Cross-Polarisation CP

Cross-polarisation has played a very important role in the development of solid state NMR, giving improvements in sensitivity compared to one pulse operation allowing C spectroscopy in polymers and other organic solids to become feasible. Normally the abundant spin is H or F, for which a general procedure can be followed to set up the CP experiment. It is helpful if a well-defined standard set-up compound can be identified for each particular pair of nuclei. 

Spin the well-defined set up sample relatively slowly (1-3 kHz). 

Observe the H/ F, set on resonance by shifting the transmitter frequency and set the power to produce the desired 90° pulse. 

Observe the X-nucleus and if a signal can be seen in decoupled one pulse operation, set X close to resonance also. 

Switch to CP mode and increment the transmitter power of the X nucleus until the maximum signal is observed (i.e. the optimum match is obtained). 

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Cross-polarisation (CP) in the rotating frame has been introduced as a means of transferring polarisation between different nuclear species in solids [168], and has become of central importance for obtaining spectra of rare spins with low gyromagnetic ratios such as 13C, since a significant sensitivity enhancement may be achieved. Cross-polarisation can be used either for direct observation of low-sensitivity nuclei or for indirect detection of such nuclear species via high-sensitivity nuclei such as protons [169]. 

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... 

To enhance the sensitivity of 13C solid state NMR, polarisation (magnetisation) transfer from the abundant proton spins to the rare carbon spins is applied. This technique is called cross polarisation (CP). This is achieved by first exciting the proton resonance and then simultaneously turning on the 13C and aH radio frequency fields. The so called Hartmann-Hahn condition should then be satisfied ... 

The greatest improvements are obtained by combination dipolar decoupling (DD) in combination with magic angle spinning (MAS) and cross polarisation (CP). 

Many multiple resonance methods such as cross-polarisation (CP), Rotational Echo Double Resonance (REDOR), and Transferred Echo Double Resonance (TEDOR) are available. Their application can be differentiated between systems involving exclusively spin- /2 nuclei and those containing quadrupolar nuclei. The presence of quadrupolar nuclei may prohibit straightforward application of these experiments and can affect the interpretation of the results. There are, however, benefits arising from the presence of quadrupolar nuclei, as illustrated by the Transfer of Populations by Double Resonance (TRAPDOR) experiment which will only work for quadrupolar nuclei. 

Figure 5.26. Two-dimensional P spectra of AIPO4-5. A. Cross polarisation (CP)...

In spite of these drawbacks, most of the more recent " Ca NMR work has been on isotopically-enriched samples. H " Ca cross-polarisation (CP) has also been inves-... 

Cross-polarisation (CP), which involves a cross-pulsing sequence. Under the appropriate conditions, magnetisation flows from protons to C during the contact time, when resonant radio frequencies are apphed to both protons and the nuclei to be observed (e.g. C). 

The solid state NMR measurements on the hard coke concentrates were carried out at 25 MHz on a Bruker DSX spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca. 6-7% of the central aromatic bands. A contact time of 1 ms was used for the cross polarisation (CP) measurements and the H decoupling and spin-lock field was ca. 60 kHz. The FIDs were processed using a Lorentzian linebroadening factor of 50 Hz. To determine the fraction of protonated and non-protonated carbon, four delay periods between 1 and 100 is were employed in dipolar dephasing experiments. 

IH spectra due to the wider range of chemical shifts. MAS spectra of 13C are mostly acquired under cross-polarisation (CP-MAS), i.e. the dipolar interaction is employed to transfer magnetisation from excited IH nuclei to the 13C nuclei and thus enhance the signal. 

Zujovic et al investigated the mobility of veiy mobile polysaccharides in plant cell walls by NMR method. Mobility of such polysaccharides as arabinan and galactan side chains of pectic polysaccharide rhamnoga-lactouronan I in two apple cultivars was evaluated by the solid-state C NMR. The authors used two pulse phase modulation (TPPM) decoupling which improved the resolution of cross polarisation (CP) and single pulse excitation (SPE) NMR spectra. This methodology facilitated the mobility estimation of mobile polysaccharides in apples cell walls. 

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