Experimental and Computational Studies on the Gas-Phase Acidity of 5,5-Dialkyl Barbituric Acids

Abstract

The gas phase acidities (GA) of 5,5-alkylbarbituric acids have been experimentally determined by electrospray ionization-triple quadrupole (ESI-TQ) mass spectrometry and by using the extended kinetic Cooks method (EKCM). The GAs of C–H (1330.9 ± 10.0 kJ mol–1) and N–H (1361.5 ± 10.5 kJ mol–1) deprotonated sites of bifunctional barbituric acid were determined from the selective production of their corresponding heterodimers. The GA value in the N–H site was confirmed by measuring the GAs of 5,5-dimethyl- and 5,5-diethyl barbituric acids (∼1368 kJ mol–1). The experimental results have been rationalized and discussed with the support of quantum chemical calculations with Gaussian-n (G3 and G4) composite methods, which confirmed the excellent consistency of the results.