Quantum Chemistry Study & Evaluation of Basis Set Effects on Prediction of Amino Acids Properties:
Summer 2006, Volume 3 - Number 2 (22 صفحه - از 111 تا 132)
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.
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