Theoretical and infrared matrix isolation study of 4(3H)-pyrimidinethione and 3(2H)-pyridazinethione. Tautomerism and phototautomerism

IR spectra of 4(3H)-pyrimidinethione and 3(2H)-pyridazinethione isolated in argon and nitrogen matrices are reported. The thiol and thione tautomeric forms of 4(3H)-pyrimidinethione were found in matrices in relative concentrations ∼5:1. IR spectra of gaseous sample showed that the ratio of tautomers in matrices corresponds to the gas-phase tautomeric equilibrium. Only the thione form of 3(2H)-pyridazinethione was observed in matrices. UV-vis irradiation of the matrices caused the conversion of the thione forms of the studied molecules into the thiol forms. The IR spectra of the thione and thiol tautomers were predicted theoretically at SCF/3-21G* level, and a comparison of the predicted and experimentally observed spectra enabled an assignment of most of the observed bands.