Quadrupolar relaxation rate

The exchange of the coordinated aqua ligand of the W(IV) aqua oxo species was qualitatively studied by NMR line-broadening as a function of temperature based on Eq. (26), where the transverse relaxation time of the bound oxygen-17 nucleus is given by 1/T2b. The l/T2Qb represents the quadrupolar relaxation rate and kmi the chemical exchange rate constant... 

Carbon-13 NMR (13C-NMR) signals of aryl carbon atoms bonded directly to boron can be broad and/or low-intensity peaks. In some cases, they may not be observed, possibly depending on the quadrupolar relaxation rate of boron nuclei (79MI1) (Fig. 1). Some special techniques are known to increase the intensity of 13C signals of these carbon atoms (77JOM(132)29 78JOMC34). 

Figure 2.41. The H spectrum of ammonium nitrate in DMSO (a) before and (b) after acidification. In (b) the NHJ ion dominates and the induced symmetry reduces the quadrupolar relaxation rate and reveals the one-bond coupling constant (51 Hz).

Fig. 15. Experimental P H NMR sjjectra of the cluster HFeCo3(CO)nPPh2H in CD2CI2. The simulated spectra are characterized by the quadrupolar relaxation rates, the chemical shift anisotropy relaxation rates, the interference term between the quadrupolar interaction and the chemical shift anisotropy, and the indirect spin-spin coupling constants J( P- Co). (Reproduced with permission from ref. 123.)...

Br, Br or I nuclei are liganded to metallic binding sites in macromolecules, the quadrupolar relaxation rates may become extremely... 

Both of the magnetically active isotopes of boron are low-Q nuclei, and while the rate of quadrupolar relaxation is slow, it is still sufficient to adumbrate or totally vitiate spin-spin couplings to neighbouring nuclei. The quadrupolar relaxation rates observed are in the region of 50 s , sufficiently slow that other mechanisms can effectively compete for some of the action. Total relaxation rates are still sufficiently slow that excessive linewidth is not a major problem in boron spectroscopy. 

Since the quadrupolar relaxation rate increases also with t, the lines are usually broadened significantly by increase in viscosity, and higher temperatures are then advantageous. 

Two theories have been developed to attempt to explain the different quadrupolar relaxation rates of these cations. The electrostatic model due to Valiev and Hertz is currently favored and considers the relaxation to be caused by the electric field gradients due to the dipole moments of the solvent molecules modulated by their rotational and translational motion. The fully random distribution (FRD) version of the model leads in the extreme narrowing limit to... 

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