Half-integer quadrupole nuclei

Since the fourth-rank anisotropic broadening can be removed by sample rotation at an angle of 30.6° or 70.1° with respect to high-resolution spectra corresponding to the removal of the residual second-rank quadrupolar broadening can be achieved by the methods of double rotation (DOR) [163] and dynamic-angle spin- 

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. 

As a specific example. Fig. 9.36 shows 5Q MAS spectra corresponding to the tetrahedral aluminium sites in the microporous aluminium methylphosphonates (a) AlMePO-a and (b) AlMePO-p [175]. In Fig. 9.36b, three distinct sites can be distinguished in the isotropic (F,) dimension. In a MAS experiment (this corresponds to the projection onto the F2 dimension), only a single peak is observed in the tetrahedral region. AlMePO-p can be converted by a thermal transformation into AlMePO-a. As discussed in [175], insight into this process is provided by the subtle but significant differences between the 5Q MAS spectra for a physical mixture of the two forms (Fig. 9.36c) and for a sample in which the thermal transformation was interrupted (Fig. 9.36d). 

Solid-state NMR spectroscopy should certainly not be used in isolation. For example, the assignment of solid-state spectra is aided by the existence of solution-state NMR spectra, while if dynamic processes are to be investigated, it is very useful if differential scanning calorometry (DSC) curves can be first obtained, so that the temperatures at which phase transitions occur are known in advance. In addition, the advances in computing power as well as the development of methodology means that the use of quantum chemical calculations of NMR parameters in the interpretation of experimental results will become ever more popular. 

SPB is supported by a Marie Curie Fellowship of the European Community programme Improving Human Research Potential and the Socio-economic Knowledge Base under contract number HPMFCT-2000-00525 . The information published does not represent the opinion of the Community, and the Community is not responsible for any use that might be made of data appearing therein. 

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Figure 1 Relationship between signal intensity and line width induced by second-order quad-rupolar interaction for typical half-integer quadrupole nuclei. Reproduced from Ref. 5.

CP between spin-1/2 and quadrupole spin systems has been investigated in recent years. Most studies focus on CP between the spin-1/2 (mostly protons) and the central transition of half-integer quadrupole nuclei including... 

Molecular motion in solids using NMR spectra of half integer quadrupole nuclei has been studied and applied to NMR spectra. ... 

Samoson, A., and E. Lippmaa. 1983a. Central transition NMR excitation spectra of half-integer quadrupole nuclei. Chem. Phys. Lett. 100, 205-208. 

As discussed above, CP involving half-integer quadrupole nucleus is not trivial and 43Ca 1H CP experiments are in general of rather low... 

A further characteristic line broadening interaction occurs for nuclei with nuclear spinsf>l/2 in addition to the interactions discussed above [seeEq.(l)]. These nuclei exhibit an electric quadrupole moment that interacts with the electric field gradient at the site of the nucleus. The NMR spectriun of quadrupole nuclei with half-integer nuclear spins (7=3/2,5/2,...) consists of the -hl/2 -1/2 central transition and the so-called satellite transitions ( l/2 3/2, 3/2 quadrupole interaction it is practically impossible to excite the whole spectriun non-selectively, i.e., the central transition is often selectively observed. 

Fig. 3. Line shape function of the central transition of a quadrupole nucleus with a half-integer nuclear spin, calculated for /Jq=0 in the limiting case of rapid magic angle spinning assuming that the electric quadrupole interaction is the dominating line broadening interaction...

The highest half-integer nuclear spin of a stable nucleus is / = 9/2. The energy of a nuclear quadrupole energy level is given as = e qQxjlA where x is a solution of the secular equation... 

When the observed nucleus is itself quadrupolar. Here the quadrupole interaction is dominant, the others being usually very minor in comparison. Normal MAS speeds are much smaller than typical quadrupole couplings (which lie in the MHz range), producing an enormous number of sidebands, except in the case of the central transition of half-integer spins. In this latter case distinctive line broadening effects appear which cannot be completely removed by MAS. Methods to overcome these difficulties will be reviewed. 

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