Polycarbon Disulfides SCnS (n = 1-3) and Their Protonated and Methylated Forms (HCnS2+ and CH3CnS2+): Tandem Mass Spectrometry and ab Initio MO Study

Abstract

Dissociative ionization of 4,5-bis(methylthio)-1,2-dithiole-3-thione (1) has allowed the detection of a series of carbon disulfide radical cations, SCnS.+ (n = 1-3), and their S-methylated forms, CH3SCnS+, in the gas phase of a mass spectrometer. The connectivities and stabilities of these ions and the corresponding neutrals have been probed by collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Moreover, the sites of protonation and cationic methylation of the polycarbon disulfides have been investigated by a combination of flash vacuum pyrolysis, chemical ionization, and CAMS (FVP/CI/MS/MS). The experimental results are supported by ab initio MO calculations at the QCISD(T)/6-311+G(2d,p) + ZPVE level. Both experiments and calculations indicate that the most favorable site of protonation and methylation is the carbon atom for C2S2 and C3S2 but a sulfur atom for CS2. The proton affinities (298 K) of C2S2 and C3S2 are predicted to be 842 and 818 kJ mol(-1), respectively.