Liquid crystals anisotropic translational diffusion

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

Since the first publications on this subject in 1963, NMR in liquid crystalline systems has been a wide and active field of research in many branches of organic and physical chemistry. In fact, NMR spectroscopy has revealed a powerful means of probing molecular structure, anisotropic magnetic parameters and dynamic behaviour of solute molecules dissolved in liquid crystals. Moreover, this technique has been successfully employed to investigate properties of mesophases themselves, such as their orientational ordering, translational and rotational diffusion and their effects on nuclear relaxation, and molecular organization in different liquid crystalline phases. 

A residence time Tq and a non-negligible jump time Ti are also introduced to describe the proton translational motion in solids but in contrast with liquids, the jump direction and length are determined by quasi-equilibrium sites which form a periodic interstitial lattice. In this case, as exemplified by studies of hydrogen diffusion in metals it becomes very interesting to look at the anisotropy of the motion by studying selected crystal orientations relative to Q, particularly when the conductivity itself is anisotropic. 

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