H 2S-CIMS: Theoretical calculations of the energies and structures of secondary ions in the ion source

Abstract

For the first time, a study has been made of H 2S as a reactant gas in positive-ion chemical-ionization mass spectrometry. To this end, the mass spectrum of an H 2S plasma was recorded, and the energies and structures of the ions were examined. The most stable structures of the plasma ions H 3S +, S 2 +, H 2S 2 +, H 3S 2 +, [H 2S···H 3S] +, H 2S 3 +, H 3S 4 + and HS n + were obtained by the ab initio G3B3 method, in which the G3 method is extended with B3LYP/6-31G( d). Of the three isomers of H 2S 3 +, the C 2V structure, with terminal hydrogen atoms, proved to be the lowest energy. The structure of the adduct [H 2S···H 3S] + was found to be comparable with that of the water adduct [H 2O···H 3O] +. An enthalpy of −59.9 kJ mol −1 was calculated for the reaction H 3S + + H 2S → [H 2S···H 3S] + and the exothermic reaction enthalpies of the polysulfonium ions, formed according to the equation HS n + + S → HS n+1 +, lie in the range 225–347 kJ mol −1 for n = 1–8.