NMR in Uniaxial Phases

FIGURE 3.8. Experimental angular dependence of the H-NMR spectrum in the Sg phase of 50.7-d4 (aniline ring is deuterated) at 31°C (after Ref. [3.36]). 

Finally, phase biaxiality has been studied by dissolving deuterated solute molecules in discotic liquid crystals [3.32]. Since discotic liquid crystals have negative Ax, an aligned sample consisting of one type of deuteron will exhibit a planar powder pattern (Fig. 3.7) without the necessity of sample rotation as in the conventional rod-like liquid crystals. Two discotic phases, Drd and have been studied by spectral pattern simulation using nonzero [3.32]. 

Equation (3.5) may be generalized for any second-rank property Aij to give the partial average of the component along the director (n 2 axis) in uniaxial mesophases 

FIGURE 3.9. Proton spectrum of a sample of paradinitrobenzene dissolved in the nematic phase of Phase 5. Molecular structure of paradinitrobenzene (after Ref. [3.21]). 

This is particularly easy for rigid solutes dissolved in uniaxial mesophases. The principal axes of Sij for a rigid solute may be located with certainty based on molecular symmetry. NMR can provide precise measurements of Szz tnd xx yy for solute molecules. However interpretation of the solute order requires modeling solute-solvent interactions [3.22]. As discussed in Section 2.3, both dipolar and quadrupolar couplings are suitable for probing orientational order of solutes and solvent molecules. Early treatment of NMR spectra [3.17, 3.41] have used a single Szz approximation by ignoring the small contribution due to Sxx - Syy. 

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