<Regular Article>Evaluation of Self-Association Energies of 2-Pyridone Tautomers by Modified MNDO Methods

We have carried out the molecular orbital calculations supported by the modified MNDO methods (MNDO/H, Modified MNDO and Am1) to study the self-associations of formic acid, 2-pyridone and its tautomer, 2-pyridinol. The greater self-association energy of 2-pyridone than that of 2-pyridinol was shown in all the methods. The calculated self-association energies of formic acid and 2-pyridone by the MNDO/H method were well compatible with the experimental values. However, the difference in energy between two tautomers of 2-pyridone was too small to reproduce the experimentally known values. On the other hand, it was found that the Modified MNDO method was invalid for the doubly hydrogen-bonded systems. The AM1 method have been shown to give a proper account of the tautomeric features of 2-pyridone, although the calculated energies of hydrogen-bonding were only about a half the experimental values