Gated decoupling, nuclear Overhauser effect

Gated decoupling The decoupler is gated during certain pulse NMR experiments, so spin decoupling occurs only when the decoupler is switched on and not when it is switched off used to eliminate either H- C spincoupling or nuclear Overhauser effect in a ID C spectrum, and employed as a standard technique in many other H-NMR experiments, such as APT and y-resolved. 

The use of reference deconvolution for the correction of artefacts in nuclear Overhauser effect difference spectroscopy [9] is illustrated by the spectra of fig. 3. The experimental technique used here differs slightly from that normally encountered in using a control spectrum in which the preirradiation is gated off rather than shifted in frequency, and in keeping the decoupler and transmitter at the same frequency. These modifications were... 

In pulsed or gated decoupling of protons (broadband decoupling only between FIDs), coupled C NMR spectra are obtained in which the CH multiplets are enhanced by the nuclear Overhauser effect (NOE, see Section 1.9). This method is used when CH coupling constants are required for structure analysis because it enhances the multiplets of carbon nuclei attached to protons the signals of quaternary carbons two bonds apart from a proton are also significantly enhanced. Figure 1.10 demonstrates this for the carbon nuclei in the 4,6-positions of 2,4,6-trichloropyrimidine. 

At least for the present, proton saturation transfer experiments will be the most tractable. For accurate measurement of the degree of saturation, adequate time must have elapsed for equilibrium to be established. This time is on the order of five times the relaxation time, or k. Hence, for the gated decoupling of the protons in (7) -allyl)Fe(CO)3l, decoupling must be carried out for 10-20 seconds before the 90° pulse to achieve reproducible results. A second feature, which can often be observed in syn and anti protons, is the signal enhancement arising from the nuclear Overhauser effect (NOE). Thus, the observed intensity is a function of both the NOE and the magnetization transferred into the site. Quantitative determinations must correct for the NOE enhancements. 

We will briefly consider in this section various aspects of homonuclear spin-de-coupling experiments and nuclear Overhauser effect (NOE) difference spectra. Obviously any detailed treatment is far beyond the size limitations of this chapter. Moving next to ID NMR techniques, we wiU briefly consider the utilization of selective spin-population transfer (SPT) and experiments which rely on these principles such as INEPT and DEPT, off-resonance proton decoupling techniques, decoupler gating experiments, and finally spin—lattice or Tj relaxation techniques. 

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