The spectroscopic (FT-Raman, FT-IR, UV and NMR), molecular electrostatic potential, polarizability and hyperpolarizability, NBO and HOMO-LUMO analysis of monomeric and dimeric structures of 4-chloro-3,5-dinitrobenzoic acid.

Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of 4-chloro-3,5-dinitrobenzoic acid (CDNBA) were carried out by using density functional (DFT/B3LYP) method with 6-311++G(d,p) as basis set. To determine lowest-energy molecular conformation of the title molecule, the selected torsion angle is varied every 10° and molecular energy profile is calculated from 0° to 360°. The optimized geometrical parameters obtained by DFT calculations are in good conformity with single crystal XRD data. The FT-IR and FT-Raman spectra were measured in the condensed state. The fundamental vibrational wavenumbers as well as their intensities were calculated and a good agreement between observed and scaled calculated wavenumbers has been achieved. The electric dipole moment, polarizability and the first hyperpolarizability values of the CDNBA have been calculated at the same level of theory and basis set. The calculation results also show that the CDNBA molecule may has nonlinear optical (NLO) behavior with non-zero values. Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using natural bond orbital analysis. Ultraviolet-visible (UV) spectrum of the title molecule has also been calculated using CIS and TD-DFT methods. The calculated energy and oscillator strength almost exactly reproduce the experimental data. The (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by the gauge independent atomic orbital (GIAO) method and compared with experimental results. The thermodynamic properties of the studied compound at different temperatures were calculated.