Proton NMR - Carbon Satellites and Spinning Sidebands

Carbon Satellites and Spinning Sidebands

Occasionally, small peaks can be seen shouldering the main 1H NMR peaks. These peaks are not the result of proton-proton coupling, but result from the coupling of 1H atoms to an adjoining carbon-13 (13C) atom. These small peaks are known as carbon satellites as they are small and appear around the main 1H peak i.e. satellite (around) to them. Carbon satellites are small because of the only very few of the molecules in the sample have that carbon as the rare NMR-active 13C isotope. As always for coupling due to a single spin-1/2 nucleus, the signal splitting for the H attached to the 13C is a doublet. The H attached to the more abundant 12C is not split, so it is a large singlet. The net result is a pair of evenly-spaced small signals around the main one. If the H signal would already be split due to H–H coupling or other effects, each of the satellites would also reflect this coupling as well (as usual for complex splitting patterns due to dissimilar coupling partners). Other NMR-active nuclei can also cause these satellites, but carbon is most common culprit in the proton NMR spectra of organic compounds.

Sometimes other peaks can be seen around 1H peaks, known as spinning sidebands and are related to the rate of spin of an NMR tube. These are experimental artifacts from the spectroscopic analysis itself, not an intrinsic feature of the spectrum of the chemical and not even specifically related to the chemical or its structure.

Carbon satellites and spinning sidebands should not be confused with impurity peaks.