Molecular structure (monomeric and dimeric structure) and HOMO–LUMO analysis of 2-aminonicotinic acid: A comparison of calculated spectroscopic properties with FT-IR and UV–vis

The experimental (UV–vis and FT-IR) and theoretical study of 2-aminonicotinic acid (C6H6N2O2) was presented in this work. The ultraviolet absorption spectrum of title molecule that dissolved in ethanol and water were examined in the range of 200–400 nm. The FT-IR spectrum of the title molecule in the solid state were recorded in the region of 400–4000 cm−1. The geometrical parameters and energies of 2-aminonicotinic acid have been obtained for all four conformers/isomers (C1, C2, C3, C4) from DFT (B3LYP) with 6-311++G(d,p) basis set calculations. C1 form has been identified the most stable conformer due to computational results. Therefore, spectroscopic properties have been searched for the most stable form of the molecule. The vibrational frequencies were calculated and scaled values were compared with experimental FT-IR spectrum. The complete assignments were performed based on the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. Also the molecular structures, vibrational frequencies, infrared intensities were calculated for a pair of molecules linked by the intermolecular OH⋯O hydrogen bond. Moreover, the thermodynamic properties of the studied compound at different temperatures were calculated. Besides, charge transfer occurring in the molecule between HOMO and LUMO energies, frontier energy gap, molecular electrostatic potential (MEP) were calculated and presented. The spectroscopic and theoretical results are compared to the corresponding properties for monomer and dimer of C1 conformer. The optimized bond lengths, bond angles, calculated frequencies and electronic transitions showed the agreement with the experimental results.

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► Molecular structure of 2-aminonicotinic acid molecule was studied using DFT/B3LYP.
► Spectroscopic properties of organic molecule were examined by FT-IR and UV techniques and DFT.
► The complete assignments are performed on the basis of the total energy distribution (TED).
► HOMO and LUMO energies, absorption wavelength and excitation energies were calculated.