Conformational equilibrium in glycine: Focal-point analysis and ab initio limit

High-level ab initio electronic structure calculations, including extrapolations to the complete basis set (CBS) limit, resulted to a highly precise energetic conformation equilibrium of glycine. The CCSD(T)/CBS relative energies of glycine (Gly, H2NCH2COOH) conformers have been estimated using CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z calculations at B3LYP/aug-cc-pVTZ geometries. An uncertainty of 17–70 cm−1 for energy difference has been reached. Despite alternative claims in recent literature, B3LYP density functional was found to be able to localize all stationary points of glycine potential energy surface (PES). This finding is supported by the vibrational analysis of glycine conformations. The importance of an accurate description of intramolecular hydrogen bond in gaseous (free) glycine is discussed.