Counting signals in ¹³C NMR
In a proton-decoupled ¹³C spectrum every non-equivalent carbon gives one single line. The number of lines tells you the symmetry, the position the type of carbon.
Rules and values
- One signal per set of equivalent carbons, no splitting (proton-decoupled)
- Monosubstituted benzene ring: 4 signals (ipso, ortho, meta, para)
- C=O of ketone/aldehyde 190–215 ppm, of acid/ester 160–185 ppm
- Aromatic C 110–160 ppm, C–O 50–90 ppm
- Alkyl C 5–45 ppm
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task
How many signals does the (proton-decoupled) ¹³C NMR spectrum of 1,2-dichloroethane show?
Number of ¹³C signals
Type whole numbers, e.g. 3. NMR shifts are typical values (±0.3 ppm), splitting follows the n+1 rule.
Worked example
How many signals does the (proton-decoupled) ¹³C NMR spectrum of ethyl methyl ether (methoxyethane) show?
Solution
- 1.3 carbon atoms; merge the equivalent ones.
- 2.alkyl C (5–45 ppm): 1
- 3.C–O (50–90 ppm): 2
- 4.3 signals in total.
Typical mistake: Simply counting the C atoms. Symmetry-equivalent C (e.g. the CH₃ of a tert-butyl group) give only one signal.
How many signals does the (proton-decoupled) ¹³C NMR spectrum of acetophenone (1-phenylethanone) show?
Solution
- 1.A monosubstituted benzene ring has a mirror plane: ipso C, 2 ortho C, 2 meta C and para C give 4 signals, not 6.
- 2.alkyl C (5–45 ppm): 1
- 3.C=O (ketone/aldehyde) (190–215 ppm): 1
- 4.aromatic C (110–160 ppm): 4
- 5.6 signals in total.
Typical mistake: Counting 6 or 1 signal for the benzene ring. With one substituent it is 4.
Common questions
Why are ¹³C integrals usually not useful?
Peak height depends strongly on how many H sit on the carbon and how fast it relaxes; quaternary carbons are often small. So you count signals rather than integrals.