MODEL FOR SPIN RELAXATION BY CORRELATED INTERNAL MOTIONS IN LIQUID-CRYSTALS

The effects of correlated internal rotations in an alkyl chain on the spectral densities of motion in liquid crystals are examined using the master-equation method. It is assumed that the overall reorientation of molecules is decoupled from the internal motions in their alkyl end chains. The pentyl chain that is attached to a cylindrical biphenyl core in 4-n-pentyl-4'-cyanobiphenyl (5CB) is studied as a specific example. The reorientation of a symmetric top in an anisotropic potential is described by the small-step rotational diffusion model. The equilibrium probability of each conformer, which is required to construct the transition-rate matrix for the master equation, is obtained using the Emsley-Luckhurst theory. The effects of the nematic mean field on the chain dynamics are discussed. Both J1(omega) and J2(2-omega) in 5CB are found to be frequency dependent, with J2(2-omega) being weaker.