Investigation of torsional potentials, molecular structure, vibrational properties, molecular characteristics and NBO analysis of some bipyridines using experimental and theoretical tools

• Most stable conformers are identified for 2,2′-bipyridine, 4,4′-bipyridine and 2,4′-bipyridine.
• Modified valence force field is evaluated based on SQM for the first time.
• Errorrms (νobs∼νcal) is around 10.0 cm−1 in DFT and Errorav is ≈ 12.0 cm−1 in IVP.
• 24BPE exhibits NLO property, whereas 2BPE and 4BPE cannot be NLO materials.
• Evaluated HOMO and LUMO energies; thermodynamic parameters and NBO analysis is also studied.

The Fourier Transform Infrared (FTIR) and Fourier Transform Raman (FT-Raman) spectra of 2,2′-bipyridine (2BPE); 4,4′-bipyridine (4BPE); and 2,4′-bipyridine (24BPE) were measured in the range 4000–450 cm−1 and 4000–50 cm−1, respectively. Torsional potentials were evaluated at various angles of rotation around the C–C inter-ring bond for the three molecules in order to arrive at the molecular conformation of lowest energy. This conformation was further optimized to get ground state geometry. Vibrational frequencies along with infrared and Raman intensities were computed. In the above calculations, DFT employing B3LYP functional with 6311++G(d,p) basis set was used. The rms error between observed and calculated frequencies was 10.0, 10.9 and 10.2 cm−1 for 2BPE, 4BPE and 24BPE, respectively. A 54-parameter modified valence force field was derived by solving inverse vibrational problem using Wilson's GF matrix method. The force constants were refined using 117 experimental frequencies of the three molecules in overlay least-squares technique. The average error between observed and computed frequencies was 12.44 cm−1. PED and eigen vectors calculated in the process were used to make unambiguous vibrational assignments of all the fundamental vibrations. The values of dipole moment, polarizability and hyperpolarizability were computed to determine the NLO behaviour of these molecules. The HOMO and LUMO energies, thermodynamic parameters and molecular electrostatic surface potentials (MESP) were also evaluated. Stability of the molecules arising from hyper conjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis.

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