Quantum mechanical study of the structure and spectroscopic (FT-IR, FT-Raman, 13C, 1H and UV), NBO and HOMO–LUMO analysis of 2-quinoxaline carboxylic acid

The FTIR and FT-Raman spectra of 2-quinoxaline carboxylic acid (2-QCA) has been recorded in the region 4000–450 and 4000–100 cm−1, respectively. The conformational analysis, optimized geometry, frequency and intensity of the vibrational bands of 2-QCA were obtained by the density functional theory (DFT) with complete relaxation in the potential energy surface using 6-31G(d) basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FTIR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The 1H and13C NMR spectra have been recorded and 1H and 13C nuclear magnetic resonance chemical shifts of the molecule were also calculated using the gauge independent atomic orbital (GIAO) method and their respective linear correlations were obtained. The theoretical UV–visible spectrum of the compound using CIS method and the electronic properties, such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. The calculated HOMO and LUMO energies show that charge transfer occurs within molecule. The Mulliken charges, the values of electric dipole moment (μ) of the molecule were computed using DFT calculations. The change in electron density (ED) in the σ* antibonding orbitals and stabilization energies E(2) have been calculated by natural bond (NBO) analysis to give clear evidence of stabilization originating in the hyper conjugation of hydrogen–bonded interactions.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► It provides the ability of the B3LYP/6-31G* computational method. ► The most stable conformer has been predicted. ► The complete vibrational assignment and spectroscopic analysis have been carried out. ► The research shows the presence of intramolecular interaction in the title compound. ► It explains a good correlation between the experimental and theoretical data.