Rearrangement and fragmentation processes on the potential energy surfacesof the (CHnS)(+) (n=1-4) systems

Stationary points on the quartet and doubler surfaces of (CH4S)(+), on the tripler 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 geomet ries have been located at second-order perturbation theory (UMP2) level usi ng a 6-311++G(d,p) basis set. Relative energies were obtained by means of e xtensive quadratic configuration interaction singles and doubles calculatio ns 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(S-4) has been identified. Owing to its large barrier (55.5 kcal/mol) for i ts 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 (CH 4S)(+) doubler surface, the conventional methanethiol radical cation (CH3SH -) is more stable than the ylide ion (CH2SH2+) and depending upon the entra nce channel, one can expect a competitive isomerization and dissociation. C leavage 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 tripler surfaces of the (CH3S)(+) syst em. While CH2SH+ is more stable on the singlet surface, CH3S+ is more stabl e on the triplet surface. The molecular hydrogen elimination requires highe r barriers from all these isomers compared to radical dissociation. CH2S+ i s 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 ac tivation energy. The latter signifies an unstable HSC+ minimum on the singl et surface.