Ab initio calculations of the energies of the ring-opened and ring-closed isomers of C2H4S+.

Ab initio calculations, including electron correlation, are reported for two C2H4S+ radical cations and the transition state for isomerization between the two forms. Electron correlation was carried out at several levels of theory to assess the possible errors inherent in these calculations. At the QCISD(full)/6-31G∗ level, the ring-closed isomer is 10.6 kcal mol−1 more stable than the ring-opened (CH2SCH2+.) radical cation. The transition state lies 43.2 kcal mol−1 above the ring-closed isomer. MP2/6-31G∗ calculations were not sufficient for thermodynamic energy differences or the activation barrier. This is attributed to spin contamination in the ring-opened isomer and the transition state.