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Count ¹³C NMR Signals
The number of signals in a ¹³C NMR spectrum equals the number of chemically distinct carbon environments: carbons related by symmetry share one signal, so a molecule usually shows fewer peaks than it has carbons. Count the unique carbon environments in each structure.
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Common Questions
How do you count the number of signals in a ¹³C NMR spectrum?
Count the number of chemically distinct (nonequivalent) carbons. Carbons related by symmetry — a mirror plane, rotation axis, or other symmetry operation — share one signal. The number of ¹³C NMR signals equals the number of unique carbon environments.
Why do ¹³C spectra usually show fewer signals than there are carbons?
Symmetry-equivalent carbons share a single ¹³C signal, so a molecule with equivalent carbons shows fewer peaks than its total carbon count. The CH carbons of a para-substituted benzene ring often collapse into two signals because of the ring's mirror symmetry.
How many ¹³C signals does benzene show?
Benzene shows a single ¹³C NMR signal. All six carbons are related by the ring's symmetry and occupy identical environments, so they resonate at the same chemical shift.
Does each ¹³C signal represent one carbon?
Not necessarily. Each ¹³C signal represents one distinct carbon environment, which may correspond to several symmetry-equivalent carbons. Standard proton-decoupled ¹³C spectra are not integrated for count, so peak height does not reliably indicate how many carbons share a signal.
Why don't bond rotations and ring flips create extra ¹³C signals?
Because they are fast. At room temperature, single-bond rotation and ring flipping are much faster than the NMR timescale, so carbons exchanged by that motion appear as one averaged signal (the three methyl carbons of a tert-butyl group, for example). Mirror-image (enantiotopic) carbons are also equivalent because the solvent is achiral.