Molecular structure and vibrational assignments of hippuric acid: a detailed density functional theoretical study

In this study, the structural properties of hippuric acid (C(9)H(9)NO(3), HA) were studied using density functional theory (DFT) employing B3LYP exchange correlation. The geometry of the molecule was fully optimized at B3LYP/6-311G(d,p) level of theory. There are four conformers, C1, C2, C3, and C4 for this molecule. The geometrical parameters and energies have been obtained for all four conformers from DFT. The computational results diagnose the most stable conformer of HA as the C1 form. The vibrational frequencies were calculated and fundamental vibrations were assigned based on the scaled theoretical wavenumbers. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by using the gauge-invariant atomic orbital (GIAO) method. A study on the electronic properties, such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach, while taking solvent effects into account. Finally, geometric parameters, vibrational bands, chemical shifts and absorption wavelengths were compared with available experimental data of the molecule.