Proton and sodium ion affinities of glycine and its sodium salt in the gas phase. Ab initio calculations

Abstract

AbstractThe energies of protonation and Na+ cationization of glycine (GLY) and its (GLY − H + Na) salt in the gas phase were calculated using ab inltio calculations. The proton affinity of GLY, valued at the MP2/6–31G*//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*//3–21G level. Calculations performed up to the MP2/6–31G*//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 Å.