WALTZ-16 pulse sequence

Vittadini et al., (2001) investigated the media systems further with the proton decoupled NMR relaxation rate measurement using a 300 MHz NMR spectrometer (Bruker MSL 300) with a WALTZ pulse sequence. Acquisition parameters were 41 ms acquisition time, 9 /rs 90° pulse width, 100 ms recycle delay and 32°C sample temperature. Transverse relaxation rate Rz) was analyzed from line shape analysis after the system was... 

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.

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). 

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. 

Fig. 1. Pulse sequences for ID- X, "Y H) polarization transfer experiments. If not stated otherwise, narrow and wide bars denote 90° and 180° hard pulses, narrow and wide ellipsoids 90° and 180° shaped pulses. Essential phase cycles for selection of the polarization transfer signal are given on top of the pulses, if no phase is indicated, pulses are applied along the x-axis A denotes a fixed delay of length (n/(X,Y))" (a) UPT (, 0 = 90°), (b) unrefocused INEPT, (c) unrefocused selective INEPT with soft pulses, (d) INEPT with selective excitation via H, "Y cross-polarization si denote WALTZ-17 spinlock pulse trains which were applied for a period t— (2/( H,Y)) ...

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 performance of these multiple-pulse sequences can be calculated using average Hamiltonian theory [23]. It has been shown that sequences with all pulses along one axis (with positive and negative amplitudes), like WALTZ... 

WALTZ-16 sequence (Chapter 10). A 0.3 Hz line broadening is used and the noise recorded over the same region, with the peak height determined for the tallest aromatic resonance. Tuning of the proton coil and appropriate calibration of the proton decoupling pulses are required in this case for optimum results. Test samples for other common nuclei are summarised in Table 3.8. Should you have frequent interests in other nuclei, a suitable standard should be decided upon and used for future measurements. 

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. 

---