37.3.1—Carbon-13 NMR interpretation

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.