THE GLYCINE ZWITTERION DOES NOT EXIST IN THE GAS-PHASE - RESULTS FROM A DETAILED ABINITIO ELECTRONIC-STRUCTURE STUDY

We have computed the properties of the zwitterionic form of glycine (NH3+CH2COO-) using ab initio molecular orbital theory. At the Hartree-Fock level, a shallow minimum on the zwitterion potential energy surface may be located with certain basis sets that do not contain p-type polarization functions on hydrogen. The small barriers (< 1 kcal/mol) for intramolecular proton transfer to nonionic glycine (NH2CH2COOH) disappear, however, when corrections are made for vibrational zero point energies or electronic correlation. The zwitterion is not a minimum at the Hartree-Fock level when the basis set contains hydrogen polarization functions nor when correlated wavefunctions are employed.