Frequency dependent biexponential 14N relaxation of methylisocyanide in nematic liquid crystals

Abstract

The dynamical behavior of methylisocyanide, CH₃NC, dissolved in two different nematic liquid crystals, has been studied for the first time by direct measurement of the spin‐lattice relaxation of its ¹⁴N nucleus, at three different field strengths. The results show a frequency dependent relaxation behavior, that can be described by director fluctuations in the nematic solvents. The parameters of the relaxation model: the reorientational correlation time τ_⊥, and the amplitude factor A and cut‐off frequency ω_c of the director fluctuation contributions, were estimated in dependence of the renormalization of the probe order parameter S_z^’z. It is shown that the influence of the mean square of fluctuations 〈θ²〉 on the relaxation model can be far from negligible. Choosing literature values for the average elastic constant K and effective viscosity η in one solution (phase V), a reasonable value for the translational diffusion constant D_tr is found. The corresponding cut‐off wavelength is L∼50 Å. For the other solution (ZLI 1167), using the same L and an estimated viscosity η, reasonable values for K and D_tr are obtained. Physical arguments are used to account for the differences between the two solutions and a comparison is made with previous studies of analogous systems. It is emphasized that the value of the director fluctuation model could be enhanced by carrying out more experiments to improve the accuracy of ω_c, and by refining the theoretical description of the cut‐off behavior.