Theory for spin-lattice relaxation in nematic liquid crystals

A theory is developed for spin-lattice relaxation in the nematic phase which includes both local and collective motion. It is found that the frequency dependence of the relaxation rate T₁^-1 depends on the correlation time for the motion at the local molecular level, τc. When ωτc1, where ω is the Larmor frequency, the theory gives the ω1/2 law characteristic of p-azoxyanisole (PAA). When ωτc1, the theory gives the more complex frequency dependence observed in the more viscous compound 4-n-methoxybenzylidene-4^'n-butylanaline (MBBA). A correlation is drawn between τc and the retarded relaxation time observed in electric dipole studies which corresponds to reorientation of the long molecular axis. The dependence of T₁^-1 on the orientation of the director in the magnetic field is included in the calculation. A model is presented to include intermolecular effects on T₁.