Hartmann-Hahn transfer defined

More generally, C = 0 if the effective coupling term 2irI,J J Iy is a zero-quantum operator in the effective field frame that is defined by and Bf (see Section II) and if the effective fields are matched (ly B l = lyyBj 1). Because these conditions are identical with the conditions for efficient Hartmann-Hahn transfer, the approximation... 

In order to assess magnetization transfer in a multiple-spin system, it is necessary to define a measure that reflects the efficiency of the transfer between two spins i and j. This parameter should reflect the amplitude of the ideal polarization transfer as well as the duration of the mixing process, because, in practice, Hartmann-Hahn transfer competes with relaxation. Relaxation effects result in a damping of the ideal polarization-transfer functions 7j . The damping due to relaxation depends not only on the structure and dynamics of the molecule that hosts the spin system of interest, but also on the actual trajectories of polarizations and coherences under a specific multiple-pulse Hartmann-Hahn mixing sequence (see Section IV.D). For specific sample conditions and a specific experiment, the coherence-transfer efficiency can be defined as the maximum of the damped magnetization-transfer function. 

Local quality factors such as qfiv, Vj) provide a detailed view of the offset dependence of the efficiency of Hartmann-Hahn transfer. For the optimization of Hartmann-Hahn sequences, the offset-dependent local quality factors must be condensed into a single global quality factor. For example, for a constant rf amplitude, the global quality factor can be defined as the minimum of the local quality factor Vj) in a predefined offset range and (Glaser and... 

Figure 22 illustrates how relatively simple global quality factors can be used as filters in the search for optimum solutions in the parameter space that defines multiple-pulse sequences. Suppose for typical coupling constants = 10 Hz a multiple-pulse sequence with a constant rf amplitude = 10 kHz is desired that effects efficient Hartmann-Hahn transfer in the offset range of +4 kHz. Here, the simple two-dimensional parameter... 

Broadband Hartmann-Hahn sequences, such as DIPSI-2 or WALTZ-16, can be made band-selective by reducing the rf amplitude of the sequences (Brown and Sanctuary, 1991). Richardson et al. (1993) used a low-amplitude WALTZ-17 sequence for band-selective heteronuclear Hartmann-Hahn transfer between N and in order to minimize simultaneous homonuclear Hartmann-Hahn transfer between and The DIPSI-2 sequence was successfully used by Gardner and Coleman (1994) for band-selective Hartmann-Hahn transfer between C d and H spins. So far, no crafted multiple-pulse sequences have been reported that were optimized specifically for band-selective heteronuclear Hartmann-Hahn transfer. Based on the results of Section X, it is expected that such sequences with well defined regions for coherence transfer and effective homonuclear decoupling will result in increased sensitivity of band-selective heteronuclear Hartmann-Hahn experiments. 

A large number of polarization-transfer experiments already exist that are based on the Hartmann-Hahn principle, and the number of Hartmann-Hahn mixing sequences is still rapidly growing. Therefore, it is important to have classification schemes that allow one to disentangle the plethora of known (and potential) mixing sequences. In the NMR literature, a number of different classification schemes have been used for Hartmann-Hahn experiments. However, the nomenclature of different authors is not always uniform (and in some cases it is even contradictory). In this section, existing classification schemes are reviewed and discussed. This discussion also defines the nomenclature that is used in this review. 

More recently, a number of Hartmann-Hahn experiments were developed, which allow one to control the transfer of magnetization within an extended coupling network and to deliberately restrict coherence transfer to a defined subset of spins (see Section X.C). The first experiments of this class were called tailored TOCSY experiments (Glaser and Drobny, 1989),... 

Second, undesired TOCSY peaks appear because some nuclei that are spin coupled experience similar fields during the application of the spin-lock and fulfil the Hartmann-Hahn condition. Since the TOCSY peaks are phase shifted by 180° with respect to the ROESY peaks, they can easily be recognized. However, the superposition of contributions from direct and indirect transfer results in a decrease of cross peak intensity and therefore in distances which are too long. When only lower boundaries are used as restraints in MD calculations this would lead to lower restraints and a less well-defined structure but would not induce wrong results. In addition, different internal correlation times, such as the above-mentioned different flexibility of the molecule have a smaller influence in ROESY than in NOESY spectra. 

---