Deuteron quadrupolar splitting

Pig. 27. Deuteron quadrupolar splitting of 1,4-tetra-deuterated butadiene oligomers from natural rubber bands for different strains. Reproduced from Ref 142, with permission from Wiley. 

Figure 20. (a) Deuterium NMR spectra of 5CB-)3d2 in nucleopore microcavities in the isotropic phase at r= 33.75 0.03°C. The solid lines are theoretical fits [210] and (b) Deuteron quadrupolar splitting versus temperature for Af = 0.1 pm. The insert shows the surface induced order parameter profile in the isotropic phase at r-r = 1.48 K [210]. 

The spin spin relaxation time T2a can be measured by a quadrupolar echo pulse train 90° — x — 90° — (2t — 90°) if the pulse spacing x < C 2/Av is satisfied (i.e., the deuteron spin relaxes as if the quadrupolar splitting Av is absent or if a non-selective excitation is achieved). Due to the large quadrupolar splitting often encountered in LC, the above condition for x cannot be fulfilled experimentally and the two lines relax independently (i.e., r> 21 Av or selective excitation of half of the doublet exists). The quadrupolar echo pulse train then measures50... 

Table 6.1 IS I order parameters as a function of temperature and position in the sn-1 chain of DM PC. The values were calculated from the quadrupolar splittings taken from ref. 100, using a deuteron quadrupolar coupling constant of 167 kHz according to ref. 101...

FIGURE 7.10 Typical examples of the H-NMR spectra of CBA-w in the nematic LC state and the definition of the molecular axis (see the text) (a) n = 9 (b) n = 10. In the upper diagrams, the quadrupolar splittings (Av ) are assigned to the individual C-D bonds of the spacer. The dipolar ( >hd) quadrupolar splittings (AVo) of the mesogenic core (lower diagrams) are due to the deuteron substituted at the ortho position. (From Abe and Furuya [1988].)... 

FIGURE 6.10. Plot of the inverse field-induced quadrupolar splittings, Ai/i, versus temperature for the chain deuterons of 5CB-di5 measured in a magnetic field... 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

Table 1. The water deuteron ( H) NMR quadrupolar splittings A) obtained in samples of the liquid crystalline phase for barium di-2-ethylhexylsulphosuccinate- H20 system at 300 K...

AVj is the quadrupolar splitting (peak for the 90 orientation of the director with respect to Bq) of the i th deuteron, is the angle between the i th C-D bond and the rotation axis, and the last term describes the anisotropic motion of the rotation axis. The last term involves only the motion of the molecule as a whole, and is therefore the molecular order parameter, It... 

In case of a deuterated sample (spin 1 case), the spectra are usually dominated by the quadrupolar interaction, that is, the coupling of the nuclear quadrupole moment with the electric field gradient of the C-2H bond. For deuterons in C-2H bonds this can lead to a splitting of about 250 kHz. As in the case of dipolar interaction, a Pake spectrum is obtained for a powder sample. The z-principal axis of the quadrupolar interaction is oriented along the bond axis which makes deuteron NMR particularly useful for studies of segmental orientations and molecular dynamics (reorientation) [1],... 

In this section we have attempted to introduce the reader to the salient features of NMR spectroscopy nuclear spin / = 1, doublets whose splittings depend on the quadrupolar interaction at the position of the deuteron, and single crystal vs. powder spectra. We have not as yet addressed the pertinent question What about molecular motion How do the rates and classes of molecular motion affect NMR spectra The second subsection of Section C will discuss some features of NMR that make it a useful technique for probing molecular motions in membranes. 

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