Infrared and Far-Infrared Studies on the Nematogenic Homologous Series Trans-4-Alkyl-(4-Cyanophenyl) Cyclohexanes

Infrared (4000–650 cm−1) and far-infrared (650–20 cm−1) spectra of the nematogenic homologous series trans-4-alkyl-(4-cyanophenyl) cyclohexanes (ethyl, propyl, butyl, pentyl and heptyl) have been examined in the solid and liquid crystalline states. An attempt has been made to correlate the variation in the intensities and frequency shifts with the molecular structures. The gradual shift towards smaller wavelength in the —C≡N stretching vibration and the relative alternation in the intensity of this frequency as the chain length increases may be correlated with odd-even effects in homologous liquid crystals and with the increase in thermal stability, as the spectra reveal an increase in the dipole-dipole interaction between the neighbouring molecules. A strong feature centred around 90 cm−1 is assigned to hindered rotation about the long axis of the molecule. Absolute orientational order parameter S has been evaluated for the series by a study of the temperature variation of —C≡N vibration in homeotropically oriented sample. The results are in very good conformity with those of NMR and optical methods