Conformational Study of Glycine Amide Using Density Functional Theory

Abstract

Seven stable stationary points, corresponding to three pairs of mirror-image conformers and one Cs symmetry conformer, have been located on the potential energy surface (PES) of neutral glycine amide at the B3LYP/6-311++G** level of theory. Accurate geometric structures, relative stabilities, and harmonic vibrational frequencies have been investigated. More importantly, the intramolecular H-bond formed from the amide to the amine plays a key role in stabilizing the global minimum, as observed in alanine amide and has been discussed qualitatively from the viewpoint of the structures, charge distributions, and vibrational analyses. As an important supplement in property for glycine amide, other property parameters, such as gas-phase basicity (GB), proton affinity (PA), and ionization potential (IP), have been predicted. The Boltzmann equilibrium distributions for the seven conformers have also been discussed qualitatively through the calculations of Gibbs free energy at various temperatures. At room temperature, the equilibrium compositions are mainly composed of conformers I and II exclusively, i.e., about 75.02% and 23.28%, respectively. As a tentative study, the conformational behaviors in aqueous solution have been explored using the Onsager model within the self-consistent reaction field (SCRF) method at the same level employed in the gas phase. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which is different from the previous theoretical reports. Moreover, the consistent results in relative energy using higher-level computations, including the MP2, MP3, MP4SDQ, and CCSD(T) methods employing the Dunning's correlation consistent basis set aug-cc-PVDZ, indicate that the B3LYP/6-311++G** level of theory may be applied to the analogous systems.