Quadrupolar splitting

Amphiphiles often have a complex phase behaviour with several liquid crystalline phases These liquid crystalline phases are often characterised by long-range order in one directior together with the formation of a layer structure. The molecules may nevertheless be able tc move laterally within the layer and perpendicular to the surface of the layer. Structura information can be obtained using spectroscopic techniques including X-ray and neutror diffraction and NMR. The quadrupolar splitting in the deuterium NMR spectrum can be... 

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... 

The potential of mean torque, parameterized by the local interaction tensors Xa and Xc (assume to be cylindrical) for the aromatic core and the C C segment, respectively, can be mapped out at each temperature by fitting the observed quadrupolar splittings in the mesophase. Furthermore, the order matrix of an average conformer of the molecule can also be calculated.17 Now pn is needed to describe the internal dynamics. 

Then averaged quadrupolar splitting is given by82... 

In addition to the use of quadrupolar splitting, the spin relaxation rate can also be used to calculate the specific surface area ratios for pulps beaten to different degrees and the results for an unbleached pulp agree closely and confirm the 250% increase in surface area measured by isotherm data (Table 5.4). 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings... 

Fig. 23. Measured quadrupolar splittings and segmental order parameter profiles of fluid d52-DPPC bilayers (left) and the dg2-DPPC-GD (4.7 mol%) bilayer mixture (right) as a function of pressure at T =55°C.

In solution, the quadrupolar coupling constant is not measured directly. Instead, the quadrupolar splitting constant is obtained from relaxation time measurements. The QSC is related to x according to equation 10 ... 

A function closely related to QCC is the Li quadrupolar splitting constant (QSC), defined as QSC = (1 -h/7 /3) x( Li), where r is the asymmetry parameter. The Li QSC values can be estimated from the Li and C(para) spin-lattice relaxation times. The QSC values are correlated with the effects of structure, solvent and temperature on association in solution for aryllithium compounds (155, 171, 172). Conclusions can be drawn about the structure of the associated species in cases where no supporting XRD evidence is available. ... 

The conformations of flexible chain molecules incorporated in a nematic environment are investigated. Proton-proton and carbon-carbon dipolar coupling constant measurements are attempted for 1,2-dimethoxyethane and 1,2-diphenoxyethane, in addition to 2H NMR observations of quadrupolar splittings. These conformation-dependent properties are analyzed according to the RIS scheme. Studies are further extended to a mixture of 1,2-diphenoxyethane with a nematic liquid crystal, 4,4 -azoxyanisole. 

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