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

Document Type : Research Paper


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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 initio
molecular orbital calculations as the starting point to carry out a force field and normal coordinate
calculation on the most stable conformer of non-zwitterionic glycine [conformer (I)]. The
calculations 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 electron
correlation), 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 the
different combinations of diffuse and polarization functions were used. Each level of theory, with
no symmetry restrictions, did fully optimization of neutral glycine. The atomic charge
distributions were obtained using the Mulliken population analysis. The structural characteristics
such as the total energies, the complete optimized geometrical parameters including bond lengths,
normal and torsion angles, as well as dipole moments, rotational constants, atomic charge
distributions, vibrational frequencies and IR intensities of the equilibrium conformation of glycine
in gas phase were calculated at a wide range of the levels of theory -as mentioned above- and the
results were compared together and with HF limit and the experimental data to examine the
reliability of the applied basis sets and to introduce the most efficient ones. We also assayed how
the strength of internal H-bonds depended on the variant parameters of basis set via the calculated
atomic charges.