PROTON AND SODIUM-ION AFFINITIES OF GLYCINE AND ITS SODIUM-SALT IN THE GAS-PHASE - ABINITIO CALCULATIONS

The energies of protonation and Na+ cationization of glycine (GLY) and its (GLY - H + Na) salt in the gas phase were calculated using ab initio calculations. The proton affinity of GLY, valued at the MP2/6-31G* parallel-to 3-21G level, is 937 kJ mol-1. The amino function is confirmed to be the most favourable site of protonation: 'proton affinities' of the carbonyl and hydroxyl functions are calculated to be 75 and 180 kJ mol-1, respectively, lower than that of NH2 at the MP2/6-31G* parallel-to 3-21G level. Calculations performed up to the MP2/6-31G* parallel-to 3-21G level give the Na+ affinity of GLY as 189 kJ mol-1 and the H+ and Na+ affinities of (GLY - H + Na) as 1079 and 298 kJ mol-1, respectively. The geometries of all neutral and protonated species optimized with the 3-21G basis set are described. Both H+ and Na+ cations complex preferably between the nitrogen atom and the carbonyl oxygen atom, leading to pseudo-five-membered ring structures in which Na-O and Na-N bonds lengths are greater than 2 angstrom.