Intermolecular dipole coupling, point

This chapter concludes by pointing out that relaxation of multispin proton systems played a major role in the early days of NMR relaxation measurements on liquid crystals [5.34]. In particular, the detection of director fluctuations [5.35] by means of its characteristic frequency dependence in proton Ti [5.36-5.39] started intensive NMR research on liquid crystals. Since there are many inequivalent proton species in a liquid crystalline molecule, it is impossible to distinguish various atomic sites from a broad proton lineshape, which is a result of strong dipolar couplings. Moreover, translation self-diffusion also modulates the intermolecular dipole-dipole interactions and contributes to proton relaxation in liquid crystals [5.40, 5.41]. Partially deuterated liquid crystals may be used to reduce the number of inequivalent proton species. Proton spin relaxation studies remain as a possible method of probing intermolecular interactions or translational motions in liquid crystals. 

For the intermolecular coupling one usually assumes a transition dipole interaction [7, 71, 97, 110, 111, 134]. While this would not be expected to be accurate at very short distances, and indeed this is not a particularly good approximation [6], there is no other convenient way to proceed. We parameterized the position of the point dipole along the OH bond by comparing to ab initio calculations [6]. 

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