The geometries, relative stabilities, proton affinities for the different tautomers of 2-, 4- and 5-pyrimidones and their thio- and azo analogs along with their fixed forms (i.e. model compounds in which proton migration is eliminated) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydroxy forms were confirmed with all three methods, as cited in the literature, with the exception of AM1 and MNDO methods which both indicate the predominance of 2- and 4-hydroxypyrimidines over oxo forms with a stability energy value of approximate to 2 and 8 kcal mol-l for the main tautomers. For azo analogs the predominance of amino forms over imino forms were confirmed with the exception of the AM1 method which indicates the predominance of 5-iminopyrimidine over amino forms with stability energy values of approximate to 28 kcal mol(-1) for the main tautomers. For the thio analogs the predominance of thiol forms over thione forms were confirmed with the PM3 and MNDO methods; the AM1 method however, indicates the predominance of the 2-thione form over the thiol form with a stability energy value of approximate to 1 kcal mol(-1). (C) 1999 Elsevier Science B.V. All rights reserved.