Hydrogen bonding and internal rotation barriers of glycine and its zwitterions (hypothetical) in the gas phase

摘要: 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[Formula: see text]. Thermodynamic properties, [Formula: see text] 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.