Why is 13C used rather than 12C in NMR?

13C has nuclear spin; 12C does not, so only 13C is detected.

What is meant by chemical shift?

The position of a peak, δ in ppm, relative to the reference compound TMS (0 ppm).

When are two carbon atoms in the same environment?

When they are bonded to the same atoms and groups in the same way, usually because of symmetry.

How many peaks does propan-2-ol give in its 13C NMR spectrum?

Two: the two equivalent CH₃ carbons give one peak and the CH(OH) carbon gives the other.

How many peaks does 1,4-dimethylbenzene give?

Three: CH₃, the two methyl-bearing ring carbons and the four ring CH carbons.

Why does a carbon bonded to oxygen have a higher chemical shift?

Oxygen is electronegative, withdrawing electron density, so the carbon is deshielded.

Can the number of peaks be greater than the number of carbon atoms?

No. It is equal to the number of carbon environments, which is at most the number of carbons.

How many peaks does 2-methylpropan-1-ol give?

Three: two equivalent CH₃ carbons, one CH carbon and one CH₂OH carbon.

How do you justify the number of peaks in an exam?

Identify the equivalent carbons using symmetry, then count the distinct environments.

Carbon-13 NMREdexcel A-Level Chemistry: Flashcards

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What does 13C NMR spectroscopy show?

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What does 13C NMR spectroscopy show?: The positions of carbon atoms in a molecule: one peak for each carbon environment.
Why is 13C used rather than 12C in NMR?: 13C has nuclear spin; 12C does not, so only 13C is detected.
What is meant by chemical shift?: The position of a peak, δ in ppm, relative to the reference compound TMS (0 ppm).
When are two carbon atoms in the same environment?: When they are bonded to the same atoms and groups in the same way, usually because of symmetry.
How many peaks does propan-2-ol give in its 13C NMR spectrum?: Two: the two equivalent CH₃ carbons give one peak and the CH(OH) carbon gives the other.
How many peaks does 1,4-dimethylbenzene give?: Three: CH₃, the two methyl-bearing ring carbons and the four ring CH carbons.
Why does a carbon bonded to oxygen have a higher chemical shift?: Oxygen is electronegative, withdrawing electron density, so the carbon is deshielded.
What is the chemical shift range for C–O carbons?: 50–90 ppm.
What is the chemical shift range for ketone and aldehyde C=O carbons?: 190–220 ppm.
What is the chemical shift range for alkyl (C–C) carbons?: 5–40 ppm.
Can the number of peaks be greater than the number of carbon atoms?: No. It is equal to the number of carbon environments, which is at most the number of carbons.
How many peaks does 2-methylpropan-1-ol give?: Three: two equivalent CH₃ carbons, one CH carbon and one CH₂OH carbon.
How do you justify the number of peaks in an exam?: Identify the equivalent carbons using symmetry, then count the distinct environments.

Exam questions on Carbon-13 NMR

A technician in a teaching laboratory has two unlabelled bottles of propanol isomers, one containing propan-1-ol and the other propan-2-ol. Both have the molecular formula C₃H₈O. She plans to tell them apart by recording the 13C NMR spectrum of each.
Propan-2-ol, CH₃CH(OH)CH₃, gives only two peaks in its 13C NMR spectrum. Explain why.2 marks
A pharmaceutical chemist records the 13C NMR spectrum of the solvent ethyl ethanoate, CH₃COOCH₂CH₃. It shows four peaks, at chemical shifts (δ) of 14, 21, 60 and 171 ppm.
Explain why the peak at δ 171 ppm is at a much higher chemical shift than the peak at δ 14 ppm.2 marks
A laboratory holds four isomeric alcohols of molecular formula C₄H₁₀O: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. A technician uses 13C NMR spectroscopy to tell them apart. Typical chemical shift ranges (ppm): C–C 5–40; C–O 50–90.
Deduce the number of peaks in the 13C NMR spectra of 2-methylpropan-2-ol and of 2-methylpropan-1-ol, and justify your answers.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).