Proton Broad Band Decoupling in 13C NMR Spectroscopy

In a 13C XH experiment, a 13C— 1H multiplet can be selectively decoupled by an B2 field whose frequency matches the Larmor frequency of the coupling protons. In routine 13C NMR, however, usually all 13C H multiplets are decoupled for reasons of sensitivity and simplicity. This is achieved when the decoupling field B2 covers the range of all proton Larmor frequencies. For the protons in organic compounds in a magnetic field Bq of 2.3 Tesla, this is at least 1 kHz at about 90 MHz. Decoupling fields with large 

6 Double Resonance Techniques used in 13C NMR Spectroscopy frequency ranges can be realized either by a very large rf power B2, so that 

The maximum NOE enhancement factor t]A (X) observable for the A signal in an A X) experiment mainly depends on the gyromagnetic ratios of A and X, if the A nuclei are 

Using the gyromagnetic ratios of 13C and H, yi3c = 6.726 and y H = 26.752 rad s-1 gauss-1, the maximum NOE enhancement factor observable in C H] spectra is almost 2, in good agreement with measurements (Fig. 2.20) [33], 

In summary, NOE sensitivity enhancements are attained in heteronuclear NMDR when the nucleus with low y is observed while the nucleus with high y is decoupled. On the other hand, negligible enhancement of H NMR sensitivity can be expected from H 13C) experiments, the enhancement factor being only jiH(13C) = 0.216 [34]. 

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