AM1, PM3 and MNDO study of the tautomeric equilibria of 2-, 4- or 5-hydroxypyrimidin derivatives and their azo- and thio-analogs

The geometries, relative stabilities, proton affinities for the different t automers 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 e liminated) were calculated with full geometry optimization using AM1, PM3 a nd MNDO methods. The predominance of oxo forms over hydroxy forms were conf irmed with all three methods, as cited in the literature, with the exceptio n of AM1 and MNDO methods which both indicate the predominance of 2- and 4- hydroxypyrimidines over oxo forms with a stability energy value of approxim ate to 2 and 8 kcal mol-l for the main tautomers. For azo analogs the predo minance of amino forms over imino forms were confirmed with the exception o f 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 for ms 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 th iol form with a stability energy value of approximate to 1 kcal mol(-1). (C ) 1999 Elsevier Science B.V. All rights reserved.