High-level theoretical study of the evolution of abundances and interconversion of glycine conformers

The relative conformer energies of glycine are evaluated by using a focal point analysis expressed as (HF -> MP2 -> MP3 -> CCSD -> CCSD(T)). The conformer abundances at various temperatures (298-500 K) are calculated based on the relative energies and Boltzmann statistical thermostatistical analysis with and without considering internal hindered rotations. A comparison between the available Raman spectrum and the electron momentum spectrum confirms that the influence of rigid-rotor hindered rotation on the conformational proportions of glycine is considerable, especially for the IIIp structure. The conformational interconversions are discussed. It is found that with increasing temperature, the mole fraction of IIn keeps constant and Ip structure can convert into IVn and IIIp, leading to the decrease in the weight of Ip and the increase in the weights of IVn and IIIp conformers, which is in accordance with experimental observations.