WALTZ sequence

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

H decoupling for FLOCK typically is performed as the C signal is acquired and is accomplished with the WALTZ sequence (Section 5-8). FLOCK data are presented in either the phase-sensitive or the absolute-value mode. Because of uncertainty concerning both the location and intensity of correlations in FLOCK contour plots, cross sections should be taken through individual chemical shifts on both the H and C axes, as with HMBC spectra. 

When NMR was performed the media hydrated with 1 1 H20 D20 were packed in 10 mm NMR tubes to reach a sample height of 8 to 10 mm. A 90° pulse WALTZ sequence was used with acquisition parameters 7.45 to 780 /AS pulse width, 1500 to 20,000 Hz pulse width, 0.012 to 0.166 sec acquisition time and recycle delay > 5Ti. Spin-spin relaxation time (T2) was determined with a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with interpulsed spacing (t) ranging from 5 to 500 ms. At least eight different T values were used for each T2 determination. 

The best way to avoid the l-cox problem (and weak nOes) is to use a rotating frame experiment, for instance ROESY (Rotating frame Overhauser Effect Spectroscopy) also named CAMELSPIN by its inventors (77). The (oXg dependence of rOes is complex and one may simply remember that rOes are always positive, never null. The ROESY sequence is similar to the sequence of HOHAHA the main difference is the power of the spinlock which is generated by a long soft pulse rather than by a WALTZ sequence. In ROESY experiments, rOe cross peaks may be accompanied by Hart-mann-Hahn correlations which are easily distinguished by their opposite sign (in phased experiments) (78). 

Composite pulses have also been used in overcoming problems due to sample overheating during broadband decoupling experiments. A widely used pulse sequence is Waltz-16 (Shaka et al., 1983), which may be repre-... 

Fig. 1. Pulse sequence for the X/Y H PFG-HSQC experiment as employed for 19F/13C correlation spectroscopy in Ref. 21. 90° and 180° hard pulses are denoted by solid and open bars, respectively groups of two solid and one open bars denote 90° 0 — 180° +9o — 90° pulse sandwiches that serve as composite 180° pulses. 2 are delays of length 1 /(2 Jx,v), and r is a short delay of the same length as the gradient pulse (typically 1 ms). Phase cycles are as in the standard HSQC experiment, and the ratio of gradient pulse strengths is set to G2/G1 = Yy/Yx- Decoupling is employed using WALTZ-16 ( H) and GARP (Y) pulse trains.

The 180° 13C pulse at the middle of the evolution period interchanges the precession frequencies of the a and /3 spins (see Fig. 9.2, bottom) and effectively decouples the spins during tu and a broadband decoupling sequence, such as WALTZ or GARP, is applied during f2.Thus, the 13C spectrum in the F2 dimension is decoupled, and the H spectrum in the Fx dimension retains homonuclear couplings but is also decoupled from 13C, as illustrated in Fig. 10.106. 

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). 

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). 

In contrast to MLEV-16, WALTZ-16 is relatively insensitive to phase errors. In the absence of amplitude imbalances, coherence transfer is isotropic. Although coherence transfer is possible over a relatively large bandwidth, the transfer efficiency decreases rapidly if the offset difference li j — Vj of two coupled spins i and j is larger than 0.6 i / (see Fig. 23A). In analogy to MLEV-17, a nonisotropic WALTZ-17 sequence was con-... 

Existing homonuclear Hartmann-Hahn mixing sequences that have been converted to clean TOCSY sequences by the introduction of delays using Method D include MLEV-17 (see Fig. 26A Griesinger et al., 1988), DIPSI-2 (see Fig. 26B Cavanagh and Ranee, 1992), and WALTZ-16 (Kerssebaum, 1990). Method C was applied to WALTZ-16, DIPSI-2, and FLOPSY-8 (Briand and Ernst, 1991). 

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. 

Phase cycling has improved procedures for broadband heteronuclear decoupling. As described in Section 5-3, modem methods use repeated 180° pulses rather than continuous irradiation. Imperfections in the 180° pulse, however, would accumulate and render the method unworkable. Consequently, phase-cycling procedures have been developed to cancel out the imperfections. The most successful to date is the WALTZ method of Freeman, which uses the sequence 90°, 180°, 270° in place of the 180° pulse (90 — 180 + 270 — 180), with significant cancelation of imperfections. The expanded WALTZ-16 sequence cycles through various orders of the simple pulses and achieves an effective decoupling result. 

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