Quadrupole-induced isotropic shift

Figure 16 2D 3Q/MAS NMR spectrum of l anite measured by the three-pulse sequence with z-filter. No shearing transformation was applied. Labels "A," "CS," and "QiS" describe anisotropic axis, isotropic chemical shift axis and axis of quadrupole-induced isotropic shift, respectively. Three different AlOg structural units are clearly defined.

Subsequently, based on the principles of 2D NMR spectroscopy, distinct sites can be clearly resolved (Fig. 16), and, using mathematical analysis, the isotropic chemical shift and quadrupole-induced isotropic shift can be separated from the anisotropic second-order quadrupolar spHtting. The principle condition for obtaining the required results or separating the anisotropic component of the quadrupolar interaction from isotropic contributions is based on the correct setup of the ratio of t and <2 detection times, during which the 3Q generally p-quantum) and IQ coherences evolve. This ratio must fulfill the following condition [72,73] ... 

As if can be seen in fhese simulations, all MAS spectra are shifted towards low frequencies wifh respecf fo fhe true isotropic chemical shift—an effect known as the second-order quadrupole-induced shift. The centre of gravity ( g) of fhese lineshapes is given by ° ... 

The isotropic shift and the quadrupole induced shift (QIS = Co(p)/Co(l), with Cs defined in Table 2.4) can readily be obtained from the data. Firstly, the 2D spectrum must be referenced correctly. To obtain the correct ppm scale in the FI dimension one must set the spectrometer frequency in the FI dimension to pVo. The shift reference is most readily set at the carrier frequency, i.e. the shift in ppm at the carrier frequency in the multiple quantum dimension Ft is the same as the shift in ppm at the carrier frequency in the single quantum dimension F2. This is valid for data obtained with or without delayed acquisition and processed with or without shearing. The isotropic chemical shift is the same for both the single quantum and the multiple quantum dimensions. The QIS is different in both dimensions, however, and is given by... 

The isotropic shift and the quadrupole-induced shift (QIS) can easily be obtained from the data. The QIS is different in both dimensions, however, and is given by... 

NMR spectra, where the v. axis nearly purely reflects the isotropic chemical shift, and the quadrupolar parameters (second-order quadrupolar broadening and quadrupole-induced shift) are separated along the V axis (Fig. 19D). 

A major advantage of MQAS is that it allows separation and measurement of the isotropic chemical shifts and quadrupolar-induced shifts by recording and analysing a single two-dimensional spectrum. Determination of quadrupole parameters and relative site populations are also possible, even for disordered and amorphous materials. In the present section we address these problems. 

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