MNDO calculations of the molecular and electronic structure of thionitroso, dithionitro and related compounds

Compounds with the formulae RNS and RNS2 (thionitroso I and dithionitro compounds II, dithionitrites III, N-thiosulfinylamines IV, dithia-aziridines V and thiazylthiols VI, R = H, CH3, C6H5)and the corresponding radical cations HNS2+ and anions HNS2- have been investigated by MNDO calculations with respect to their molecular geometries, relative stabilities, ionization and electron capture properties and dipole moments. For the simplest representatives (R = H) the infrared spectra have been calculated. The lengths of NS triple (in VI), double (in I, III, IV) and partial double (in II, III) bonds are predicted to be in the region of 147-150, 152-155 and 158-162 pm, respectively. Within these regions systematic shifts result from different substituents R. Contraction of NS bonds (1-6 pm) is observed in the case of ionization whereas electron capture causes bond elongation (2-6 pm). The MNDO sequence of stability deviates from that of ab initio calculations for compounds with tetravalent sulfur the stabilization effect of which is obviously underestimated by the MNDO approach. All compounds studied are characterized by negative non-vertical electron affinities suggesting vigorous reactivity with nucleophilic reactants. The patterns of infrared spectra enable recognition, discrimination and characterization of these mostly unknown compounds which might be accessible as short-living transients.