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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 — symmetry is the key.
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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 two methyls of an isopropyl group, for example). Mirror-image (enantiotopic) carbons are also equivalent because the solvent is achiral.