Tautomerism of 2- and 4-Thiouracil. Ab Initio Theoretical Study

Ab initio quantum mechanical studies on tautomerism are extended to the 2-thiouracil and 4-thiouracil molecules. We considered the protomeric tautomerism in vacuum and conclude that for both species the oxo-thione forms should prevail in full agreement with recent experimental matrix isolation studies. Surprisingly, the next most stable structure appears to be the hydroxy-mercapto form. The relevance of the relative stability order of 2-thiouracil and 4-thiouracil to the tautomerism of various derivatives is discussed. Major features of the vibrational spectra of various tautomeric forms are presented and compared with the experiment.