Gas Phase Intramolecular Proton Transfer in Cationized Glycine and Chlorine Substituted Derivatives (M–Gly, M=Na + , Mg 2+ , Cu + , Ni + , and Cu 2+ ): Existence of Zwitterionic Structures?

The intramolecular proton transfer in cationized glycine and chlorine substituted derivatives with M=Na + , Mg 2+ , Ni + , Cu + , and Cu 2+ has been studied with the three parameter B3LYP density functional method. The coordination of metal cations to the oxygens of the carboxylic group of glycine stabilizes the zwitterionic structure. For all monocations the intramolecular proton transfer occurs readily with small energy barriers (1–2 kcal mol −1 ). For the dication Mg 2+ and Cu 2+ systems, the zwitterionic structure becomes very stable. However, whereas for Mg 2+ , the proton transfer process takes place spontaneously, for Cu 2+ the reaction occurs with an important energy barrier. The substitution of the hydrogens of the amino group by chlorine atoms decreases the basicity of nitrogen, which destabilizes the zwitterionic structure. For monosubstituted glycine complexed with Na + , the zwitterionic structure still exists as a minimum, but for disubstituted glycine no minimum appears for this structure. In contrast, for Mg 2+ complexed to mono‐ and disubstituted glycine, the zwitterionic structure remains the only minimum, since the enhanced electrostatic interaction with the dication overcomes the destabilizing effect of the chlorine atoms.