Investigation of torsional potentials, hindered rotation, molecular structure and vibrational properties of some biphenyl carboxaldehydes using spectroscopic techniques and density functional formalism

Fourier Transform infrared spectra and Fourier Transform Raman spectra, of biphenyl-4-carboxaldehyde (B4A), biphenyl-3-carboxaldehyde (B3A), and biphenyl-2-carboxaldehyde (B2A), were recorded in the spectral range 4000-450 cm−1 and 4000-50 cm−1, respectively. 1H NMR and 13C NMR spectra of the three samples were also measured. Quantum chemical calculations were performed using density functional theory (DFT) and B3LYP functional in conjunction with 6–311++G(d,p) basis set in order to determine torsional potentials, barrier to hindered rotation around C-C inter-ring bond and C-Cα aromatic-carbonyl bond, optimized structure parameters, general valence force field, harmonic vibrational frequencies, potential energy distribution(PED) and infrared and Raman intensities. Its time-dependent variant (TD-DFT) was used to simulate 1H NMR and 13C NMR spectra of the three molecules under investigation. Observed and calculated frequencies agreed with an rms error 11.4, 10.4, and 11.4 cm−1 for B4A, B3A, and B2A, respectively. Further, measured infrared and Raman spectra agreed with their computed counterparts with fair degree of accuracy. Unambiguous vibrational assignments were made for all fundamentals using PED and eigenvectors. Experimental chemical shifts agree well with their theoretical counterparts. Geometry optimization was made for dimers of the three molecules at the same level of theory as employed for the monomers. Existence of intra-molecular and inter-molecular hydrogen bonds was predicted.