Theoretical and matrix-isolation experimental study on 2(1H)-pyridinethione/2-pyridinethiol

Tautomerism and infrared spectra of 2(1H)-pyridinethione were studied with theoretical and experimental methods. The relative stabilization energy of thiol and thione tautomers was calculated by using the SCF + MBPT(2) method of quantum chemistry with DZP basis set. The thiol form was predicted to be predominant in the gas phase. Vibrational frequencies and intensities of the IR bands were calculated at the SCF 3-21G* level. In reported experimental IR spectroscopic studies of Ar and N₂ matrix-isolated compounds, the thiol and thione tautomers were found in relative concentrations of about 30:1. The phototautomeric reaction - the conversion of the thione form into the thiol form after UV-vis irradiation of the matrix - was observed and used to separate the spectra of the two forms. Comparison of the experimental IR spectra of matrix-isolated and crystalline compounds with the spectra predicted theoretically gave an assignment for the majority of the absorption bands in the spectra of both tautomers.