High and low frequency torsional absorptions in nematic K21

Abstract

The far infrared absorption of 4 cyano-4′n-heptyl biphenyl (K21) has been measured in the nematic phase at 296 K in the frequency range 10–200 cm^–1. An incompletely resolved narrow band at 148 cm^–1 has been assigned to whole-molecule torsional oscillation on the evidence supplied by (a) dilution in cyclohexane, (b) application of an aligning field and (c) measurement of the f.i.r. intramolecular modes of 4-cyanobiphenyl, 4-n-heptyl biphenyl and biphenyl.

With a dipole moment aligned in the long axis, rotational absorptions may arise only from torsion about the mutually perpendicular short axes, an energetic process which is simulated here by an expansion of the orientational autocorrelation function in terms of successive memory functions. These are autocorrelation functions of the Langevin random torque, its time derivative, etc. Assuming that the latter decays exponentially (Markov process), equations for the dielectric loss, permittivity, and power absorption coefficient are deduced which describe the MHz absorption observed by Moutran in aligned nematic K21, and which upon extrapolation yield a narrow librational peak at 137 cm^–1, the result of an abnormally large mean square torque and an abnormally long residence at the bottom of deep and steep intermolecular potential wells.