Abstract
Nuclear Magnetic Resonance (NMR) has been shown to be an extremely powerful technique for investigating molecular orientational order and dynamics in partially ordered systems such as thermotropic and lyotropic liquid crystals (LC) [1]. In this Chapter, nuclear spin relaxation of orientationally ordered molecules is described. In addition, theoretical models explaining NMR observables are outlined for various dynamical processes in LCs. It is known that nuclear spin-lattice relaxation rates contain information on how a nuclear spin system exchanges energy with its surrounding “lattice”, i.e., all degrees of freedom in the physical system of interest except those of the nuclear spins. Pulsed NMR provides a highly versatile tool for measuring various spin relaxation rates which can probe the entire spectrum of molecular motions in LCs. As in ordinary liquids, mesogenic molecules can reorient and translate, as well as execute internal motions if they are non-rigid. Furthermore, these molecules align preferentially in a certain direction labeled by the director n̂0 and possibly also arrange spatially to form various layered structures. When these molecules move collectively, the local director fluctuates both spatially and temporally. These unique motions are known as order director fluctuations (ODF). All the dynamical processes mentioned can contribute to the spin relaxation in LCs. In addition, cross relaxation due to possible couplings between different motions may also exist.
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Dong, R.Y. (2003). Spin Relaxation in Orientationally Ordered Molecules. In: Burnell, E.E., de Lange, C.A. (eds) NMR of Ordered Liquids. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-0221-8_16
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DOI: https://doi.org/10.1007/978-94-017-0221-8_16
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