Couplings, short relaxation

The signal due to the 1-proton is broad owing to its coupling with the 14N nucleus (spin= 1) which has a short relaxation time presumably by... 

Correct setting of the magic angle is essential in solid-state NMR spectroscopy. The most commonly used method relies on the Br resonance in solid KBr. Br is a quadrupolar nucleus (spin 3/2) ensuring a short relaxation time. The Br first-order quadmpole coupling on the satellite transition ( 3 /2 1 /2) produces a set of distinct spinning... 

Since the short relaxation times associated with a quadrupolar nucleus drastically reduce the time delay to be applied in an NMR experiment between two pulses, measuring times are short or, in other words, distinct NMR signals can often be detected with a limited time spent down to micromolar concentrations. Along with this apparent advantage, quadrupolar nuclei provide information in addition to the classical parameters chemical shift (or shielding) and nuclear spin-spin coupling constants. Variations in linewidths for quadrupolar nuclei are another sensitive quantity allowing for the evaluation of the electronic and the steric situation in the first coordination sphere of a vanadium compound, its periphery, its (local) symmetry and its interaction with the matrix, i.e. counter-ions, solvent molecules and other constituents present in solution. 

Fig. 5. 62.8MHz ovpbjiH NMR spectra of a cyclic bis(amino)plumbylene (=15% in [Dg]toluene), showing the competition between scalar relaxation of the second kind [short relaxation times caused by partially relaxed scalar coupling C N ... 

Relatively few relaxation measurements have been made with this nucleus, though they should not be a very time-consuming operation, owing to the generally short relaxation times involved. Indeed, linewidth measurements should prove satisfactory in some cases. and Al relaxation has recently been studied in detail for AlEt3 and correlation times decrease in the order A1>CH2>CH3. Mixed scalar and quadrupole relaxation can be detected in symmetrical species and the scalar part can be used to estimate coupling constants between A1 and other quadrupolar nuclei. 

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