Theoretical Calculations of the Proton Affinities of n-Alkyl Systems

Int. J. Mass Spectrom. 243 (3), 241-247, 2005.

H₂S-CIMS: Theoretical calculations of the energies and structures of secondary ions in the ion source

Axel Patrick Ligon^a, and Siegmar Gäb^b

^aInstitute of Environmental Research (INFU), University of Dortmund, Otto-Hahn-Strasse 6, 44221 Dortmund, Germany
^bDepartment of Analytical Chemistry, University of Wuppertal, Gauss-Strasse 20, 42097 Wuppertal, Germany

Received 25 February 2005

Accepted 21 March 2005

Abstract:

For the first time, a study has been made of H₂S as a reactant gas in positive-ion chemical-ionization mass spectrometry. To this end, the mass spectrum of an H₂S plasma was recorded, and the energies and structures of the ions were examined.

The most stable structures of the plasma ions H₃S⁺, S₂⁺, H₂S₂⁺, H₃S₂⁺, [H₂S···H₃S]⁺, H₂S₃⁺, H₃S₄⁺ 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 H₂S₃⁺, the C_2V structure, with terminal hydrogen atoms, proved to be the lowest energy. The structure of the adduct [H₂S···H₃S]⁺ was found to be comparable with that of the water adduct [H₂O···H₃O]⁺. An enthalpy of −59.9 kJ mol⁻¹ was calculated for the reaction H₃S⁺ + H₂S → [H₂S···H₃S]⁺ 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.

Keywords: H₂S plasma; Chemical-ionization mass spectrometry; Ab initio; G3B3