Relation between structure and tautomerism in diazinones and diazinethiones. An experimental matrix isolation and theoretical ab initio study

Tautomerism of 2-pyridinone, 2-pyrimidinone, 2-pyrazinone, 4-pyrimidinone, 3-pyridazinone, and their thio analogs isolated in low-temperature inert gas matrices has been investigated by means of IR spectroscopy. The UV-induced phototautomeric reaction has been used to separate the IR spectra of the tautomers. It is found that the relative stability of the oxo and hydroxy tautomers of the above compounds depends in a systematic way on the relative position of the lactam group and the second nitrogen atom in the ring. In the analogous compounds with an exocyclic sulfur atom, the stability of the thiol forms (with respect to the corresponding thione forms) is considerably higher than the stability of the hydroxy forms (with respect to the corresponding oxo forms) in the compounds with exocyclic oxygen. The trend, which emerges from the experimental observations, is correctly reproduced by the theoretical ab initio calculations performed at the SCF + MBPT(2) level with the DZP basis set.