Deuteron spin-spin relaxation study of second-order director fluctuations in the nematogen 5CB

We report on a study of deuteron spin-spin relaxation times (T-2) at 46 MHz in the perdeuterated liquid crystal 4-n-pentyl-cyanobiphenyl (5CB). By combining with previously measured deuteron spin-lattice relaxation rates, we have derived the spectral densities of motion J(0)(O),J(1)(omega(0)), and J(2)(2 omega(0)) for various carbon sites of 5CB. In our global target analysis of all the spectral density data, a model that relies on molecular reorientations, internal rotations in the flexible chain, and director fluctuations as relaxation mechanisms is used. We found that director fluctuations make an insignificant contribution to J(1)(omega(0)) because 5CB has a high cutoff frequency smaller than our Larmor frequencies, but make a nonzero contribution to J(0)(O). According to the theory, director fluctuations can only affect J(0)(O) via a second-order contribution. Our T-2 data seem to be consistent with the prediction of the director fluctuation theory. Model parameters that include rotational diffusion constants, internal jump constants, and the prefactor A of director fluctuations have been determined as a function of temperature.