Double cross polarisation

Figure 36 Double cross-polarisation spectrum ( H — 13C —> 3,P) of [,3C. 15N] pamidronate on human bone. Taken form Ref. [123].

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... 

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. 

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. 

Additional work using 2D-double quantum 1H/31P cross polarisation heteronu-clear correlation (HETCOR) NMR spectroscopy confirmed that the phosphorous position D possibly represents an oxyapatite, Ca10(PO4)6O SRO structure (Hartmann et al., 2000). Because this chemical shift is identical to that of crystalline... 

An improved variant of the popular double cross-polarisation (DCP) experiment for heteronuclear dipolar re-coupling in solid state NMR spectroscopy under MAS was introduced by Bjerring and Nielsen. By simple phase and amplitude modulation of the RF irradiation at the Hartman-Hahn sideband conditions, the new pulse sequence, dubbed iDCP, was shown to enable broadband excitation with the high efficiency of y-encoded coherence transfer. The efficiency and robustness of iDCP towards isotropic chemical shift variations and chemical shift anisotropies in the case of uniformly C, N-labelled proteins has been demonstrated numerically and experimentally by N to C coherence transfer for N-labelled N-Ac-L-valyl-L-leucine and C, N-labelled-L-threonine. 

With the advent of high resolution techniques in solids it became of interest to measure site-specific relaxation times. For the typical double-resonance type of experiments, there are no real problems in measuring T s under MAR conditions, except possibly the often very large values. When it comes to Tj p, however, for the dilute spin species, then there can be difficulties in interpretation. Suppose we have created some spin-locked magnetisation say for in a solid, probably via cross polarisation. To measure T p we would switch off the... 

A detailed analysis of the cross-polarisation behaviour showed that quantitative results can be obtained. The amount of unreacted units, typically 0-15%, was found to depend on the polymerisation parameters. Conditions favouring mobility, i.e., higher temperatures or increased solvent quality, resulted in lower content of residual double bonds. Bromine addition values are 2-3% higher than the NMR data. The reactivity toward bromine further indicates that the mobility is reasonably high. This has also been confirmed by measurements of the rotating-frame relaxation time constant, Tj ( C) Most likely, Tjp is dominated by spin-lattice processes i.e., it can be interpreted in terms of molecular dynamics. The values obtained for C=0 and >C =CH2 in unreacted units are about twice that of C=0 in reacted units, indicating increased mobility. The reactivity of the remaining double bonds in a radical polymerisation with a chiral monomer was also demonstrated. 

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of... 

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