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

6 pags., 5 figs., 1 tab. -- This work is dedicated to Prof. Rafael Notario (IQFR-CSIC).
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.
A.F.L. acknowledges the support from the Brazilian funding agencies CNPq, FAPESP, and CAPES. J.M.O.-E. acknowledges support from the Spanish MICINN, Grant Number CTQ2018-094644-B-C22.