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.

中文翻译：

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5,5-二烷基巴比妥酸气相酸度的实验和计算研究。

5,5-烷基巴比妥酸的气相酸度 (GA) 已通过电喷雾电离-三重四极杆 (ESI-TQ) 质谱和扩展动力学 Cooks 方法 (EKCM) 进行实验测定。双功能巴比妥酸的 CH (1330.9 ± 10.0 kJ mol-1) 和 NH (1361.5 ± 10.5 kJ mol-1) 去质子化位点的 GAs 是通过选择性生产它们相应的异二聚体来确定的。通过测量 5,5-二甲基-和 5,5-二乙基巴比妥酸 (~1368 kJ mol-1) 的 GA 来确认 NH 位点的 GA 值。在高斯-n（G3和G4）复合方法的量子化学计算的支持下，对实验结果进行了合理化和讨论，证实了结果的良好一致性。