37.3 Carbon-13 NMR spectroscopy

A carbon-13 NMR spectrum has one signal for each chemically distinct carbon environment, ignoring usually small isotope effects. Symmetry can make several carbons equivalent and reduce the signal count.

Count environments in the structure, then compare the expected number and approximate chemical shifts with the spectrum. Use shift regions as support, not as a substitute for connectivity.

Propane gives two carbon environments because the two terminal CH₃ groups are equivalent; propanone gives two environments for the methyl carbons and the carbonyl carbon.

The number of carbon atoms is not automatically the number of signals, and signal intensity is not a reliable direct carbon count in routine 13C spectra.

The number of carbon-13 NMR signals equals the number of chemically distinct carbon environments in the molecule. Symmetry and rapid equivalence reduce the count.

Label carbons, compare their attached groups and surroundings, then merge positions related by a symmetry operation. Do not count equivalent carbons twice.

Butan-2-one has four carbons but three environments: the terminal methyl groups are not equivalent because one is next to C=O and the other is not.

Signal count is a structural count, not a carbon atom count and not a direct measure of signal height.