Experimental (FTIR and FT-Raman) and theoretical investigation of some pyridine-dicarboxylic acids

• FTIR and FT-Raman spectra are recorded for 23PDA, 24PDA and 34PDA.
• Most stable rotational conformer is identified for each of the three molecules.
• Unambiguous vibrational assignments are made using the results of quantum chemical and classical approaches.
• NLO behavior of the three molecules is established by computing hyperpolarizability.

The FTIR and FT-Raman spectra of 2,3-pyridine-dicarboxylic acid (23PDA); 2,4-pyridine-dicarboxylic acid (24PDA); and 3,4-pyridine-dicarboxylic acid (34PDA) were recorded in the range 4000–450 cm−1 and 4000–50 cm−1, respectively. The optimized geometries and vibrational frequencies along with intensities were computed using DFT employing B3LYP functional with 6-311++G(d,p) basis set. The rms error between observed and calculated frequencies was 11.76, 12.79 and 9.8 cm−1 for 23PDA, 24PDA and 34PDA, respectively. A 74-parameter modified valence force field was evaluated by solving inverse vibrational problem using Wilson's GF matrix method and 99 experimental frequencies of the three molecules were used to refine the force constants in overlay least-squares technique. The average error between observed and computed frequencies was found to be 9.21 cm−1. PED and eigen vectors calculated in the process were used to make unambiguous vibrational assignments. The values of dipole moment, polarizability and hyperpolarizability were computed to determine the NLO behaviour of these molecules. The HOMO and LUMO energies and thermodynamic parameters were also evaluated.

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