Quadrupolar broadening nuclei

A further factor to be taken into account when assessing the relative ease with which a quadrupolar nucleus may be observed is the width of the line which is determined for a given electric field gradient by the quadrupole moment (also listed in Table 1.2). Often only the central transition is observed, for which the linewidth is determined by its second-order quadrupolar broadening and is proportional to [Q (I(I +... 

Figure 2.9. The quadrupole perturbed powder patterns for an I = nucleus. A. The outer satellite transitions to first-order. B. The second-order quadrupolar broadening of the central transition with...

The resonance frequencies of the isotopes Mo and Mo are very similar, differing by only 2%, but the sensitivity of Mo and its significantly smaller second-order quadrupolar broadening make it the preferred Mo NMR nucleus even though Mo has the advantage of relaxing much faster in cases where quadrupole relaxation is dominant. 

It should be noted that the CHj group and the nitrogen-bound carbon atom of the carbazole moiety ue subject to the same effect, arising from the dipolar coupling to the quadrupolar N nucleus. In this case, however, since the quadrupole coupling constant is much smaller than for the bromine isotopes (2.16 MHz for N-methyl pyrrole (29)) the interaction is much weaker and manifests itself only as a moderate broadening of the peaks with some ill-resolved structure. For these asymmetric resonances, the chemical shifts were measured at the center of gravity. 

Nitration of poly-DCH. In contrast to the bromination reaction, the nitration reaction cannot be as easily controlled, and the resultant products are all highly nitrated. The results of some preliminary studies are shown in Fig. 5, where the spectra of a sample of composition poly-DCH(N02)7.6 and the model compound N-ethyl-3,6-dinitrocarbazole are compared. The sites of nitration, unlike those of bromination, can be directly observed in the CP-MAS NMR spectra, although they are somewhat broadened by the dipolar interaction with the quadrupolar nitrogen nucleus as discussed above. No triple bond resonance is detected in the nitrated polymer, while some poorly resolved absorption in the region near 130 ppm in the spectra obtained with delayed decoupling indi-... 

The REDOR experiment has been proposed for spin-1/2 pairs. For quadrupolar nuclei-spin-1/2 pairs, the use of this experiment is limited. The main reason behind this is the fact that a significant quadrupolar broadening (in order of several MHz) makes the Six pulses inefficient for the inversion of S populations. For such systems REDOR experiments can be applied if the n pulses are applied to the spin-1/2 nucleus and only central transition is observed for a halfinteger spin. The transfer of population in double resonance (TRAPDOR)" or rotational-echo adiabatic passage double resonance (REAPDOR)" experiment offer a much better choice for the measurement of quadrupolar nuclei-spin-1/2 pair dipolar couplings. 

F . 9.4 Simulated static powder spectrum first-order quadrupolar coupling of a spin / = 1 (with added noise) for the anisotropic broaden- nucleus, e. g., H. (Reproduced by permission of ing due either to a dipolar coupling between the SocieU Italiana di Fisica from (5).) an isolated pair of spin / =1/2 nuclei or to the... 

In 1995, Frydman and Harwood presented a 2D MQMAS experiment, which, by means of the formation of an echo corresponding to the refocusing of the fourth-rank second-order quadrupolar broadening, yields 2D spectra in which anisotropi-cally broadened ridges are resolved on the basis of their different isotropic shifts [168]. The resolution of the three distinct Rb sites in RbN03 in an isotropic MQMAS spectrum is demonstrated in Fig. 9.34c. It is to be noted that the experiment is only applicable for odd MQ orders (e. g. 3Q or 5Q), for which there is no first-order quadrupolar broadening. Moreover, as compared to the spin 1=112 MQ methods described earlier in this article, MQC can be excited for a single isolated nucleus. 

NMR resonance of solid platinum metal. The chemical shift of 42.S MHz lies between those of platinum metal (41.5 MHz) and HaPtQt (43.0 MHz), and is interpreted as being a consequence of the submetallic nature of the 1.6 nm platinum particles. Thus the size issue for the transition from metallic properties to molecular properties is introduced. The symmetry of the band is surpriang since more than 50 % of the platinum atoms are surface atoms at this particle size and might be expected to be resolved from the resonances for the internal platinum atoms. [234, 236] The surface atoms can, in fact, be resolved after the addition of CO to the colloid, which results in the appearance of a low intensity resonance at 42.46 MHz (Fig. 6>29b), that is, up field (lower frequency) of the main band. This signal is assigned to the surface platinum atoms as the adsorption sites for CO. Displacement of CO by iodide broadens this resonance through the interaction with the quadrupolar iodide nucleus. 

The only naturally occurring isotope of sulfur with a nuclear spin is with properties that are far from ideal for NMR work, but nonetheless some hundreds of compounds have now been examined successfully. These are usually ones with a fairly high degree of symmetry so as to reduce the electric field gradient at the sulfur nucleus and hence the quadrupolar broadening experienced. 

The receptivity of a nucleus affords an estimate of the strength of signal to be expected. The receptivity R at constant field and for equal numbers of nuclei is the product of the natural abundance (N.A./%) and the NMR sensitivity, taken as ly l 7(/4-1) for that nucleus. The table gives values of R, the receptivity relative to that of C. The low receptivity of low-frequency nuclei is well known the increase in receptivity for nuclei with large I values (e.g., by a factor of 33, relative to /= 1/2 nuclei, for 7=9/2 nuclei such as Ge or Nb) is advantageous, if quadrupolar broadening is not excessive. 

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