Proton Decoupled Difference Spectroscopy

Difference spectroscopy, as the name implies, requires spectral subtraction. Two spectra are acquired. One in which the proton of interest is decoupled and a second reference spectrum in which the irradiation is appHed in an isolated region far from the nearest proton resonance. The two spectra are then subtracted from one another, the resulting difference spectrum highHghting protons that were affected by the decoupling process [34]. Several excellent examples of applications of this technique are found in the monograph by Nakanishi [24]. 

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FIGURE 23 The sp lsp bonding ratio i.s determined by proton-decoupled NMR spectroscopy. The solid symbols represent results reported by different authors [83]. 

The signal dispersion is much greater than that observed in PMR spectroscopy. This, combined with the fact that the signals in the proton-decoupled spectra are usually sharp singlets, is invaluable for the detailed analysis of complex systems, of mixtures or for detecting subtle structural differences (see, e.g., Section V). 

NOE difference spectroscopy has proved to be a useful method for studying the spatial proximity of protons in a molecule. In this experiment the H NMR spectrum is recorded during the decoupling of a particular proton (measurement 1) an additional... 

Figure 2.19 illustrates NOE difference spectroscopy with a-pinene (1) decoupling of the methyl protons at 7.27ppm (experiment c) gives a significant NOE on the proton at... 

One of the most useful proton nuclear magnetic resonance ( H-NMR) one-dimensional (a single-frequency axis) techniques to be applied in structure determination is difference spectroscopy. This approach involves subtracting a spectrum obtained under the conditions of some perturbation from a normal, unperturbed spectrum. The resulting difference spectrum reveals the spectral changes caused by the perturbation, whereas those signals that remain unaffected are nulled. The perturbation can be a decoupling irradiation, a nuclear Overhauser enhancement, or even a population transfer experiment (135, 328). 

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