Photoionization mass spectrometry of CH3SH, CD3SH, and CH3SD: Heats of formation of CH3S+(CH2SH+), CH2S+, CH2S, and HCS+

Photoionization yield curves have been obtained for the major ions from CH3SH. Auxiliary studies with selectively deuterated species reveal that CD3S+ has a slightly lower threshold than CD2SH+ from CD3SH; but CH2SD+ has a lower threshold than CH3S+ from CH3SD. An analysis of alternative interpretations is presented. The isotopic studies also imply that CH2S+ is more stable than HCSH+, and HCS+ is more stable than CSH+. The value ΔH°f,0(HCS+)≤259.2±0.9 kcal/mol obtained in this study is in good agreement with a bracketed proton affinity for CS, but differs significantly from a recent ab initio calculation. This result confirms the view that HCS+ can be formed in interstellar clouds by exothermic reactions. The reaction CH3S+/CH2SH+→HCS++H2 proceeds weakly at threshold, and much more strongly after overcoming an activation barrier of ∼0.9 eV.