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.
