DFT study of the intramolecular double proton transfer of 2,5-diamino-1,4-benzoquinone and its derivatives, and investigations about their aromaticity

Density functional theory method at a B3LYP/aug-cc-pvtz theoretical level was used to investigate the process of intramolecular double proton transfer of 2,5-diamino-1,4-benzoquinone (A) and 3,6-diaminopyrazine-2,5-dione (B). Two mechanisms, stepwise (TS1) and concerted (TS2), are proposed for the proton transfer process. Also, the pathways and the produced resonance forms in the conversion processes have been studied. In addition, proton transfer process was studied by infrared spectra analysis, which shows the process dynamics. The solvent effects are simulated by the self-consistent reaction field method using the polarizable continuum model. These calculations showed that both compounds perform the process of proton transfer through stepwise mechanism and that the compound A has less energy barrier than compound B for the proton transfer process. In addition, aromaticity of the two compounds was evaluated using the harmonic oscillator model of aromaticity and the nucleus-independent chemical shift values to predict dominant resonance structures and the charge distributions in the ring.