Cross-polarisation experiment

In the case of the aromatic protonated carbons, cross-polarisation experiments can be performed. The t /2 cross-polarisation time (defined as the contact time required to produce half the maximum value of magnetisation that is possible by cross-polarisation) significantly increases with temperature, as shown in Fig. 17. 

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 catalytic activity of aluminosilicate zeolites and aluminas appears to be directly related to the concentration of 30 ppm sites present, leading to the development of super-five materials displaying large NMR signals at this position (Wood et al. 1990). Since catalysis depends on the chemical nature of the Al at the surface, which may not be the same as in the bulk, cross-polarisation experiments between H and Al have been used to distinguish between the surface and bulk species (Coster et al. 1994). Since chemisorption of water provokes extensive surface reconstruction, a more suitable proton source for the CP experiments was found to be ammonia adsorbed on the surface. The results showed the presence of two kinds of surface Lewis sites associated with the non-framework Al (a tetrahedral site at ca.58 ppm with a xq of about 6 MHz, and an Af site at ca. 40 ppm with a slightly smaller xq)- Lewis sites either... 

A similar but very weak resonance reported at 30 ppm in 7-AI2O3 heated at 600°C has been attributed to 5-coordinated aluminium atoms on the surface of the oxide particles (Pecharroman et al. 1999). Cross-polarisation experiments between Al NMR and the protons from surface-adsorbed pyridine (Morris and Ellis 1989) or ammonia (Coster et al. 1994) have been used to investigate the surface state of cat-alytically active 7-AI2O3. No NMR evidence has been found of the hypothetical 3-coordinated Al theoretically predicted to occur at the alumina surface. Even assuming for this species a 8iso of 95-100 ppm, a Xq of 10 MHz and an t value of one, Coster et al. (1994) found no experimental evidence for its existence, prompting a recent explanation based on density functional calculations that the 3-coordinated surface Al atoms... 

Fluorine-containing aluminosilicate glasses have been examined by Al MAS NMR and F- A1 cross-polarisation experiments (Kohn et al. 1991) which demonstrate the presence of Al in (IV), (V) and (VI) coordination states. The presence of F in the glasses does not exert a large effect on the chemical shift of the Al(VI) suggesting that this coordination is relatively remote from the F. By contrast, the A1(V) signal is... 

The final product of thermal treatment of VPI-5 is the compound AIPO4-8, which itself exists in both hydrated and dehydrated forms. The dehydrated form has a broad, featureless P MAS NMR resonance centred at about — 29 ppm (Martens etal. 1991). The P MAS NMR spectrum of hydrated AIPO4-8 contains a major resonance at -31 ppm, with smaller features at about - 26 and - 23 ppm (Martens et al. 99 ). The 3 distinct P environments in hydrated AIPO4-8 have been shown by two-dimensional cross-polarisation experiments between the P and Al (Figure 7.28B) to be connected to both the tetrahedral and octahedral Al sites (Fyfe et al. 1993). 

Fig. 18.22. Evolution of the F magnetisation as a function of the contact time in a standard cross-polarisation experiment for a commercial Viton sample.

The addition of a chemical species with a large dielectric constant to induce desired microwave effects in matrices devoid of such substances, or lacking substances with significantly different dielectric constants, can be compared, on a conceptual basis, to cross-polarisation experiments carried out in nuclear magnetic resonance spectroscopy (see Chapter 6). In that case, a nucleus that relaxes relatively rapidly is excited selectively and allowed to transfer that excitation energy to neighbouring nuclei with low or relatively lower relaxation rate (e.g., nuclei being cross-polarised to nuclei). 

We used modifications of the standard solid-state CP-MAS (cross-polarisation, magic-angle spinning) experiment to allow the proton relaxation characteristics to be measured for each peak in the C spectrum. It is known that highly mobile, hydrated polymers can not be seen using either usual CP-MAS C spectrum or solution NMR (6). We found, however, that by a combination of a long-contact experiment and a delayed-contact experiment we could reconstruct a C spectrum of the cell-wall components that are normally too mobile to be visible. With these techniques we were able to determine the mobility of pectins and their approximate spatial location in comparison to cellulose. 

So that the results from the delayed-contact and variable-contact data could be compared, they were normalized to give equal signal intensities at a contact time of 0.5 ms (zero delay in the delayed-contact experiment) after the variable-contact data had been adjusted to allow for the fact that full equilibration of proton and polarisation had not quite been reached after 0.5 ms. The difference in normalised signal intensity between the variable-contact and delayed-contact experiments, at a given time point t, is then a measure of the amount of 13C cross-polarising between 0.5 ms and t. 

Cross-polarisation (CP) is a solid-state NMR experiment designed to achieve a higher sensitivity for the rare nucleus through the transfer of polarisation, driven by the heteronuclear dipolar interaction, from an abundant ( ll) to a dilute spin (13 C).118 Transfer of magnetisation is possible... 

While the original CP experiments were designed for the proton-to-rare spin polarization transfer, cross-polarisation in the spin system which do not involve protons (i.e. 31P —> 27A1 or even 27A1 —> 31P) have also been described.22,23 These experiments have been widely applied for characterization of the structure of molecular sieves as well as the catalytic activity of their acidic sites.5,22... 

Deconvolution of these peaks would yield a distribution of Q2 and Q3 sites as a function of temperature. However, cross-polarisation spectroscopy is not automatically a quantitative technique, since the signal s intensity depends -amongst others- upon the proximity of protons and upon the contact time, used in the experiment. 

Cross Polarisation not only provides a better signal-to-noise ratio, but the experiment can also be done faster, because the repetition rate of the pulses is now determined by the proton relaxation times, which are a factor 10-100 smaller than those of the carbons. 

This example is illustrated in Figure 13.9 in which the MAS spectrum (a) is represented with the spectrum obtained from cross polarisation (b). The comparison of the two clearly shows a relative increase in certain peaks these are the Si (OH) (Si)4, , n taking the values shown on the spectrum (b). The Si (OSi)4 signal is still present this is simply due to an extended contact time which permits the polarisation to be extended over 3 bonds. It may thus be necessary to vary the contact time in order to fully discriminate the atoms bearing hydroxyls, Quantitative analysis is not possible but the experiment can be used to determine the presence or absence of silanol groups. 

In the same way, it is possible to analyse the coke deposited on used catalysts. The experiment has been successhilly attempted on cokes in which the graphite structure is poorly developed, thus containing hydrogen. The results were obtained using marked, and thus expensive, molecules. For an evolved coke, whose low hydrogen content makes them particularly suited for cross polarisation, it is not possible to detect less than 4-5% coke. 

The H- Si cross-polarisation time constants (Ts,h) associated with the Q species of both the 5% V-MCM and the standard MCM-41 calcined samples have been derived from variable-contact-time experiments. These values can be viewed as an indication of how fast the transfer of polarisation from protons to silicons occurs. The value for the 5% V-MCM sample is of the order of 0.15 ms, whereas for the standard sample it is 20 times greater, about 3 ms. 

The dipolar coupling is also central to many schemes for magnetisation transfer between nuclei using double resonance techniques such as cross-polarisation. The data become more difficult to unravel when a nucleus experiences numerous different dipolar interactions at the same time. Even an increase to just three spins causes the lineshape... 

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