The Mills-Nixon effect on enol-enol tautomerism in beta-dicarbonyl compounds and on annular tautomerism in NH-pyrazoles: A semi-empirical study

AM1 semiempirical calculations, both Delta H and Delta S, were carried out on the enol/enol tautomerism of 25 beta-diketones and 8 beta-ketoaldehydes. In the first case, delta Delta G(exp) was determined by Hansen (Magn. Reson. Chem. 1996, 34, 467) and his values correlate reasonably well with delta Delta G(calc), delta Delta G(exp) = - 0.23 + 0.29 delta Delta G(calc), r(2) = 0.83. The calculated differences in energy are linearly related to some geometrical characteristics of the ketoenol, namely the angles about the substituent on the central carbon. To check if this geometrical dependence is related to the Mills-Nixon effect, parallel AM1 calculations on the tautomerism of beta-ketoaldehydes and 3(5),4-disubstituted NH-pyrazoles were carried out confirming the influence of the Mills-Nixon effect on the enol/enol tautomerism of beta-dicarbonyl compounds. (C) 1997, Elsevier Science Ltd.