Investigations of double proton transfer and one-electron oxidation behavior in double H-bonded glycine-formamide complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycine-formamide complex have been investigated employing the B3LYP/6-311++G** level of theory. The relevant quantities involved in the DPT process, such as interaction energies, tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. Compared with the intramolecular proton transfer (PT) in isolated glycine, the participation of a formamide molecule favors the PT of glycine kinetically. The DPT process proceeds with a concerted mechanism rather than a stepwise one since no zwitterionic complexes have been located during the DPT process. Correspondingly, the calculated forward and reverse barrier heights are 12.62 and 2.01 kcal/mol, respectively. However, both of them have been reduced to 9.35 and −1.16 kcal/mol when zero-point vibrational energy (ZPVE) corrections are included, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously. Additionally, the one-electron oxidation behavior for the double H-bonded glycine-formamide complex has also been investigated. The oxidation product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to formamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.20 and 8.26 eV, respectively.