Vibrational, electronic and quantum chemical studies of 5-benzimidazole carboxylic acid

Detailed structural and vibrational investigations of 5-benzimidazole carboxylic acid have been carried out with FTIR and FT-Raman spectral techniques. The electronic structure of the molecule has been analysed by 1H and 13C NMR spectroscopies. The DFT studies were carried out with B3LYP method utilising 6-31G**, 6-311++G** and cc-pVTZ basis sets to determine the structural, thermodynamical, vibrational, electronic characteristics of the compound and also to understand the electronic and steric influence of the carboxylic group on the skeletal frequencies. Complete NBO analysis was also carried out to find out the intramolecular electronic interactions and their stabilisation energy. The extreme limits of the electrostatic potential is +1.435e × 10−2 and −1.435e × 10−2. The frontier orbital energy gap in the case of 5BICA is found to be 5.2742 eV. In 5BICA molecule, the lone pair donor orbital, nN→πCN∗ interaction between the N1 lone pair and C2N3 antibonding orbital is seen to give a strong stabilisation of 43.32 kJ mol−1.

► FTIR, FT-Raman and NMR studies of 5–benzimidazole carboxylic acid were carried out. ► The down field signal of hydroxy proton strongly indicates the influence of –COO–. ► The n → π∗ interaction between N1 and C2–N3 has stabilisation of 43.32 kJ.mol–1. ► The extreme limits of the electrostatic potential are +1.435e × 10–2 and –1.435e × 10–2. ► 1H and 13C NMR chemical shifts of the molecule were discussed.