Decoupler gating

Si NMR has been used to study some polytrimethylsilylated sugars. (537, 538) Selective decoupling, gated to remove the negative... 

Figure 4.13. J-modulated spin-echo sequences, (a) The decoupler-gated variant and (b) the pulsed variant.

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. 

Figure 9 Pulse sequence and response for a single-pulse experiment (a) with decoupling (b) with decoupler gating.

As mentioned in Sec. II.D.3, the NOE is also a consideration for quantitative analysis. The use of decoupler gating ensures that the NOE will be suppressed, but it is not always necessary, or even desirable, to eliminate the S/N enhancement that the NOE affords. If the molecules in the sample move relatively slowly (as do polymers in solution), and if all or most of the X nuclei are directly bonded to Y s (e.g., if most carbons are protonated), then most NOE s tend to be equivalent. This happens because of spin diffusion, a process by which magnetization migrates to neighboring spins. Correct relative peak areas are maintained for most peaks, with a significant decrease in the experiment time. Some analogous concerns apply to quantitation in solid-state NMR these will be discussed in Sec. II.E.2. 

Quantification in 1-NMR spectroscopy necessitates optimisation of experimental conditions, such as pulse width, recycle delay and decoupler gating, as well as determination of the inherent relaxation... 

Figure 1.10. NMR spectra of 2,4,6-trichloropyrimidine [CeDe, 75% v/v 25 °C, 20 MHz], (a) NMR spectrum without proton decoupling (b) NOE enhanced coupled NMR spectrum (gated decoupling)...

Figure 2.9. NMR spectra of 3,4-dimethoxy-p-methyl-p-nitrostyrene (9) [CDCI3, 25 C, 20 MHz], (a, b) H broadband decoupled, (a) complete spectrum with CH3 quartets at Sc = 14.1 and 56.0 (b, c) decoupled and coupled partial spectrum of benzenoid and alkene carbon atoms, (c) obtained by gated decoupling...

From which compound were the INADEQUATE contour plot and C NMR spectra 21 obtained Conditions (CD3)2CO, 95 % v/v, 25 °C, 100 MHz. (a) Symmetrised INADEQUATE contour plot with C NMR spectra (b) H broadband decoupled spectrum (c) NOE enhanced coupled spectrum (gated decoupling) (d) expansion of multiplets of (c). 

Conditions CDCI3, 25 °C, 100 MHz ( C), 400 MHz H). (a-e) C NMR spectra (a,b) //broadband decoupled spectra (c,d) NOE enhanced coupled spectra (gated decoupling) with expansion (e) of the multiplets in the sp shift range (f) //NMR spectrum with expanded multiplets. 

Conditions (CD3)2CO, 25 °C, 400 MHz (H), 100 MHz ( C), 40.55 MHz ( N). (a) //NMR spectrum with expanded partial spectra and integrals (b, c) C NMR spectra, in each case showing proton broadband decoupled spectrum below and gated decoupled spectrum above, (b) ali-phatle resonances and (c) heteroaromatic resonances (d) N NMR speetrum, coupled, with expanded sections and integrals. 

Conditions CDCI3, 25°C, 200 MHz ( //), 50 MHz ( C). (a) NMR spectrum with expanded multiplets (b) NOE difference spectrum, irradiated at Sff = 1.87, (c) C NMR partial spectra, each with H broadband decoupled spectrum below and NOE enhanced coupled speetrum (gated decoupling) above (d) CH COSY diagram ( empty shift ranges omitted). 

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