Ab initio SCF and configuration interaction calculations of the multi-reference double-excitation (MRD-CI) type are reported for the HCS+-CSH+ system using two different basis sets of double-zeta plus polarization quality. Potential surface calculations find an absolute minimum for linear HCS+ with CS = 2.814 ao and CH = 2.062 ao but only a saddle point for the corresponding CSH+ species, lying 110 kcal/mole above the most stable HCS+ structure. Because of large distinctions in the minimum IP's of CS and CO it is seen that the HCS+ ground state decomposes into CS+(2Σ+) + H(2S) whereas the corresponding products for HCO+ are proton plus CO(1Σ+); similar differences are noted among their excited state products. The lowest excited states of HCS+ are found to be (π,π*) species with nearly the same ground state transition energies as in CS, while the corresponding 3,1 Π(σ,π*) species lie considerably higher in HCS+ as a result of the effects of σ MO stabilization relative to the unprotonated diatomic species. Comparison with the appropriate quantities for the HCO+-COH+ system is made wherever possible and qualitative arguments are given to rationalize the distinctions which occur. The heat of formation is calculated to be approximately 25 kcal/mole higher for HCS+ than for HCO+. © 1978.
