Nuclear spin-lattice relaxation rates in liquid crystals

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Abstract
The spin-lattice relaxation rates R (i) have been measured for the deuterons in 4-n-pentyl-d11-4′-cyanobiphenyl-d4 (5CB-d15) and 4-n-pentyl-d1-4′-cyanobiphenyl (5CB-d1) at 30·7 MHz and over the temperature range 257 to 400 K which includes the nematic-isotropic transition, T NI, at 308 K. Both samples display a small biphasic region because of the presence of a low concentration of an unknown impurity. The variation of both quadrupolar splittings and R (i) values have been carefully measured at 0·1 K intervals close to and in this interesting region. A discontinuity in the relaxation rate of the deuteron at the first position in the alkyl chain has been detected at T NI. The observed discontinuity is in good agreement with the change in R (i) predicted to occur because of the onset of long range orientational order at this temperature. There are regions of temperature in both phases where in R (i) varies linearly with T -1 and from which apparent activation energies can be determined for C-D bond reorientational motion. The orientations of the C-D bonds fluctuate because of overall as well as internal modes of motion and to separate these effects a model of superimposed motion is applied which yields activation energies for C-C bond rotation in the nematic phase at different positions along the alkyl chain.