Gas phase proton affinity and basicity of methylated uracil-derivatives

Abstract

The gas phase proton affinity (PA) and basicity (GB) of methylated uracil derivatives have been evaluated experimentally by the Extended Kinetic Method (EKM) using ESI-TQ mass spectrometry. The experimental PA values, in kJ·mol-1, for 1,5-dimethyluracil (890.0 ± 8.4), 5,6-dimethyluracil (896.4 ± 8.4) and 5,6-dihydro-6-methyluracil (856.0 ± 8.4) were in the same order as those determined theoretically by calculations performed at the B3LYP/6–311++G(3df,2p) level of theory. From the results, it can be concluded that most stable protonated tautomers correspond to the enol structures. Furthermore, it should be noted that the methylation of uracil and 5,6-dihydrouracil has little effect on the proton affinity and basicity of their corresponding methylated derivatives.