HYDROGEN-BONDING AND INTERNAL-ROTATION BARRIERS OF GLYCINE AND ITS ZWITTERION (HYPOTHETICAL) IN THE GAS-PHASE

The structures of the major glycine conformers, and several transition state structures, were optimized at HF/6-31G* and HF/6-31+G* levels of theory and the correlation energies were calculated at MP2/6-31G* and MP2/6-31+G** levels on these geometries, respectively. The heat of formation of gas phase glycine is calculated to be DELTAH(f)0 (298.15) = -389.6 kJ/mol. Thermodynamic properties, C(p)0, S0, H-0 - H-0(0), DELTAH(f)0, etc. are calculated as functions of temperature using standard statistical thermodynamic methods. MP2/6-31+G* optimization was carried out on the zwitterion, found by HF/6-31+G* optimization to be a local minimum. The results suggest that glycine in the zwitterionic form in the gas phase is not a stationary point. Zwitterionic glycine does not exist in the gas phase.