Carbon-13 NMR spectroscopy

How it works (spec 15.18)

About 1% of carbon atoms are carbon-13, which have nuclear spin. In a strong magnetic field they absorb radio waves at a frequency that depends on their chemical environment. The chemical shift, δ in ppm, is measured relative to TMS, tetramethylsilane Si(CH3)4, set at δ = 0. Samples are dissolved in a solvent with no interfering signals, such as CDCl3.

Number of peaks (spec 15.19 ii)

Each peak is a different carbon environment. Carbons related by symmetry give one peak.

• propan-1-ol CH3CH2CH2OH: 3 peaks; propan-2-ol (CH3)2CHOH: 2 peaks
• butanone CH3COCH2CH3: 4 peaks; pentan-3-one: 3 peaks
• benzene: 1 peak; methylbenzene: 5 peaks

In 13C spectra peak heights are not proportional to numbers of carbons.

Chemical shifts (spec 15.19 i)

Approximate ranges (use the data booklet in the exam):

Worked example

How could 13C NMR distinguish propanal from propanone?

1. Propanal CH3CH2CHO: 3 carbon environments, so 3 peaks, one near 200 ppm.
2. Propanone CH3COCH3: the two CH3 groups are equivalent, so 2 peaks, one near 205 ppm.

Answer: Propanal gives 3 peaks; propanone gives 2.

Check yourself

1. How many peaks are in the 13C NMR spectrum of propan-2-ol?
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   2. The two CH3 carbons are equivalent; the CH(OH) carbon is different.

2. How many peaks are in the 13C NMR spectrum of butanone, CH3COCH2CH3?
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   4. Every carbon is in a different environment.

3. How many peaks are in the 13C NMR spectrum of methylbenzene, C6H5CH3?
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   5. CH3, the ring carbon bonded to it, two ortho, two meta and one para: 5 environments.

4. A 13C NMR spectrum shows a peak at δ 208 ppm. Which group is present?
   • A C-O in an alcohol
   • An alkyl CH3
   • An arene ring carbon
   • The C=O of a ketone or aldehyde
   Show answer

   The C=O of a ketone or aldehyde. Aldehyde and ketone C=O carbons appear at about 190 to 220 ppm.

5. What is TMS used for in NMR spectroscopy?
   • As the solvent for all samples
   • As the reference compound, defined as δ = 0
   • To make the sample magnetic
   • To split the peaks
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   As the reference compound, defined as δ = 0. Shifts are measured relative to TMS.

6. How many peaks are in the 13C NMR spectrum of 2,2-dimethylpropane, C(CH3)4?
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   2. The four CH3 carbons are equivalent; the central carbon is different.

7. Which isomer of C4H10O has only two peaks in its 13C NMR spectrum?
   • Butan-1-ol
   • 2-methylpropan-2-ol
   • Butan-2-ol
   • 2-methylpropan-1-ol
   Show answer

   2-methylpropan-2-ol. (CH3)3COH has three equivalent CH3 carbons and one central carbon.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.