Tautomerism in 4‐substituted 1‐phenyl‐3‐methyl‐pyrazolin‐5‐ones—a theoretical ab initio and 13C NMR study

Abstract The tautomeric equilibria between the CH, OH and NH forms in a series of 4‐substituted 1‐phenyl‐3‐methyl‐pyrazolin‐5‐ones have been studied using ab initio calculations at various levels of theory and comparison made with the experimental results obtained from NMR measurements. Quantitative comparison of both the relative energies and the 13C chemical shifts of the tautomers constituting the tautomeric equilibria were made by calculation of both sets of parameters. The influence of the solvent was included by employing various models of the self‐consistent reaction field theory. Initially, the solvent was included in the calculations by applying a continuum of differing polarity over the surface of isolated tautomers (self‐consistent isodensity polarized continuum model method), then later by assuming formation of an adduct between the solute and the solvent (thereby simulating effectively the hydrogen bonding in the OH and NH tautomers) and finally by calculating dimer or trimer complexes of the various tautomers. In this manner, the agreement between the theoretically calculated and the experimentally determined tautomeric equilibria was improved significantly. The theoretically calculated 13C chemical shifts of the tautomers were found to be viable for the assignment of the tautomers, particularly the preferred tautomer in the OH/NH equilibrium, which remains fast on the NMR time scale even at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd.