The gas-phase conformations of (N–Cl)-glycine: some theoretical observations

Abstract

B3LYP and MP2 methods were employed to optimize thirteen conformers of (N–Cl)-glycine using 6-311++G** basis set. Single-point MP n ( n=2–4) calculations using the same basis set were carried out in order to confirm the reliabilities of B3LYP and MP2 on energies and relative stabilities for the glycine derivative. On the other hand, the effects of zero-point vibrational energy and electron correlation on the relative stabilities of different (N–Cl)-glycine conformers were discussed by comparing results from these theoretical calculations. It was found from the calculations above that B3LYP method could provide geometry structure very similar to MP2 theory for (N–Cl)-glycine. But the relative stabilities of the conformers of (N–Cl)-glycine predicted by MP2 method are in more agreement with high-level MP4(SDTQ) than B3LYP. The zero-point vibrational energy plays an important role on relative stabilities of some conformers for MP2 methods, but not for B3LYP. Significant effects of electron correlation on the relative energies of (N–Cl)-glycine conformation were observed by means of different treatments for electron correlation. Natural Bond Orbital and Atoms in Molecule analyses were performed to determine the existence of intramolecular hydrogen bond, which indicate that only three conformers have intramolecular hydrogen bonds. Furthermore, NBO is also employed to investigate quantitatively hyperconjugation in the conformers in order to account for conformational preference. Our results point out that hyperconjugations are responsible for the relative stability of most conformers. While for conformers containing hydrogen bond, the origin of relative stability comes from the balance of H-bond and hyperconjugative effect.