Selective Hartmann-Hahn coherence transfer

Fig. 9. Experimental (A)-(C) and corresponding simulated (A )-(C ) coherence-transfer functions for broadband, selective, and two-step selective Hartmann-Hahn transfer in the spin system of 1,2-dibromo-propanoic acid. The experimental coherence-transfer functions in (A), (B), and (C) are cross sections through the experimental spectra at the resonance frequencies of spins A, M, and X in Fig. 8. Experimental details are given in the caption to Fig. 8. (Adapted from Glaser and Drobny, 1991, courtesy of Elsevier Science.)...

In general, a given pulse sequence can act as a TOCSY or as a TACSY mixing sequence, depending on the rf amplitude, the irradiation frequency, and the spin system to which is it applied. Therefore, it is important to consider the offset dependence of the coherence-transfer efficiency of a Hartmann-Hahn sequence. In this respect, a rough distinction between highly selective and band-selective Hartmann-Hahn experiments is useful. 

Because every broadband Hartmann-Hahn mixing sequence has only a finite bandwidth, it can, in principle, be turned into a band-selective Hartmann-Hahn mixing sequence by scaling down the rf amplitude of the sequence. Then coherence transfer is restricted to the scaled active bandwidth and coherence transfer to spins that are well outside of this... 

Both limitations can be avoided if tailor-made multiple-pulse sequences are used for band-selective Hartmann-Hahn transfer. The so-called tailored TOCSY sequences TT-1 and TT-2 (see Table 4) were the first crafted band-selective Hartmann-Hahn sequences to be reported in the literature (Glaser and Drobny, 1989). Both phase-alternated sequences do not use any supercycling scheme. The TT-1 sequence with vf = 10 kHz was developed for band-selective coherence transfer between the offset ranges R- (-2.5 kHz < < —1.5 kHz) and Rj (1.5 kHz < Vj < 2.5 kHz). 

In principle, this type of band-selective Hartmann-Hahn transfer can be used to restrict coherence transfer to spins with resonances in certain frequency ranges. However, in these experiments the effective coupling constant is reduced by a factor of 1/2, which requires longer mbdng times. However, in broadband homonuclear isotropic-mixing experiments as discussed in Section IV.C.2 coupling constants are scaled by a factor... 

Fig. 31. Ideal coherence-transfer functions Tam> nd T/x> during multiple-step selective Hartmann-Hahn transfer in a three-spin system. In the first mixing step of duration T, = 1/(2/ ) coherence is selectively transferred from spin A to spin M. In a second mixing period of duration Tj = 1/(27mx) coherence is selectively transferred from spin M to spin X. (Adapted from Glaser and Drobny, 1989, courtesy of Elsevier Science.)...

Multiple-step homonuclear Hartmann-Hahn transfer is not limited to highly selective transfer steps. As suggested by Glaser and Drobny (1989), several consecutive, band-selective Hartmann-Hahn mixing steps can be used to transfer coherence selectively in coupling networks with selected ranges of chemical shifts and coupling constants. 

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. 

One-dimensional subspectra also may be obtained by combining selective excitation and broadband homonuclear Hartmann mixing with het-eronuclear polarization-transfer steps like INEPT, DEPT (distortionless enhancement by polarization transfer), or heteronuclear Hartmann-Hahn transfer (Doss, 1992 Gardner and Coleman, 1994 Willker et al., 1994). Related experiments with multiple-step selective Hartmann-Hahn mixing in combination with heteronuclear coherence transfer were used by Kupce and Freeman (1993a). 

Isotropic mixing [29] known also as Hartmann-Hahn polarization transfer [30,31] is a unique and very efficient method of coherence transfer between spins. Non-selective isotropic mixing is widely used in different types of correlation experiments. Selective Hartmann-Hahn transfer has been introduced quite recently [32-37] and provides the means for multiplet-selective [33-37] or band-selective [32] correlation experiments. 

In this chapter multiple-pulse sequences for homonudear Hartmann-Hahn transfer are discussed. After a summary of broadband Hartmann-Hahn mixing sequences for total correlation spectroscopy (TOCSY), variants of these experiments that are compensated for crossrelaxation (clean TOCSY) are reviewed. Then, selective and semiselective homonudear Hartmann-Hahn sequences for tailored correlation spectroscopy (TACSY) are discussed. In contrast to TOCSY experiments, where Hartmann-Hahn transfer is allowed between all spins that are part of a coupling network, coherence transfer in TACSY experiments is restricted to selected subsets of spins. Finally, exclusive TACSY (E.TACSY) mixing sequences that not only restrict coherence transfer to a subset of spins, but also leave the polarization state of a second subset of spins untouched, are reviewed. 

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... 

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