Why is there no coupling to protons in normal 13C NMR spectra
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص 418
2025-06-01
601
We get the singlets consistently seen in carbon spectra because of the way we record the spectra. The values of 1JCH are so large that, if we recorded 13C spectra with all the coupling constants, we would get a mass of overlapping peaks. When run on the same spectrometer, the frequency at which 13C nuclei resonate turns out to be about a quarter of that of the protons. Thus a ‘400 MHz machine’ (remember that the magnet strength is usually described by the frequency at which the protons resonate) gives 13C spectra at 100 MHz. Coupling constants (1JCH) of 100–250 Hz would cover 2–5 ppm and a CH3 group with 1JCH of about 125 Hz would give a quartet covering nearly 8 ppm (see the example on the previous page).
Since the proton-coupled 13C spectrum can so easily help us to distinguish CH3, CH2, CH, and quaternary carbons, you might wonder why they are not used more. The above example was chosen very carefully to illustrate proton-coupled spectra at their best. Unfortunately, this is not a typical example. More usually, the confusion from overlapping peaks makes this just not worthwhile. So 13C NMR spectra are recorded while the whole 10 ppm proton spectrum is being irradiated with a secondary radio frequency source. The proton energy levels are equalized by this process and all coupling disappears. Hence the singlets we are used to seeing. For the rest of this chapter, we shall not be introducing new theory or new concepts; we shall be applying what we have told you to a series of examples where spectroscopy enables chemists to identify compounds.
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