Polarisation CPMAS Experiments

In cross-polarisation experiments two nuclides are excited at the correct frequencies to satisfy the Hartmann-Hahn condition (Chapter 2) so that the magnetisation of the more abundant spin system (typically H) is transferred to the system with the smaller signal ( Si). The FID of the nuclide of interest ( Si) is acquired with a stronger signal and at the usually shorter T i value of the protons, allowing the spectrum to be obtained more quickly and with an enhanced signal/noise level. 

As well as providing improved sensitivity, cross-polarisation experiments between H and Si can also be used to provide additional structural information, since the Si atoms in closest proximity to protons are preferentially enhanced. The data from CPMAS experiments should, however, be regarded as qualitative rather than quantitative. If improved quantification is required, it is essential to ensure that the signal is not 

The improved sensitivity of H- Si CP MAS has been exploited by Alma et al. (1984) to record the Si spectra of synthetic mica and montmorillonite and by Kodama et al. (1989) in a study of ground kaolinite. In these studies, the non-CP and CP spectra were identical in shape and position, indicating that all the Si sites are in similar proximity to protons. 

The discrimination of protonated Si sites by CP MAS was also used by Yang and Kirkpatrick (1989) in a study of the hydrothermal decomposition of albite and sodium aluminosilicate glass, and rhyolitic glass (Yang and Kirkpatrick 1990), and as a means of differentiating between the various sites in acid-treated montmorillonites (Tkac et al. 1994). CP MAS NMR has also been used to identify a Si site at -100 ppm in a microporous material derived from acid-leached metakaolinite as the unit Si(0Si)30H (Okada et al. 2000). 

A1 atoms this is related to differences in the relaxation rate and the cross-polarisation rate of the — 98.1 ppm site (Shore et al. 1999), and illustrates the necessity for caution when drawing conclusions from CP intensities. 

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Cross polarisation from protons to fluorine nuclei becomes feasible with the double-resonance probes developed for F- H work. Figure 6.6.3 shows a CP profile as a function of contact time. This experiment offers similar possibilities for discrimination within spectra that are traditional with C CPMAS spectroscopy. Thus, the dipolar dephasing pulse sequence (Fig. 

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