Quadrupolar line-broadening

Due, in the main, to quadrupolar line-broadening effects, together with the poor sensitivity, there has been relatively little general interest in 14N magnetic resonance studies and, although the quadrupolar broadening has been shown to be minimized by the use of low viscosity solvents,465 the 14N chemical shifts of only four pyrroles have been reported.465,466 The 14N chemical shift of pyrrole has been reported as either 230465 or 227466 ppm relative to the N02 ion 14N resonance, and values of 227, 230, and 225 ppm have been recorded for 1-methylpyrrole, 2,5-dimethylpyrrole, and l-methyl-2-methoxy-carbonylpyrrole, respectively.465 The associated effect of electric... 

Figure 5.21. Stacked and contour representations of the A1 MQMAS NMR spectra of lanthanum aluminate gels of (A) low-lanthanum content, in which three resonances assigned to Al, Al and Al can be distinguished, and (B) high-lanthanum content gel, in which only the octahedral and tetrahedral A1 resonances are observed. The solid line indicates the isotropic chemical shift, the dashed line shows the direction of the anisotropic quadrupolar line broadening and the dotted-dashed line indicates the direction of the quadrupolar-induced shift. From luga et al. (1999), by permission of the American Chemical Society.

Is the spectrometer/probe capable of variable temperature operation Dynamic samples need to be measured at low temperatures to freeze out the dynamic processes. Quadrupolar nuclei are often best observed at high temperatures the increased molecular tumbling rates help to average out the quadrupolar line-broadening interaction. 

Halide ions show a weak association with cyclo-dextrins which can be visualized by the quadrupolar line broadening of the halide resonances. Competition between bromide ions and other ions for the... 

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. 

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 nucleus is quadrupolar (spin 7/2, natural abundance 99.76%), and thus, the spectra can be affected by both the first- and second-order quadru-pole interaction, though the second-order broadening is generally not the largest source of line broadening in these materials. In general, three major anisotropic interactions influence the line shapes seen in the NMR spectra of solid samples (i) the qua-... 

MHz relative to the protons in Mc4Si at exactly 100 MHz. The effect of quadrupole line broadening is attenuated to some extent by the magnitude of / (/= 9/2), and the line widths are among the narrowest of all quadrupolar nuclei studied to date. One would expect efficient relaxation via the quadrupole moment, giving very short T2 times and consequently broad NMR signals. Also if the electronic distribution around the nucleus is symmetrical, sharp resonances can be obtained. 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

Plenio and Diodone have also reported fluorocrown ethers (Table 5), which exhibit chemical shift response upon binding Ca + [314]. Of course, calcium could potentially be analyzed directly by " Ca NMR, however, its natural abundance is <0.2%, its sensitivity is <1% that of H, and being quadrupolar, it is liable to extensive line broadening [59]. Thus, the application of F NMR with appropriately designed reporter molecules gives insight into cytosolic [Ca +]. 

Broad Lines. - The width of an n.m.r. line, At>1/2, is defined as the width in Hz at half signal height. Narrow lines, i.e., Ap1/2 < 10 Hz, are desirable in order to make use of chemical shift information and to follow chemical change. N.m.r. line widths in the liquid and the physisorbed state tend to be very narrow, with Ar>1/2 of the order of 10-1Hz. This fortuitous state arises because the molecular motion is sufficiently rapid and random in a liquid to average out the line broadening features present in solids, namely dipolar interactions, chemical shift anisotropy, quadrupolar interactions, and paramagnetic interactions which render the spectrum unusable under conventional or liquid-state experimental conditions. The mechanisms of each of these features will be described. The treatment will perforce be cursory, but an indication will be given to where a full theoretical treatment can be found. 

The specific line broadening effects which MAS reduces most effectively are the chemical shift anisotropy and quadrupolar coupling. Although dipolar broadening is also reduced, it is not entirely eliminated, and other techniques... 

It is useful to emphasise from the outset that alternative techniques for investigating solid samples, other than NMR spectroscopy of the quadrupolar halogen nuclei, tend to be considered first this is likely due to the (perceived) difficulty of the technique and may be related to the issues of sensitivity and line broadening mentioned above. However, the information gained is often complementary, and hence solid state nuclear magnetic resonance (SS NMR) spectroscopy is primarily used when it can provide unique insight unavailable with other techniques. 

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