Rearrangement and Fragmentation Processes on the Potential Energy Surfaces of the (CH_nS)⁺ (n = 1−4) Systems

Stationary points on the quartet and doublet surfaces of (CH₄S)⁺, on the triplet and singlet surfaces of (CH₃S)⁺, on the doublet surface of (CH₂S)⁺, and on the singlet and triplet surfaces of (CHS)⁺ have been examined by ab initio molecular orbital theory. Equilibrium and saddle point geometries have been located at second-order perturbation theory (UMP2) level using a 6-311++G(d,p) basis set. Relative energies were obtained by means of extensive quadratic configuration interaction singles and doubles calculations with a 6-311++G(2df,2pd) basis set. On the quartet (CH₄S)⁺ surface, an association complex stabilized by 25.2 kcal/mol with respect to CH₄ and S⁺(⁴S) has been identified. Owing to its large barrier (55.5 kcal/mol) for its dissociation, it is expected to be long-lived as assumed by Zakouril et al. (J. Phys. Chem. 1995, 99, 15890) in their experimental work. On the (CH₄S)⁺ doublet surface, the conventional methanethiol radical cation (CH₃SH⁺) is more stable than the ylide ion (CH₂SH₂⁺) and depending upon the entrance channel, one can expect a competitive isomerization and dissociation. Cleavage of the C−H bonds in the ylide ion involves higher barriers compared to that in CH₃SH⁺. Three stable isomers, viz., CH₃S⁺, CH₂SH⁺, and CHSH₂⁺, have been located on the singlet and triplet surfaces of the (CH₃S)⁺ system. While CH₂SH⁺ is more stable on the singlet surface, CH₃S⁺ is more stable on the triplet surface. The molecular hydrogen elimination requires higher barriers from all these isomers compared to radical dissociation. CH₂S⁺ is predicted to be more stable than trans-HCSH⁺ with a barrier of 51.9 kcal/mol for the rearrangement to the less stable isomer. A significant barrier to 1,2 hydrogen shift isomerization is predicted on the triplet surface of the HSC⁺ while that on the singlet surface is predicted to occur without activation energy. The latter signifies an unstable HSC⁺ minimum on the singlet surface.