Observation of various conformers of 4-cyano-4′-alkylbiphenyl by IR spectroscopy and density functional theory calculations

Infrared (IR) absorption spectra of 4-cyano-4′-alkylbiphenyls (nCBs; 1≤n≤5) with various alkyl chain conformations were calculated with density functional theory (DFT) of B3LYP/6-31G(d) level. Normal coordinate analysis base on the results of DFT calculations were also carried out. Calculated spectral patterns of phenyl ring C-H out-of-plane (CHOOP) deformation modes in the 880-750 cm−1 region, which are coupled with C-H rocking and C-C stretching vibrations of alkyl chain, show marked differences with conformational changes in alkyl chains. The observed IR spectral patterns of CHOOP bands in nCBs (3≤n≤5) are well explained by the overlap of the calculated spectra of possible conformers. The peak wavenumber shifts and intensity changes of the observed spectra in this region with phase transition from isotropic (40 °C) to nematic (25 °C) phase for 5CB are also well explained by the population changes within possible 5CB conformers.