AB-INITIO PREDICTIONS OF THE STRUCTURES AND SPECTRA OF SOME SIMPLE THIOSULFENO (XS(2)) FREE-RADICALS

Ab initio theory with split-valence plus polarization function and lar ger basis sets at the self-consistent field (SCF) and second order Mol ler-Plesset (UMP2) levels was used to predict the geometries and spect roscopic parameters for the ground and first excited electronic states of the HS2, FS2, and ClS2 free radicals. The ground-state species are predicted to be bent and may be described as having a sulfur-sulfur d ouble bond, a sulfur-substituent single bond, and an unpaired electron which is delocalized principally over the sulfur centers. The first e lectronic transition involves an (n,pi) electron promotion which lead s to a longer S-S bond, smaller bond angle, and greater localization o f the unrepaired electron on the terminal sulfur atom. The adiabatic t ransition energies are predicted to be at similar to 6700, 13700, and 10600 cm(-1) for HS2, FS2, and ClS2, respectively. The theoretical res ults are in good agreement with the rather limited amount of experimen tal data available. These ab initio predictions provide critical infor mation to aid in future experimental studies of the matrix isolation o r gas-phase spectra of the thiosulfeno radicals.