Cross polarisation

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) 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]. 

Signal enhancement in liquids (NOE) and solids (H-C cross-polarisation). 

Carbon-13. The molecular structure of phosphino-thioformamides (27) and their chalcogenides have been studied in the solid and liquid states.57 The, 3C n.m.r. spectra of solid methylphenyl phosphonium salts have been studied using high power decoupling cross-polarisation and slow magic angle rotation. 

CROSS BEAM, 195 CROSS HEAD DIE, 365 CROSS-POLARISATION, 101 315 CROSSLINK DENSITY, 66 168 169 262... 

Equilibrium Phase Behaviour. Phase studies were performed using approximately 10 g samples of oil-surfactant mixture diluted sequentially by the weighed addition of water. The initial binary mixture contained 5-70 w/w surfactant at 5 intervals. Phase boundaries were determined to + 0.5 water. The ternary mixtures in Pyrex glass tubes fitted with PTFE lined caps were equilibrated to the required temperature (20-65 0.1°C) for 2 hours and then thoroughly mixed for 5 minutes using a Fisons orbitsil whirlimixer. The tubes were then returned to the waterbath and left undisturbed for 48 hours before identification of the phase type using a crossed polarised viewer and an optical microscope. 

MHz MAS CP DD (magic-angle spinning with cross-polarisation and H dipolar decoupling) 13 C NMR spectra performed at room temperature for the three poly(cycloalkyl methacrylates) (cyclopentyl, cyclohexyl, cyclohep-tyl) are shown in Fig. 8. 

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. 

Fig. 17 Cross-polarisation contact times, fi/2, for the aromatic carbons in the phenyl rings, in both the pure PET and the PET/TPDE blend O aromatic carbons in PET and aromatic carbons in the blend that contains 10% TPDE (from [12])...

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... 

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