(f298) (7~+) = 798 kJ mol~(-1) (1998, J. Phys. Chem. Ref. Data, Suppl. 1). The symmetric conformer of protonated vinyl thiol CH_2CHSH_2~+ (ΔH_(f0) (9~+) = 857 kJ mol~(-1)) is slightly more stable than the non-symmetric one (8~+) by 8 kJ mol~(-1). The C-C protonated thiirane 11~+ with ΔH_(f0) = 989 kJ mol~(-1) is an intermediate for the 1,3-H shift 4~+ → 4~+. The isomerization reactions 8~+/9~+ → 2~+/3~+ → 7~+ → 4~+ are energetically facile compared to the unimolecular dissociation processes. The ΔH_(f0) of the transition state structures for these isomerizations lies within a narrow range of 960-1000 kJ mol~(-1). The calculated energetics of [C_2H_5S~+] are in line with the observation (1979, Org. Mass Spectrom. 14, 543) that both 2~+/3~+ and 4~+ isomerize to a common structure 7~+ and decompose through the same reaction channels.