Quantum Chemistry Study & Evaluation of Basis Set Effects on Prediction of Amino Acids Properties:

Abstract

The potential energy surface of gaseous glycine determined years ago in the ab initio B3LYP/6-<br />311++G** calculations is composed of thirteen stable conformers. We performed the ab initio<br />molecular orbital calculations as the starting point to carry out a force field and normal coordinate<br />calculation on the most stable conformer of non-zwitterionic glycine [conformer (I)]. The<br />calculations were carried out at different levels of theory using two methods, namely, the Hartree-<br />Fock (HF) and the Möller-Plesset second order perturbation (MP2) method (including electron<br />correlation), and using the Pople's basis sets, namely, STO-nG (n=2, 3 and 6), 3-21G, 6-21G, 6-<br />31G, 6-311G and also cc-pVnZ to obtain HF limit. This different basis sets accompanied with the<br />different combinations of diffuse and polarization functions were used. Each level of theory, with<br />no symmetry restrictions, did fully optimization of neutral glycine. The atomic charge<br />distributions were obtained using the Mulliken population analysis. The structural characteristics<br />such as the total energies, the complete optimized geometrical parameters including bond lengths,<br />normal and torsion angles, as well as dipole moments, rotational constants, atomic charge<br />distributions, vibrational frequencies and IR intensities of the equilibrium conformation of glycine<br />in gas phase were calculated at a wide range of the levels of theory -as mentioned above- and the<br />results were compared together and with HF limit and the experimental data to examine the<br />reliability of the applied basis sets and to introduce the most efficient ones. We also assayed how<br />the strength of internal H-bonds depended on the variant parameters of basis set via the calculated<br />atomic charges.