Hyperconjugation versus intramolecular hydrogen bond: origin of the conformational preference of gaseous glycine

Three experimentally detected low-energy conformers of gaseous glycine are selected as models to investigate the origin of the conformational preference of nonionized glycine, employing the atoms in molecules (AIM) and natural bond orbital (NBO) analysis methods. At the B3LYP/6-311++G(3d,3p) theory level, it is found that the importance of intramolecular hydrogen bond was overemphasized in the previous studies and it is hyperconjugation not intramolecular hydrogen bond that determines the order and relative energy of the conformers considered in this Letter.