Phase cycling coherence transfer pathway

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

Comparison of the results of the one-dimensional gradient supported 31P/15N 1H -se-HSQC experiment with phase-cycled HSQC and HMQC experiments gave relative S/N-ratios of 0.75 0.92 1 which was, under consideration of the suppression of one of the two possible coherence transfer pathways by the field gradients and the longer duration of the pulse sequence, interpreted in terms of a very good performance.25 The main benefits of the PFG-es-HSQC sequence were seen, however, in the excellent level of artefact suppression which allowed one to observe correlations via very small couplings even in cases where the active isotopomer is present in low natural abundance and its lines are normally obscured by residual parent signals (Fig. 3). 

Use a coherence transfer pathway for a noncoupled 13C similar to that in Eq. 12.4 or prepare a suitable vector diagram to show its coherence state after the evolution period of Fig. 12.4. Verify that the phase cycling procedure described for INADEQUATE cancels the single quantum coherence from this 13C. 

If the phase cycling used selects only one coherence transfer pathway, a two-dimensional spectrum with phase-twist lineshapes is obtained. However, the experiment can be easily modified to ensure that pure absorption (pure phase) lineshapes are obtained. The most commonly used modified acquisition schemes that allow pure-phase MQMAS spectra to be obtained include amplitude-modulated experiments, with hyper-complex (States) acquisition, and phase-modulated experiments, with delayed acquisition such as shifted echo or antiecho and split-methods. 

Fig. 22 Pulse sequence, coherence transfer pathway and phase-cycling for standard HET-COR experiment...

In solution-state NMR, many important experiments incorporate the creation and evolution of MQ coherence (MQC).5,6,84-86 Since MQC cannot be directly detected, experiments that follow the evolution of a MQC are inherently at least two-dimensional. This is the case with H- H DQ MAS spectroscopy. Figure 7 shows a corresponding pulse sequence and coherence transfer pathway diagram first, a DQC is excited, which subsequently evolves during an incremented time period q the DQC is then converted into observable single-quantum (SQ) coherence (SQC), which is detected in the acquisition period, q. To select the desired coherence transfer pathways, e.g., only DQC during q, a phase cycling scheme is employed.79,80 Pure absorption-mode two-dimensional line shapes are ensured by the selection of symmetric pathways such that the time-domain... 

Figure 5.40. The DQF-COSY experiment and coherence transfer pathway. The pulses are phase-cycled as described in the text to select the pathway shown with quadrature detection observing the p = — 1 magnetisation. The period 8 allows for rf phase changes and is typically of only a few microseconds.

Figure 5.41. The gradient-selected DQF-COSY experiment and coherence transfer pathway. No phase-cycling is needed as the required pathway is selected with gradient ratios of 1 2. Both gradient pulses are applied within spin-echoes for phase-sensitive presentations. Note only one pathway is retained from the double-quantum filter.

Like phase cycling, field gradient pulses can be used to select particular coherence transfer pathways. During a pulsed field gradient the applied... 

Phase cycling selects coherence transfer pathways which are characterized by a particular change of coherence orders and further pathways with Aptotai = XApi N-r and N = 0, 1, 2, 3... (N, r see equation [2-16]). 

Due to the application of several gradients a lost of signal intensity must be accepted because of the suppression of coherence transfer pathways which otherwise would contribute to the detected signal and which are not suppressed by phase cycling. 

Figure 5.41. The gradient-selected DQF-COSY experiment and coherence transfer pathway. No phase cycling is needed as the...

Figure 5.75. The INADEQUATE sequence and the corresponding coherence transfer pathway. The experiment selects double-quantum coherence during the evolution period (analogous to the selection in the DQF-COSY experiment, Section 5.6.1) with a suitable phase cycle or optionally via gradient selection (shown greyed). In doing so, it rejects all contributions from all uncoupled spins.

Phase cycling is a fundamental procedure in most NMR experiments and is used not only for removing instrument artifacts, but also for selecting or suppressing signals, specially for achievement of specific coherence transfer pathways [5,13]. In NMR experiments, one must be aware of the importance of phase cycling, which sometimes is more difficult to understand than the basic aspects of the pulse sequences. 

Fig. 11. Left. Experimental schemes for the acquisition of 3D DACSY spectra on quadrupolar spins. The pulse sequence ((a) top) is analogous to the one used in 2D DAS experiment it contains three 90° pulses with relative phase cycled so as to select antiecho coherence transfer pathways, and initial and final sample spinning angles chosen so as to average out second and fourth rank anisotropies, (b) Sequence of r-space cross-...

As explained in depth in the next sections, most of the advanced high-resolution experiments on quadrupolar nuclei are based on the selection of specific coherences and on the transfer of these coherences along the selected pathways, which always terminate at the observable SQ coherence p = — 1. Most of the time, the selection is done using nested phase-cycling of the radiofrequency (rf) pulses included in the pulse sequence [34]. Recently, new methods have been proposed to optimize the nested phase-cycling procedure, including the schemes referred to as cogwheel [36-39] and multiplex [40—421. 

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