C2H2S radical cations: Application of tandem mass spectrometry methodologies

Attempts have been made to characterize ionic and neutral C2H2S+/o isomers, viz., thioketene (1+), ethynylthiol (2+), and thiirene (3+) using various tandem mass spectrometric methodologies. Dissociative electron ionization of thiophene, 1-propane thiol and thiirane were used to produce the C2H2S+ (m/z 58) ions. The collisional activation (CA) mass spectrum of m/z 58 ions from thiophene is highly compatible with thioketene ions which has been supported by the CA mass spectrum of metastably generated m/z 58 ions and by comparison with the previously reported CA mass spectrum of thioketene ions. Theoretical calculations at the B3LYP/6-311G(d,p), and G2/G2(MP2) levels predict that low energy thioketene ions equilibrate with other isomeric ions viz., ethynylthiol and thiirene prior to dissociation by loss of H (m/z 57) and CH (m/z 45). The CA spectra of C2H2S+ ions from 1-propane thiol and thiirane are nearly identical indicating a facile isomerization of the initial structures at energies lower than the dissociative threshold. The low energy CID spectra of the m/z 58 ions from three precursors are found to be similar. However, significant differences are noticed in the relative abundances of product ions in the NR spectra of m/z 58 ions from the three precursors. While the NR mass spectra of m/z 58 ions from thiophene and thiirane confirms the stability of thioketene neutrals, the NR spectra of m/z 58 ions from 1-propane thiol suggests formation of a mixture of ethynylthiol and thioketene ions. Calculations predict a different thermo-chemistry for 1+-3+, but, due to a mixture of reacting isomeric ions whatever the precursor molecule is, similar results were obtained for the associative ion/molecule reactions between the C2H2S+ ions with methylisocyanide.