Rearrangement and Fragmentation Processes on the Potential Energy Surfaces of the (CH<i><sub>n</sub></i>S)<sup>+</sup> (<i>n</i> = 1−4) Systems

Stationary points on the quartet and doublet surfaces of (CH<sub>4</sub>S)<sup>+</sup>, on the triplet and singlet surfaces of (CH<sub>3</sub>S)<sup>+</sup>, on the doublet surface of (CH<sub>2</sub>S)<sup>+</sup>, and on the singlet and triplet surfaces of (CHS)<sup>+</sup> 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 (CH<sub>4</sub>S)<sup>+</sup> surface, an association complex stabilized by 25.2 kcal/mol with respect to CH<sub>4</sub> and S<sup>+</sup>(<sup>4</sup>S) 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. (<i>J. Phys. Chem.</i> <b>1995</b>, <i>99</i>, 15890) in their experimental work. On the (CH<sub>4</sub>S)<sup>+</sup> doublet surface, the conventional methanethiol radical cation (CH<sub>3</sub>SH<sup>+</sup>) is more stable than the ylide ion (CH<sub>2</sub>SH<sub>2</sub><sup>+</sup>) 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 CH<sub>3</sub>SH<sup>+</sup>. Three stable isomers, viz., CH<sub>3</sub>S<sup>+</sup>, CH<sub>2</sub>SH<sup>+</sup>, and CHSH<sub>2</sub><sup>+</sup>, have been located on the singlet and triplet surfaces of the (CH<sub>3</sub>S)<sup>+</sup> system. While CH<sub>2</sub>SH<sup>+</sup> is more stable on the singlet surface, CH<sub>3</sub>S<sup>+</sup> is more stable on the triplet surface. The molecular hydrogen elimination requires higher barriers from all these isomers compared to radical dissociation. CH<sub>2</sub>S<sup>+</sup> is predicted to be more stable than <i>trans</i>-HCSH<sup>+</sup> 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<sup>+</sup> while that on the singlet surface is predicted to occur without activation energy. The latter signifies an unstable HSC<sup>+</sup> minimum on the singlet surface.