Decoupling composite pulse

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-... 

Composite pulses reduce the error introduced due to the delay between the start of the pulse and when it reaches full power. They have also been used to overcome problems of sample overheating during broadband decoupling and in experiments in which pulses have to be applied for long durations. 

Figure 4.7 Pulse schemes representing separation of decoupling effects from the nOe during X nucleus acquisition. The decoupler is programmed to produce noise-modulated irradiation or composite pulse decoupling at two power levels. Suitable setting of the decoupler may produce either (a) nOe only, (b) proton decoupling only, or (c) both nOe and proton decoupling.

Exploitation of the TROSY effect is rather straightforward. In contrast to 15N-HSQC (Heteronuclear Single Quantum Coherence) or standard triple-resonance experiments based on 15N-HSQC, no radio frequency pulses or composite pulse decoupling should be applied on amide protons when HN spin is not in the transverse plane. Likewise the 15N decoupling should be... 

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]. 

The undesirable off-resonance effects become important in high field spectrometers. Schenker and von Philipsbom114 analysed them and found that they can be partly diminished by phase cycling and composite pulses, but a better solution would be increased rf intensities to shorten the pulses in both observing and decoupling channels. 

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

Multiple-pulse sequences are indispensable tools for the practical implementation of Hartmann-Hahn experiments in liquid state NMR. They often consist of thousands of defined rf pulses with or without intermediate delays and allow one to create a desired form of the effective Hamiltonian for a given class of spin systems. In the field of high-resolu-tion NMR, multiple-pulse sequences are also used for broadband het-eronuclear decoupling (Levitt et al., 1983 Shaka and Keeler, 1986). Composite pulses (Levitt, 1986) and shaped pulses (Warren and Silver, 1988 Freeman, 1991 Kessler et al., 1991) may be considered as special classes of multiple-pulse sequences. 

In addition to multiple-pulse sequences that were derived from heteronuclear decoupling experiments, a number of rf sequences have been specifically developed for homonuclear Hartmann-Hahn transfer. A systematic search for phase-alternated composite 180° pulses R expanded in an MLEV-16 supercycle was reported by Glaser and Drobny (1990). Several clusters of good sequences were found for the transfer of magnetization in the offset range of 0.Av. However, substantially improved Hartmann-Hahn sequences were found after the condition that restricted R to be an exact composite 180° pulse on-resonance was lifted. For example, the GD-2 sequence is based on R = 290° 390° 290°, which is a composite 190° pulse on-resonance and is one of the best sequences based on composite pulses of the form R = (Glaser and Drobny, 1990). 

Numerous such composite pulse decoupling sequences have been developed along these lines over the years (Table 9.2), the most widely used being WALTZ-16 whose basic inversion element is 90 180 c270 c (or more succinctly 123, hence the name). This provides very effective decoupling over a band-... 

Table 9.2. Selected composite-pulse sequences for broadband decoupling and spin-locking...

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