Liquid crystals quadrupole splittings

Study of correlation times in polymers. Study of degree of ordering in nematic phase of liquid crystal quadrupole splittings detected by ENDOR. 2 2 ) X = halogen. ... 

The order and mobility of a labeled flexible alkyl spacer in the linear thermotropic polymeric nematic liquid crystal poly(2,2 -dimethyl-4,4 -dioxyazoxybenzenedodecanedioyl-dj0) (poly[oxy(3-methyl-1,4-phenylene)azoxy 2-methyl-1,4-phenylene)oxy(1,12-dioxo-1,12-dodecanediyl-d2oll) is explored with deuterium NMR. The quadrupol splittings of the spacer methylene segments in the nematic melt of the polymer are reported as a function of the temperature and are contrasted with observations on model compounds solubilized in a nematic solvent. 

Similar to the situation for quadrupole-induced relaxation, the quadrupole splitting in liquid crystals is zero if the molecular symmetry is tetrahedral or higher. Electric field gradients are zero for such symmetries so there can be no quadrupolar interaction. However, one expects to see small splittings from tetrahedral or octahedral derivatives because of structural distortions. These predominately arise from specific interactions with extraneous materials such as lipophilic headgroups in surfactant systems, as seen, for instance, in both cationic and anionic octahedral cobalt(III) species [23], Much larger splittings will be expected from other structure... 

Wide-line spectra are observed for solids, or for systems In solution with very long (with respect to the Inverse of the quadrupole splitting) correlation times for re-orlentatlon. Examples of the latter are collagen fibres, membranes, and liquid crystals. As many examples of this type of study were given In our earlier review (2), we shall confine ourselves to collagen and membranes. 

Liquid crystals confined into cylindrical cavities are the most suitable systems for deirteron NMR studies of the interfacial liquid crystal-substrate interactions. There are, however, other systems of confined liquid crystals, which are more important for applications (PDLC and H-PDLC materials), or have been the object of intensive theoretical studies. The latter are composite systems with a random network of pores like liquid crystals in nano-pore glasses, aerogels and aerosils. Unfortunately, in all these cases NMR spectroscopy alone cannot yield accurate information on the surface-or constraint-induced order in the high-temperature phase as no quadrupole splitting is observed. The nature of the spectral line-broadening (static or dynamic) has to be established using NMR relaxometry. Current NMR results related to these systems will be briefly discussed in Sect. 2.4. 

The coupling constant J(P-D) = 29.0 0.1 Hz was determined from the ip NMR spectrum of PD3 in the isotropic phase CCI4 and the dipolar coupling constant in the liquid-crystal solvent EBBA (the doublet splitting in the NMR spectrum led to the deuterium nuclear quadrupole coupling constant see p. 157) [3]. 

Liquid crystals provide a useful approach to the investigation of quadrupolar nuclei since quadrupole interactions give rise to extra splittings in ENDOR spectroscopy. 

It should be noted that for anisotropic liquid crystals the NMR signal is split into 21 component signals due to first-order static quadrupole interactions but that, in the absence of second-order quadrupole effects, the width of the central line gives the transverse relaxation rate both for powder samples and for macroscopically aligned samples (see further Chapter 7). 

Halogen Quadrupole Splittings of Covalent Compounds in Liquid Crystals... 

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