Quadrupolar nuclei sensitivity

The centric scan, one-dimensional, DHK SPRITE measurement was used to study the ingress of lithium. This measurement technique was selected due to the low absolute sensitivity of 7Li (27% of [36]), the small amounts that are present and the short signal lifetimes (bulk Tx of 10 ms and T2 of 120 ps). In addition to the robust, quantitative nature of this technique, lithium is a quadrupolar nucleus and interpretation of the image intensity is more complex than spin % nuclei. Once again Eq. (3.4.2) is quantitatively correct for even quadrupolar nuclei due to the fact the longitudinal steady state does not influence the image intensity. 

Although you would expect deuterium (D) NMR to be every bit as useful as NMR, it is not carried out routinely as deuterium has a natural abundance of 0.015% and very low sensitivity. Furthermore, as the spin is greater than %, it is a quadrupolar nucleus and the signals are usually broad. Deuterium shows signals in the same chemical shift range as H (5 0-12), and NMR is most often used in experiments using D-labelled compounds to investigate reaction mechanisms. 

Vanadium-51 is a spin 7/2 nucleus, and consequently it has a quadrupole moment and is frequently referred to as a quadrupolar nucleus. The nuclear quadrupole moment is moderate in size, having a value of -0.052 x 10 2S m2. Vanadium-51 is about 40% as sensitive as protons toward NMR observation, and therefore spectra are generally easily obtained. The NMR spectroscopy of vanadium is influenced strongly by the quadrupolar properties, which derive from charge separation within the nucleus. The quadrupole moment interacts with its environment by means of electric field gradients within the electron cloud surrounding the nucleus. The electric field gradients arise from a nonspherical distribution of electron density about the... 

N-NMR spectroscopy can be useful for ammine and amine complexes, but 14N is a quadrupolar nucleus, and quadrupolar relaxation is dominant when the environment of 14N has a low symmetry. This can lead to very broad lines and a consequent reduction in sensitivity. On the other hand, short relaxation times also have the advantage of allowing rapid pulsing so that a large number of transients can be acquired. Thus it is possible to follow reactions of cisplatin in blood plasma and cell-culture media at milli-molar drug concentrations and to detect ammine release [6],... 

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. 

The rare gas xenon contains two NMR-sensitive isotopes in high natural abundance Xe has a spin of 1/2 and Xe is a quadrupolar nucleus with a spin of 3/2. The complementary NMR characteristics of these nuclei provide a unique opportunity for probing their environment. The method is widely applicable because xenon interacts with a useful range of condensed phases including pure liquids, protein solutions, and suspensions of lipid and biological membranes. It was found that the range of chemical shifts of Xe dissolved in common solvents is = 200 ppm, which is 30 times larger than that found for in methane dissolved... 

Some appreciation for the nitrogen nuclear characteristics may be obtained from Table 1. Because the relative sensitivities are comparable, the approximately 300-fold higher natural abundance of would seem to make it the nucleus of choice. It is even more sensitive than at natural abundance. However, like all nuclei with spin quantum number I > 1/2, possesses an electric quadrupole moment that arises from a nonspherical electric charge distribution in the nucleus itself. When placed in an electric field gradient, such as that characteristic of most molecular electron distributions, a quadrupolar nucleus experiences random fluctuating electric fields. The characteristic frequencies of these motions have components at the resonance frequency and hence afford an efficient relaxation mechanism. As a result, spin-lattice relaxation times (Tj ) are very short, 0.1-10 ms. Because Tj = To for in most molecules Lie in solution, linewidths are corres-... 

As a low-abundance, quadrupolar nucleus, 0 has often been overlooked as a viable NMR nucleus for structure determination. The 0 nucleus has, however, proved to be a surprisingly convenient nucleus for study by NMR techniques due to the availability of water and dioxygen enriched in 0 and the relatively small magnitude of the 0 quadrupole moment. This chapter focuses on two practical aspects of 0 NMR spectroscopy. First, methods for optimizing sensitivity and resolution are outlined. Correlations between chemical shift and structural environment are then presented in the context of examples selected from the organometallic, organic, inorganic, and biochemical literature. 

One can envision a "worst case" substance of unknown structure which is amorphous, insoluble, and has as one of its principle components a quadrupolar nucleus. The alucones produced by the reaction of diethyMuminume oxide and ethylene glycol fit into diis category. Since the polymers are potential precursors for phase-specific aluminas, die primary structural concern is the relationship between the observed coordination environment present around the aluminum sites in the polymer and the corresponding ceramic (5 - 8). XRPD has shown diat the structures are amorphous there is no crystalinity within the microstructure which would diffract X-rays and (Uvulge hints related to the structure (Fig. 13). 27 1 is a relatively sensitive NMR nucleus, but the polymers are insoluble, and the aluminum nucleus is quadrupolar until recently NMR was all but useless for structural determination of these polymers (Fig. 14). 

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