Off-resonance effects

Fig. 9.8. Irradiation scheme used in NOE experiments to minimize off-resonance effects. The r.f. is alternately placed at vn, vo + 5, vo — <5. The spectra obtained upon irradiation at Vo are subtracted from those obtained upon irradiation at Vo-...

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. 

Thus, we can avoid off-resonance effects by making B as strong as possible (highest power pulse possible, shortest duration) and by making v0 — vr as small as possible (avoid having resonances peaks very far from the center of the spectral window). 

Typical 13C rectangular pulses with y Blmax = 20 kHz (12.5 and 25 /is for 90° and 180° pulses) generate large off-resonance effects for chemical shift range of 200 ppm required for 13C. These effects can lead to a significant attenuation or complete loss of signal in multi-pulse INADEQUATE pulse sequences. Practical implementations of INADEQUATE pulse sequences must, therefore, include some sort of compensation of off-resonance effects. Several approaches have been reported to date. 

Because the 17th pulse is not compensated for off-resonance effects, the amplitude of the effective field for a spin i increases with the offset p, ... 

Relaxation Rate Measurements. - Measurement and analysis of traditional laboratory frame heteronuclear relaxation parameters R, R2 and NOE were summarised by Palmer in a review. The publication concentrates on the dynamic processes on ps-ns time scale in proteins. The theoretical part has a general character, while the experimental section and examples deal mainly with N relaxation. A source of systematic error in the commonly used T2 CPMG measurements was highlighted by Korzhnev et al The authors observed offset-dependent difference between T2 and Tip values. This difference was attributed to the off-resonance effects of N 180° pulses. When uncorrected, the error can be misinterpreted as a contribution from a slow conformational exchange. The authors suggested a numerical correction procedure. 

Thus, the procedure is to mn an experiment with x = 0 and adjust (phase) the spectrum to be negative absorption. After having waited >5 Ti repeat the experiment with an incremented x using the same phase adjustments, until the signal passes through the null condition (see Fig. 2.23), thus defining x uii, which may be a different value for each resonance in the spectrum. Errors may be introduced from inaccurate 180° pulses, from off-resonance effects (see Section 3.2) and from waiting for insufficient periods between acquisitions, so the fact that these values are estimates cannot be overemphasised. 

Probe tuning is necessary for a number of reasons. Other than the fundamental requirement for maximising sensitivity, it ensures pulse-widths can be kept short which in turn reduces off-resonance effects and minimises the power required for broadband decoupling. A properly tuned probe is also required if previously calibrated pulse-widths are to be reproducible, an essential feature for the successful execution of multipulse experiments. 

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