Relaxation, quadrupolar

The quadrupolar relaxation mechanism is only directly relevant for those nuclei that have a nuclear spin quantum number, I, greater than Vi (quadrupolar nuclei) and is often the dominant relaxation process for these. This can also be a very efficient mechanism, and the linewidths of many such nuclei can be hundreds or even thousands of hertz wide. The properties of selected nuclei with I Vi are summarised in Table 2.3. Whilst the direct observation of these nuclei may not be routine for many organic chemists, their observation can, at times, prove very enlightening for specific problems, and the indirect effects they have on the spectra of spin-14 nuclei should not be overlooked. 

The relaxation rates of a quadrupolar nucleus are dictated by two new factors not previously considered. The first is the magnitude of the quadrupole moment itself (Table 2.3). Larger values contribute to more efficient spin relaxation and hence broader linewidths, whereas smaller values typically produce sharper lines. Thus, those nuclei with smaller quadrupole moments are usually more favoured for NMR observation. As before, for the mechanism to be effective, molecular tumbling must occur at an appropriate 

Isotope Spin (1) Natural abundance(%) Quadrupole moment (10 m ) NMR frequency (MHz) Relative sensitivity 

The broad resonances of many quadrupolar nuclei means field inhomogeneity makes a negligible contribution to linewidths, so the methods described previously for measuring relaxation times are no longer necessary. For small molecules at least, T2 and Ti are identical and can be determined directly from the half-height linewidth. The broad resonances together with the sometimes low intrinsic sensitivity and low natural abundance of quadrupolar nuclei are the principal reasons for their relatively low popularity for NMR studies relative to spin-Vi nuclei. The very fast relaxation of certain quadrupolar nuclei can also make their direct observation difficult with conventional high-resolution spectrometers see Section 4.5. 

Probably of more relevance to the practising organic chemist is the influence quadrupolar nuclei have on the spectra of spin- /2 nuclei, by virtue of their mutual scalar coupling. Coupling to a quadrupolar nucleus of spin I produces, in theory, 27-1-1 lines, so, for example, the carbon resonance of CDCI3 appears as a 1 1 1 triplet ( H has 7 = 1) by virtue of the 32 Hz coupling. However, more generally, if the relaxation of the 

In the simplest case when the frequency of modulation of the quadrupolar interaction is very rapid compared to the Larmor frequency of the halogen nucleus, u),., the relaxation of the quadrupolar nucleus 

When extreme narrowing conditions are no longer valid, i,e, when 

---

Huntress WT 1970 The study of anisotropic rotation of molecules in liquids by NMR quadrupolar relaxation Adv. Magn. Reson. 4 1-37... 

Roberts J E and Schnitker J 1993 Ionic quadrupolar relaxation in aqueous solution—dynamics of the hydration sphered. Rhys. Chem. 97 5410-17... 

The signal from H(3) is broadened because N-H(3) spin coupling is not completely washed out by quadrupolar relaxation of the nitrogen atom (68MI41701>. This broadening niay be useful in assigning signals in the spectrum and it is usually reduced in solvents of... 

Other good news comes in the shape of the 13C nucleus having a spin quantum number of /2. This means that 13C signals are generally sharp as there are no line-broadening quadrupolar relaxation issues to worry about and we don t have to deal with any strange multiplicities. 

Quadrupolar relaxation Rapid relaxation experienced by quadrupolar nuclei. 

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... 

The possibility of obtaining spin-spin coupling information from measurements involving the 14N nucleus is generally impossible, because rapid quadrupolar relaxation of 14N... 

In the standard mathematical expressions for the contribution of quadrupolar relaxation to the relaxation rates of the quadrupolar nucleus (in nuclear magnetic resonance), rapid isotropic motion is assumed to occur. This behavior, in most cases, will not be true in the solid or liquid crystalline state ". ... 

Table 14 summarizes the magnitudes and absolute signs of a variety of couplings involving nitrogen in oximes. The entries include examples of spin coupling between other nuclei and and N. Spin coupling between and H nuclei is not usually observable because of the rapid quadrupolar relaxation of the nucleus. 

Both [14N1- and [lsN]-pyrrole have been subjected to extensive analyses in the pursuit of accurate f3C chemical shifts and heteronuclear coupling constants. The resonance due to the a-carbons in [14N]pyrrole is broadened as a consequence of non-zero coupling to, and quadrupolar relaxation by, the 14N nucleus. 

The H NMR spectra of the 1,3,5-triazines are quite simple. The ring protons give sharp lines which are not broadened by nitrogen quadrupolar relaxations (65BSB119). The chemical shifts of the ring protons are 1 to 2 p.p.m. downfield from benzene protons, presumably... 

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

Porion, P., Faugere, M.P., Lecolier, E., Gherardi, B. and Delville, A. (1998) 23Na Nuclear Quadrupolar Relaxation as a Probe of the Microstructure and Dynamics of Aqueous Clay Dispersions An Application to Laponite Gels. The Journal of Physical Chemistry B 102, 3477-3485... 

Delville, A., Porion, P. and Faugere, A.M. (2000) Ion Diffusion within Charged Porous Network as Probed by Nuclear Quadrupolar Relaxation. Journal Physical Chemistry B... 

---