AM1, PM3 and MNDO study of the tautomerism of 2-, 4- or 5-imidazolones and their thio- and azo- analogs

Abstract The geometries, relative stabilities, ionization potentials and dipole moments for the different tautomers of 2-, 4- and 5-imidazolones and for their thio- or azo-analogs along with their 1-methyl derivatives were calculated with fully geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydoxy forms were confirmed with all three methods, as sited in the literature, with the exception of MNDO method which indicates the predominance of 2- and 4-hydroxyimidazoles over oxo forms with a satability energy values of approximately 3 and 4 kcal mol −1 for the main tautomers and for their 1-methyl derivatives respectively. For azo analogs the predominance of amino forms for both main tautomers and their 1-methyl derivatives were predicted with all three methods as sited in the literature. For the thio analogs however, thiol forms were found to be more stable with all three methods contradicting to literature reports in which the existence of thione forms indicated experimentally.