Computational studies on electron and proton transfer in phenol-imidazole-base triads

The electron and proton transfer in phenol-imidazole-base systems (base = NH(2)(-) or OH(-)) were investigated by density-functional theory calculations. In particular, the role of bridge imidazole on the electron and proton transfer was discussed in comparison with the phenol-base systems (base = imidazole, H(2)O, NH(3), OH(-), and NH(2)(-)). In the gas phase phenol-imidazole-base system, the hydrogen bonding between the phenol and the imidazole is classified as short strong hydrogen bonding, whereas that between the imidazole and the base is a conventional hydrogen bonding. The n value in sp(n) hybridization of the oxygen and carbon atoms of the phenolic CO sigma bond was found to be closely related to the CO bond length. From the potential energy surfaces without and with zero point energy correction, it can be concluded that the separated electron and proton transfer mechanism is suitable for the gas-phase phenol-imidazole-base triads, in which the low-barrier hydrogen bond is found and the delocalized phenolic proton can move freely in the single-well potential. For the gas-phase oxidized systems and all of the triads in water solvent, the homogeneous proton-coupled electron transfer mechanism prevails.