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

The potential energy surface of gaseous glycine determined years ago in the ab initio B3LYP/6-311++G** calculations is composed of thirteen stable conformers. We performed the ab initiomolecular orbital calculations as the starting point to carry out a force field and normal coordinatecalculation on the most stable conformer of non-zwitterionic glycine [conformer (I)]. Thecalculations were carried out at different levels of theory using two methods, namely, the Hartree-Fock (HF) and the Möller-Plesset second order perturbation (MP2) method (including electroncorrelation), and using the Pople’s basis sets, namely, STO-nG (n=2, 3 and 6), 3-21G, 6-21G, 6-31G, 6-311G and also cc-pVnZ to obtain HF limit. This different basis sets accompanied with thedifferent combinations of diffuse and polarization functions were used. Each level of theory, withno symmetry restrictions, did fully optimization of neutral glycine. The atomic chargedistributions were obtained using the Mulliken population analysis. The structural characteristicssuch as the total energies, the complete optimized geometrical parameters including bond lengths,normal and torsion angles, as well as dipole moments, rotational constants, atomic chargedistributions, vibrational frequencies and IR intensities of the equilibrium conformation of glycinein gas phase were calculated at a wide range of the levels of theory -as mentioned above- and theresults were compared together and with HF limit and the experimental data to examine thereliability of the applied basis sets and to introduce the most efficient ones. We also assayed howthe strength of internal H-bonds depended on the variant parameters of basis set via the calculatedatomic charges.