8.22:
¹³C NMR: ¹H–¹³C Decoupling
The low natural abundance of carbon-13 results in a negligible probability of there being two adjacent carbon-13 atoms, practically eliminating carbon–carbon J-coupling.
However, neighboring protons split the carbon signal according to the n+1 rule.
These complex signals are simplified by the broadband proton decoupling technique.
In this method, one transmitter produces radio frequency pulses that cause carbon-13 resonance.
A second transmitter generates a continuous broadband of radio frequencies that cause all the protons to flip rapidly between their two energy states.
So, the carbon-13 nuclei sense one average field from the protons, nullifying the spin-spin interactions.
The carbon-13 signals are now decoupled from the protons, and the peaks are not split into multiplets, as seen in the proton decoupled spectrum of 1-hexanol.
8.22:
¹³C NMR: ¹H–¹³C Decoupling
The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a multi-channel pulse sequence, where the sample is irradiated with two radio frequencies simultaneously. One transmitter produces pulses that excite carbon-13 nuclei, while another transmitter generates a continuous broadband radiofrequency that excites all the protons. The continuous irradiation of protons causes rapid transitions of protons between their spin states. As a result, carbon-13 nuclei can sense only one average spin state from the protons, and the interactions are effectively averaged to zero. All the signals are singlets in the resulting broadband proton-decoupled spectrum of carbon-13.