Q.3. (c) What do you understand by terms “Broad band decoupling” and “off resonance decoupling” in 13C NMR? Discusses their role in structure elucidation of organic compounds
Ans.3.(c) Broad band decoupling:To eliminate the complicating effects of the proton couplings in the 13C spectra, we must decouple the 1H nuclei by double irradiation at their resonant frequencies (80 MHz at 1.9 T, etc.).
This is an example of heteronuclear decoupling, but we do not wish merely to decouple specific protons; rather we wish to double irradiate all protons simultaneously while recording the 13Cspectrum. A decoupling signal is used that has all the 1H frequencies spread around 80 MHz, and is therefore a form of radiofrequency noise; spectra derived thus are 1H-decoupled, or noise decoupled. The alternative name broad band decoupled spectra simply takes cognizance of the fact that a widespread of decoupling radiofrequency can be produced by several electronic techniques, other than by simple noise modulation. The convenient notation 13C-{1H] can be used to identify proton decoupled carbon 13 NMR spectra; in the same way 31P-{1H} spectra are phosphorus-31 NMR spectra with all proton coupling to phosphorus removed by broad band or noise decoupling, and 15N-{1H} corresponds for nitrogen-15, etc
Off resonance decoupling:In an off resonance-decoupled 13C spectrum, the coupling between each carbon atom and each hydrogen attached directly to it is observed. The n+1 rule can be used to determine whether a given carbon atom has three, two, one, or no hydrogens attached. However, when off resonance decoupling is used, the apparent magnitude of the coupling constants is reduced, and overlap of the resulting multiplets is a less frequent problem. The off-resonance-decoupled spectrum retains the couplings between the carbon atom and directly attached protons (the one-bond couplings) but effectively removes the couplings between the carbon and more remote protons. In this technique, the frequency of a second radiofrequency transmitter (the decoupler) is set either upfield or downfield from the usual sweep width of a normal proton spectrum (i.e. off resonance). In contrast, the frequency of the decoupler is set to coincide exactly with the range of proton resonances in a true decoupling experiment. Furthermore, in of resonance decoupling, the power of the decoupling oscillator is held low to avoid complete decoupling. The off resonance decoupled spectrum is usually obtained separately, along with the proton decoupled spectrum.Off resonance decoupled spectrum of 1-propanol, in which the methyl carbon atom is split into a quartet, and each of the methylene carbons appears as a triplet. The observed multiplet patterns are consistent with the n+1 rule. If TMS has been added, its methyl carbons would have appeared as a quartet centered at δ = 0 ppm.
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