Research in the pyrimidine series XXVI. Calculation of the in-plane normal vibrations of the “enol” forms of barbituric acids

Abstract

The frequencies and forms of the in-plane normal vibrations of the “enol” forms of barbituric and 5-chloro- and 2-thiobarbituric acids and their deutero derivatives were calculated for the C_s symmetry point group. On the basis of an analysis of the forms of the normal vibrations, the principal frequencies observed in the IR spectra of 5-substituted (methyl, chloro, and bromo) and 2-thiobarbituric acids were assigned with respect to the types of vibrations. The force constants were also determined. The results of the calculations also confirm and refine the previously proposed interpretation of the IR spectra of crystalline samples of the investigated compounds and make it possible to form a judgment regarding the presence of tautomers in the crystalline state and in solutions from the IR spectra.