Hartmann-Hahn cross polarization

Fig. 11.3 Schematic pulse sequence of Hartmann-Hahn cross polarization to transfer polarization from the / spins to the S spins by matching the rf-field amplitudes such that the condition co ( = coiS is fulfilled.

Hartmann-Hahn cross polarization between two low-y nuclei has been successfully used to record chemical-shift correlation spectra between 13C and 15 N nuclei. Cross polarization between two low-y nuclei suffers from a high sensitivity to the exact matching condition at one of the side bands of the Hartmann-Hahn condition [101]. Adiabatic methods (APHH-CP) can eliminate most of this sensitivity and lead to high transfer efficiencies [34, 62, 90]. 

M. Baldus, D. G. Geurts, S. Hediger and B. H. Meier, Efficient N- C polarization transfer by adiabatic-passage Hartmann-Hahn cross polarization. J. Magrc Reson.. 4. 1996, 118, 140-144. 

S. Hediger, B. H. Meier and R. R. Ernst, Adiabatic passage Hartmann-Hahn cross polarization in NMR under magic angle sample spinning. Chem. Phys. Lett., 1995, 240, 449-456. 

Finally, in this section, the simple Hartmann-Hahn cross polarization experiment, and in particular, the JH-19F CP build up curve was used to determine the residual JH-19F dipolar coupling constant due to motion in an ethylene/tetrafluoroethylene co-polymer.31 This curve for the crystalline... 

S.R. Hartmann and E.L. Hahn reported that pulsed DNP experiments based on coherent polarization transfer such as the Hartmann-Hahn cross polarization (HHCP) are more efficient and do not show a field dependence of the polarization transfer with increasing magnetic field strengths. 

Lobo and Ramanathan have combined adiabatic and Hartmann-Hahn cross-polarization for sensitivity enhancement in solid-state separated local field 2D NMR experiments of partially ordered systems The magnetization in double- and zero-quantum reservoirs of an ensemble of spin-1/2 nuclei has been examined and their role in determining the sensitivity of a class of separated local field NMR experiments based on Hartmann-Hahn cross-polarization has been described. Lobo and Ramanathan report that for the liquid crystal system studied, a large dilute spin-polarization, obtained initially by the use of adiabatic cross-polarization, can enhance the sensitivity of the above experiment. The signal enhancement factors, however, are found to vary and depend on the local dynamics. The experimental results have been utilized to obtain the local order-parameters of the system. 

Hartmann-Hahn cross polarization (CP) olfers an alternative approach. Recent Xe- C CP experiments on methoxylated silica demonstrate the utility of this technique not only is it more selective toward surface nuclei, it is applicable at temperatures near and above room temperature and applicable to materials with short Ti relaxation times (surface areas (<5m /g). ° As such, selective enhancement of the magnetization of surface nuclei is possible provided a coupling exists between the adsorbed HP Xe and the nuclei of interest. 

At the time of this writing, a rapidly growing area of research within NMR is the high resolution study of solids brought about by the development of certain multiple pulse sequences (such as the WAHUHA and MREV-8) on the one hand and an adaptation of the Hartmann and Hahn cross polarization experiment to obtain spectra of dilute spins on... 

In addition, the technique of cross polarization introduced and developed by Pines, Gibby and Waugh (9) is used to increase the signal-to-noise ratio of the spectrum. The proton magnetization is spin-locked along the y axis with a spin-locking field % and the carbons subjected to an RF pulse chosen such that the two fields fulfill the Hartmann-Hahn condition (10), equation [3] (Figure 2). 

Hartmann-Hahn match, and cross-polarization mixing time are discussed in relation to obtaining quantitative NMR results. 

Figure 3. Cross polarization magnetization for the PIP-cured epoxy under the SL (Hartmann-Hahn) condition. The cross polarization contact time is rcp. The decay corresponds to proton T,p relaxation.

These intermolecular correlation peaks mean that intermolecular cross polarization (CP) occurs between the carbon and proton of intermolecular -interacting polypeptides in blend. There may be two pathways for the observed intermolecular CP. One is that a direct transfer from proton to carbon exists, and another is that a change in the magnetization by spin diffusion (homonuclear Hartmann Hahn transfer) exists. It is thought that the former is much more efficient than the latter because the former comes from only one magnetization transfer process, but the latter comes from two... 

These questions were resolved with the use of the same relatively simple epoxy system. All C-13 nuclei in contact with the proton bath were counted when moderate spinning rates were used and in spin-lock cross polarization in rf fields not close to any Tle minimum. The molecular motion determines the relaxation rate, under the Hartmann-Hahn condition when T, = T2. The spin-spin effects determine relaxation when Tle does not equal T2 under the same conditions 62). The spin-spin fluctuations are in competition with the spin-lattice fluctuations in producing an effective relaxation time. To discriminate against the spin-spin fluctuations large rf fields are mandatory. It was pointed out that, with great care, C-13 NMR spectra can reflect molecular motion. 

Cross-polarization is a well-known method for enhancement of low-/ nuclei signals in solids (CP MAS experiments)145. The theory and first cross-polarization experiment in liquid were presented by Hartmann and Hahn in the same paper that laid the foundation of the famous CP experiments in solids in 196274. It took almost two decades until CP experiments in liquids were investigated in more detail by Ernst and coworkers146 and Bertrand and coworkers147. 

Cross-polarization in tilted frame has been proposed.275 In this scheme, the two effective fields matching the Hartmann-Hahn condition have a large offset comparable to two RF fields. This method was named SPECIFIC CP (spectrally induced filtering in combination with cross-polarization Fig. 20) because the matching depends on the resonance offset of the involved nuclei. This method is clearly useful for spectral simplification, as demonstrated in Fig. 21. Under fast spinning conditions, 7-coupling-mediated CP can be observed as shown by Verhoeven et al.216... 

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