Whereas 13C spectroscopy without decoupling is time-consuming and sometimes difficult to interpret, decoupling the protons from the carbon nuclei while obtaining the carbon spectrum yields stunning results. We obtain a singlet for each magnetically-unique carbon. 13C spectroscopy garners information about the backbone of an organic molecule. The wonders of Fourier transform instruments allow us to surmount S/N problems associated with the low isotopic abundance of 13C and its rather poor magnetogyric ratio.
In CPD (Composite Programmed Decoupling) technique, we decouple the protons from the carbon by means of an intricate radio-frequency pulse sequence directed at the protons. This means that the protons are continually changing spin state during the time the carbon spectrum is acquired. As a result, the carbon “sees” only one net proton environment, yielding only one peak for each magnetically-unique carbon.
The area under the peaks is generally not proportional to the number of carbons present in these experiments, due to relaxation effects. In order to achieve good results in integration, you must wait 10-20s between pulses. Typically we are too impatient to do so…
This is the fastest 13C experiment to carry out. As a result, it is the “standard” 13C experiment on newer spectrometers. In the spectrum below, the peak at 15 ppm is the methyl carbon Ca and the peak at 28 ppm is the methylene carbon Cb. The quarternary carbon in the benzene ring Cc (the one without an attached proton) is at 144 ppm. The remaining three peaks at 125, 127, and 128 ppm are the remaining aromatic carbons. Note that even though there are five additional carbons in the ring, there are only three magnetically unique signals. Also note the triplet due to CDCl3 (solvent) at 77 ppm in the spectrum below.
Spectrum of Ethyl Benzene: