Hartmann cross polarization

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. 

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]. 

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... 

Various authors have used different names for Hartmann-Hahn-type experiments that emphasize distinct experimental or theoretical aspects. For example, heteronuclear Hartmann-Hahn transfer in liquids has been called coherence transfer in the rotating frame (Muller and Ernst, 1979), J cross-polarization (JCP Chingas et al., 1981), heteronuclear crosspolarization (Ernst et al., 1991), HEHAHA (heteronuclear Hartmann-Hahn transfer Morris and Gibbs, 1991), and hetero TOCSY (total correlation spectroscopy Brown and Sanctuary, 1991). Homonuclear Hartmann-Hahn transfer has been referred to as TOCSY (Braunschweiler... 

Bax and co-workers demonstrated that a homonuclear Hartmann-Hahn transfer of net magnetization can be obtained by the application of a spin-lock field, using CW irradiation (Bax and Davis, 1985a Davis and Bax, 1985) or by the DB-1 sequence that consists of a series of phase-alternated spin-lock pulses (Davis and Bax, 1985). The homonuclear Hartmann-Hahn effect caused by CW irradiation was discovered when artifacts in ROESY experiments were analyzed (Bax and Davis, 1985a). CW irradiation can be regarded as a homonuclear analog of spin-lock experiments for heteronuclear cross-polarization (Hartmann and Hahn,... 

If the coupling constants are known in advance, the total mixing time can be reduced in multiple-step selective coherence-transfer experiments by using the selective homonuclear analog of the optimized heteronuclear two-step Hartmann-Hahn transfer technique proposed by Majumdar and Zuiderweg (1995). In this technique [concatenated cross-polarization (CCP)] a doubly selective transfer step (DCP) is concatenated with a triple selective mking step (TCP). For the case of a linear three-spin system with effective planar coupling tensors, a CCP experiment yields complete polarization transfer between the first and the third spin and the total transfer... 

Zuiderweg, 1995). In the case of a linear ISQ system, concatenated cross-polarization (CCP Majumdar and Zuiderweg, 1995) is more efficient than two sequential double-resonance (DCP) Hartmann-Hahn transfer steps. The CCP scheme is a combination of a DCP and TCP experiment with optimized mixing times Tj and Tj that depend on the magnitudes of and J Q (Majumdar and Zuiderweg, 1995). 

Homonuclear and heteronuclear coupling constants can be determined from the mixing-time dependence of Hartmann-Hahn transfer (see Sections II and VI). For example, for two heteronuclear spins 1 /2, the ideal polarization-transfer frequency under planar Hartmann-Hahn mbdng is 7,5/2 (see Section VI). Heteronuclear H- Si coupling constants have been determined in IS, I2S, and I S spin systems by Fourier analysis of the cross-polarization intensity as a function of the mbdng time (Bertrand et al., 1978a) and by an iterative fitting procedure (Murphy et al., 1979). 

Finally, we have to address the problem of how we create observable magnetization for nuclei such as C, " Sn, Pb. In solids, T relaxation times of dilute spin-1/2 nuclei can be prohibitively long. Cross polarization (CP) from abundant nuclei with shorter T relaxation characteristics ( H) is then the method of choice. Amongst various CP techniques, Hartmann-Hahn CP has become the most popular method [lf,9d] often used in combination with MAS... 

Cross-polarization between spin 1=112 and S=l/2 systems is a relatively simple and well-understood process. For a static sample, under on-resonance conditions, there is a unique Hartmann-Hahn matching condition [73] at which polarization is transferred between the two sets of nuclei, given by... 

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