N Quadrupolar Relaxation

As shown by Table 4, N linewidths vary widely, depending on two factors, the nitrogen bond type and the molecular or ionic mobility. High-resolution work is possible if the local electronic symmetry, and the mobility, are both high and N is a useful probe nucleus for studies of molecular motions. 

The linewidth (fFi/2) is related to the quadrupolar relaxation time and to the structural and motional factors by 

Thus linewidths for the different kinds of nitrogen in the N2O molecule are found to be in the ratio of the respective x values. NQCCs are obtained from microwave spectroscopy of gases and NQR spectroscopy of solids, also by NMR methods on solids and liquid crystals. Values in the liquid phase are usually close to those in the solid, and 10%-15% less than in the gas phase. Determining the NQCC from estimates of t, requires knowledge of the molecular motions. Anisotropic or additional internal motions may be monitored by relaxation measurements on two or more nuclei in the molecule, as of N, 0, and in dimethyl formamide.  

Motional factors determining the linewidths can be envisaged in terms of a frictional model (Debye-Stokes-Einstein theory) according to which 

Increases in linewidth and in t, with molecular size, as from NHj or MeCN to ammine or cyano complexes, or from Me4N to long-chain surfactants, are evident in Table 4. A direct dependence of linewidth on viscosity is observed for a wide range of molecules. N NMR of fluoronitrogen compounds is favored by their high mobility. Linewidths may be sharply reduced by warming the sample, since viscosity decreases exponentially with increase in temperature. 

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Brown, M.F. 1982. Theory of spin-lattice relaxation in lipid bUayers and biological membranes. and N quadrupolar relaxation. J. Chem. Phys. 77 1576-1599. 

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