IMINOETHENETHIONES, RN=C=C=S - CHARACTERIZATION BY NEUTRALIZATION-REIONIZATION MASS-SPECTROMETRY AND G2(MP2) THEORY

(Methylimino)ethenethione (2) and iminoethenethione (4) are stable molecules on the microsecond time scale of neutralization-reionization mass spectrometry experiments. The corresponding radical cations were generated by fragmentation of thiazolopyrimidinedione molecular ions 1(.+) and 3(.+). Iminoethenethione (4) does not tautomerize to thioformyl cyanide (H-CS-CN, 5) under the wall-less conditions of the MS experiment, but it does so under FVP conditions when generated from isoxazolones 6. Thioformyl cyanide was unequivocally identified by IR and mass spectra. The structures and stabilities of 2, 4, and 4(.+) were investigated by ab initio calculations at the G2(MP2) level of theory. Both 2 and 4 are predicted to have a singlet ground state, in contrast to O double bond C double bond C double bond S, for which a triplet state is preferred. The singlet-triplet gaps are approximately 40 kJ mol(-1). In agreement with experimental findings, both iminoethenethiones are calculated to be thermodynamically and kinetically stable species, lying in energy wells with at least a 100 kJ mol(-1) barrier to dissociation into HNC (or CH3NC) + CS. The IR and UV spectra and ionization energies of 2 and 4 are predicted. The iminoethenethione radical cation (4(.+)) is found to be the global minimum on the C2HNS.+ potential energy surface and stable toward all possible fragmentations: the most favorable fragmentations into H-. + NCCS+ and HNC + CS.+ are in accord with the mass spectrometric observations.