Gas-Phase Basicity of 2,7-Dimethyl-[1,2,4]-Triazepine Thio Derivatives

The gas-phase proton affinities of 3-thio-5-oxo, 5-thio-3-oxo, and 3,5-dithio derivatives of 2,7-dimethyl-[1,2,4]-triazepine have been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The structures and vibrational frequencies of all the stable protonated tautomers and all the transition states connecting them have been obtained by means of the B3LYP density functional method, together with a 6-31G* basis set expansion. The final energies were obtained at the B3LYP/6-311+G(3df,2p) level. In contrast with the results from the analogous thiouracils, our results indicate that all of these compounds behave as sulfur bases in the gas phase. For 5-thio-3-oxo-[1,2,4]-triazepine and 3,5-dithio-[1,2,4]-triazepine, the thiol−enol and the dithiol forms are the most stable protonated species, respectively. Conversely, for 3-thio-5-oxo-[1,2,4]-triazepine, the thiol−ketone form is the most stable one. For 5-thio-3-oxo-[1,2,4]-triazepine and 3,5-dithio-[1,2,4]-triazepine, as it was found for thiouracils, a comparison between theoretical and experimental proton affinities suggests the formation of dimers between protonated and neutral species, which favors proton-transfer mechanisms leading to the formation of the most stable protonated species.