Intramolecular proton transfer in glycine radical cation

The effect of ionization on the relative stabilities of the four lowest con formers of glycine and on the intramolecular proton transfer process has be en studied using density functional and MP2 methods. Single-point calculati ons at the CCSD(T) level have also been performed. The energy ordering of t he radical cations differs from that observed for the neutral conformers, m ainly due to the changes in the basic and acid character of the NH2 group u pon ionization. Ionization favors the intramolecular proton-transfer proces s. For the ground ionic state,(2)A', both reactant and product have similar energies, the energy barrier being about 9.0 kcal/mol. For the first excit ed state, the proton transfer appears to be spontaneous. However, in both c ases, the final product is a distonic [NH3+-CH2-COO.] radical cation. This is in contrast to what is observed for neutral glycine, for which the zwitt erionic structure is not stable in the gas phase.