Gated experiments, spin decoupling

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 spin-echo experiment is particularly simple to set up as it does not require proton pulses or their calibration, a desirable property when the experiment was first introduced but of little consequence nowadays. The same results can, in fact, be obtained by the use of proton 180° pulses rather than by gating of the decoupler [23] (Fig. 4.15b). In this case the A period is broken in two periods of 1/27 separated by the simultaneous application of proton and carbon 180° pulses. These serve to refocus carbon chemical shifts but at the same time allow couplings to continue to evolve during the second A/2 period (Section 2.2). Hence, the total evolution period in which coupling is active is 1/7, as in the decoupler-gating experiment above, and identical modulation patterns are produced. It is this shorter pulsed form of the heteronuclear spin-echo that is widely used in numerous pulse sequences to refocus shift evolution yet leave couplings to evolve. 

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. 

In the carbon-13 experiments so far discussed, only a single radio-frequency pulse has been used to irradiate the spin system. This gave us information on the chemical shifts of the carbon nuclei in the molecule. The coupled spectrum obtained using gated decoupling (1.2.2) told us how many protons are bound to any one carbon atom however, this experiment requires a lot of time. There are however other experiments which give us this information... 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Fig. 2.42. I3C NMR spectra of D-camphor in tetradeuteriomelhanol at 15.08 MHz (a). /-modulation of aliphatic carbon signals depending on the decoupling delay z, a verification of Fig. 2.41 (b) proton broadband decoupled spectrum (c-e). /-modulated spin-echo experiments with z = 4, 6, and 8 ms for CH multiplicity analysis (f-g) spectra with off-resonance (0 and gated decoupling of protons (g) for comparison.

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Fig. 5.1. C NMR spectra of 5a-cholcstan-3-onc in dcutcriochloroform (50 mg/0.5 mL) (a) proton broadband-decoupled, 400 scans (b). /-modulated spin-echo experiment for quaternary carbon selection, 1000 scans (c-e) CH, Cl I2, and CH3 subspectra generated from linear combination of three DEPT experiments (see Section 2.9.3.2), 200 scans per experiment (f) gated proton-decoupled experiment for comparison.

Introducing a variable evolution time x, a series of experiments with gated Li decoupling leads to /-resolved spectra where the C, Li multiplets appear parallel to the F axis [13]. A similar experiment has been applied to P, Li spin systems to separate the P, Li coupling constants from the homonuclear P, P interactions in lithiated organophosphorous compounds [108]. 

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