Quadrupolar parameters

The chemical shift and quadrupolar parameters can also be easily extracted from the unsheared spectrum [8],... 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Another approach to deducing the structures of materials from their solid state NMR spectra is to make an ab initio theoretical calculation of the electric field gradient at the nucleus of an atom in a known crystal structure environment (Figure 1.3, pathway 1), from which the nuclear quadrupolar parameters and hence the expected NMR spectrum can be calculated and compared with the experimental spectrum (Figure 1.3. pathway 2). The difficulties inherent in making such ab initio calculations are such that relatively few real compounds have so far been solved completely, but developments in ab initio theory, and mathematical algorithms and computing techniques should make this approach much more widely accessible in future. 

Even for nuclei where the satellite transitions are broader than the central transition (Table 2.6), if their second-order quadrupolar structure can be observed, they act as an independent check of quadrupolar parameters deduced from the central transition. The larger shift of the satellite transitions for I = /2 has been extremely useful in identifying different boron species from B NMR of borate glasses. 

Since both excitation and conversion are dependent on r, MQ spectra are normally not directly quantitative, so great care has to be taken to interpret the spectra. The situation will generally improve as the applied rf field increases. Since the 2D data set contains the anisotropic information, lineshapes can be analysed and the quadrupolar parameters deduced. The lineshape will show distortion as there will be crystallites whose orientation is such that their quadrupolar frequency is zero and will not contribute. Apart from the excitation and the conversion, the actual coherence pathway... 

Numerical analysis of MQMAS spectra permits the determination of the isotropic chemical shift and quadrupolar parameter distribution. The Simula-... 

The three parameters (ocs> (6cs)> q) that describe the chemical shift and quadrupolar distributions are obtained by simulation of the eiqierimental spectrum, which is described by a sum of individual two-dimensional spectra over all possible combinations of chemical shifts and quadrupolar parameters as follows ... 

Fig. 9a-c Schematic representation of a Gaussian distribution of a chemical shifts b quadrupolar parameters c experimental and simulated triple-quantum MAS NMR... 

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