DIPSI

Composite-pulse decoupling schemes like WALTZ [36, 37], DIPSI [38], or GARP [39], which are used in solution-state NMR, have failed to offer any significant improvements in the solid state compared to CW decoupling. The residual line width in CW-decoupled spectra is dominated by a cross term between the chemical-shielding tensor of the protons and the heteronuclear dipolar-coupling tensor [40, 41]. 

Fig. 18. The pulse sequence of a ID ge-NOESY-TOCSY experiment, tnoe is the NOE mixing time, 5 are optional delays which can be used for z-filtration [81] or for suppression of ROE effects in macromolecules (2 x (5 + Tgrad) = 0.5 x mixing time). DIPSI-2 [78] sequence was used for isotropic mixing. Phases were cycled as follows 0i = 2x, 2(—x) (j)2 = X, —x Ip = X, 2 —x), X. Rectangular PFGs, G = 6 Gauss/cm and Gi = 7 Gauss/cm, were applied along the axis for Xpad = 1 ms.

Fig. 1. Basic pulse sequence and CP diagram for gradient-based spin-locked ID exf>eriments. A 1 (— 1) 2 gradient ratio selects N-type data (solid lines) while 1 (— 1) (—2) selects P-type data (dashed lines). When SL stands for a -filtered DIPSI-2 pulse train, a ge-lD TOeSY is performed. On the other hand, when SL stands for a T-ROESY pulse train, a GROESY experiment is performed. S stands for the gradient length.

Other strategies to achieve this task are provided by the broadband (BB) saturation sequences (like MLEV16 (Fig. 9.1J) [21], WALTZ16 [22], GARP [23], DIPSI [24], etc.). The power of the BB saturation pulse can be adjusted in such a way as to effectively saturate the slowly relaxing signals and only maiginally the... 

As stated in the introduction of this section, we use Hartmann-Hahn experiment as the generic term for transfer experiments that are based on the Hartmann-Hahn principle, that is, on matched effective fields. Because two vanishing effective fields are also matched, Hartmann-Hahn sequences need not have finite effective fields. Examples of Hartmann-Hahn sequences without effective spin-lock fields are MLEV-16 (Levitt et al, 1982), WALTZ-16 (Shaka et al., 1983b) and DIPSI-2 (Shaka et al., 1988). Note that the term Hartmann-Hahn sequence has also sometimes been used in the literature in a more restricted sense for experiments with matched but nonvanishing effective spin-lock fields (see, for example, Chandrakumar and Subramanian, 1985, and Griesinger and Ernst, 1988). 

Phase-modulated multiple-pulse sequences with constant rf amplitude form a large class of homonuclear and heteronuclear Hartmann-Hahn sequences. WALTZ-16 (Shaka et al., 1983b) and DIPSI-2 (Shaka et al., 1988) are examples of windowless, phase-alternating Hartmann-Hahn sequences (see Table II). 

Homonudear Hartmann-Hahn sequences with delays were developed for clean TOCSY experiments (see Section X.B). Examples are delayed MLEV-17 (Griesinger et al., 1988), delayed DIPSI-2 (Cavanagh and Ranee, 1992), and clean CITY (computer-improved total-correlation spectroscopy Briand and Ernst, 1991). The MGS sequences (Schwendinger et al., 1994) are examples of broadband heteronuclear Hartmann-Hahn mbdng sequences with delays and variable rf amplitudes. 

Fig. 8. Experimental spectra of the AMX spin system of 1,2-dibromo-propanoic acid with coupling constants / m = 10 Ha Jax = 4.6 Hz, nd = H Hz and offset differences l X mI 232 Hz, x xl 497 Hz, and — i/ l = 265 Hz at a spectrometer frequency of 500 MHz. The spectra were obtained by selectively exciting spin A followed by a mixing period of increasing duration The mixing schemes were broadband DIPSI-2 with...

A number of theoretical transfer functions have been reported for specific experiments. However, analytical expressions were derived only for the simplest Hartmann-Hahn experiments. For heteronuclear Hartmann-Hahn transfer based on two CW spin-lock fields on resonance, Maudsley et al. (1977) derived magnetization-transfer functions for two coupled spins 1/2 for matched and mismatched rf fields [see Eq. (30)]. In IS, I2S, and I S systems, all coherence transfer functions were derived for on-resonance irradiation including mismatched rf fields. More general magnetization-transfer functions for off-resonance irradiation and Hartmann-Hahn mismatch were derived for Ij S systems with N < 6 (Muller and Ernst, 1979 Chingas et al., 1981 Levitt et al., 1986). Analytical expressions of heteronuclear Hartmann-Hahn transfer functions under the average Hamiltonian, created by the WALTZ-16, DIPSI-2, and MLEV-16 sequences (see Section XI), have been presented by Ernst et al. (1991) for on-resonant irradiation with matched rf fields. Numerical simulations of heteronuclear polarization-transfer functions for the WALTZ-16 and WALTZ-17 sequence have also been reported for various frequency offsets (Ernst et al., 1991). 

With vf = 10 kHz, these sequences create only a small effective field that is on the order of a few hertz in the offset range between 11.5 kHz (MLEV-16) and between 9 kHz (DIPSI-2), respectively. Spins in this range of offsets have approximately matched effective fields Iv Kv,) and efficient Hartmann-Hahn transfer is possible,... 

Under the unrealistic assumption (vide infra) of an offset-independent effective coupling constant = 10 Hz), the two-dimensional function Vj) is shown for MLEV-16 and DIPSI-2 in Fig. 21A and A, respectively. 

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