Chemical shift anisotropy , quadrupolar nuclei

The magnitude of the chemical shift anisotropy depends on the bonding situation and the nucleus gyromagnetic ratio. Since the bonds formed by lithium in organolithium compounds or other lithiated systems are mainly ionic, the anisotropy of the lithium chemical shift is generally small. It is more pronounced for Li than for Li. Li spectra are dominated by the quadrupolar effect and the CSA contribution to the Li lineshape is often negligible. Exceptions are compounds with poly-hapto bound lithium, such as... 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

Static line shapes arising from chemical shift anisotropy (CSA) and quadrupolar coupling also reflect the local symmetry of the nucleus. Isotropic lines occm for high-symmetry sites, such as tetrahedral, cubic, or octahedral, where these interactions are zero [e.g., in the Cu-NMR of tetrahedral copper centers in N(CH3)4-CuZn(CN)4 and N(CH3)4CuCd(CN)4 guest framework-], -whereas the lines have characteristic shapes for sites with axial symmetry or for lower-symmetry sites. In the case of the sign of an axial CSA line... 

Certain surfactants carry phosphate or ammonium polar headgroups, offering P and nuclei, respectively, for relaxation studies. The phosphate (7=1/2) has a relatively large chemical shift anisotropy. is an 7 = 1 nucleus, and its relaxation is dominated by the strong quadrupolar interaction and can be most useful for studies of microemulsion droplet size. 

MAS NMR spectroscopy of the spin nuclei H, Si and P has been particularly useful in studies of microporous solids but many other nuclei of interest in the area of microporous solids are quadrupolar, and indeed 74% of NMR-active nuclei have a spin greater than Among these, is the only nucleus of major chemical interest with an integer spin (I = 1), and is of particular relevance to porous solids for studies of mobility. By contrast, many quadrupolar nuclei with non-integral spin are important, including Na, A1 and Ga. The central (mi= + 5 -5) transition of quadrupolar nuclei is to first order unaffected by quadrupolar coupling, and since dipolar coupling and chemical shift anisotropy are removed by MAS, reasonable... 

The CS interaction arises from a small magnetic field induced by the interaction of the external magnetic field (Bq) with the electric environment of the nucleus of interest. Similar to the quadrupolar coupHng, the CS is a three-dimensional tensor interaction. In its PAS, the CS tensor is described by three principal components, n, 22 and 33 ( 11 > 22 33)- The chemical shift anisotropy (CSA) can be defined by the parameters using Herzfeld—Berger/Maryland convention [19]... 

Nuclear relaxation is caused by interaction between the nuclear magnetization M and small incoherent magnetic fields which arise from random Brownian motions of molecules (Abragam, 1961 Farrar and Becker, 1971 Becker, 1974). In the case of quadrupolar nuclei, electrical fields interact with the electric quadrupole moment of the nucleus. The fluctuating fields can arise from a number of processes, including (1) magnetic dipole-dipole interaction, (2) electric quadrupole interaction, (3) scalar coupling, (4) spin-rotation interaction, and (5) chemical shift anisotropy. 

Under anisotropic conditions, NMR lineshapes for a quadrupolar nucleus are dominated by chemical shielding and (first and second order) quadrupolar interactions. Dipolar interaction is usually a minor contribution only. First-order quadrupole interaction lifts the degeneracy of the allowed 21 (i.e. seven in the case of V / = V2) Zeeman transitions as shown in Figure 3.7, giving rise to seven equidistant lines, viz. a central line (mj = + V2 -V2. unaffected by quadrupole interaction) and six satellite lines. The overall breadth of the spectrum is determined by the size of the nuclear quadrupole coupling constant Cq the deviations from axial symmetry and hence the shape of the spectral envelope are governed by the asymmetry parameter. Static solid-state NMR thus provides additional parameters, in particular the quadrupole coupling constant, which correlates with the electronic situation in a vanadium compound. [ 1 The central component reflects the anisotropy of the chemical shift. 

It must be stressed that there are fundamental differences in the natures of the different nuclei discussed above. Si and P are spin 1/2 nuclei, and MAS yields particularly simple spectra with complete averaging of the chemical shift tensor components. The average isotropic shift values are field independent and correspond to the solution chemical shifts. AI and O, however, are quadrupolar nuclei with nonintegral spins greater than 1, and their solid-state spectra are often much more complex. Since the quadrupolar interaction depends on local electric field gradients at the nucleus studied, solid-state NMR of quadrupolar nuclei can yield further information about the local environment. More complicated spinning techniques have been introduced to average the anisotropies in the spectra of quadrupolar systems, and these new methods have increased interest in the use of such nuclei as probes of local microstructure. 

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