Density functional theory studies on molecular structure, vibrational spectra, AIM, HOMO-LUMO, electronic properties, and NBO analysis of benzoic acid monomer and dimer

In this study, the molecular structure, vibrational frequencies, and atom-in-molecule (AIM) analysis of the benzoic acid monomer and dimer were investigated. Geometry optimization and vibrational frequency calculations for the monomer and dimer were carried out at the density functional theory (DFT) level with the 6-311++G(2d,p) basis set. UV-Vis spectroscopy and electronic properties (i.e., excitation energies, oscillator strengths) were calculated by time-dependent DFT (TD-DFT) in different solvents. The calculated absorption values mainly represented excitation from HOMO-1 (H-1)  LUMO and HOMO  LUMO+2 (L+2). The structure-activity relationship was interpreted by its molecular electrostatic potential (MEP) surface, which is very useful to the research of molecular structures and their physiochemical-property relationships. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy was analysed. The HOMO-LUMO energy gap and other related molecular properties were also calculated. In addition, natural bond orbital (NBO) analysis was carried out to investigate the various intra-intermolecular interactions in the molecular system.