Why are several analytical techniques used together?

Together they allow the structures of new compounds to be confirmed.

Why is ¹³C NMR simpler than ¹H NMR?

Each peak is a single line with no splitting, and there are fewer peaks.

What is chemical shift?

The position of a peak, in ppm on the delta scale, relative to TMS.

What does the chemical shift depend on?

The molecular environment of the atom, such as nearby electronegative atoms or C=O groups.

What is the standard used in NMR, and what is its δ value?

Tetramethylsilane, TMS, Si(CH₃)₄, at δ = 0.

Why does TMS give one peak?

All four carbon atoms are equivalent.

Give two other reasons why TMS is a suitable standard.

It is unreactive and volatile, so it does not react with the sample and is easily removed.

How many peaks does propanone give in ¹³C NMR?

Two: the two CH₃ carbons are equivalent, and the C=O carbon is different.

What are the ¹³C shift ranges for C=O in esters/acids and in aldehydes/ketones?

Esters and acids 160–185 ppm; aldehydes and ketones 190–220 ppm.

What are equivalent carbon atoms?

Carbon atoms in identical environments, which give a single peak.

Principles of NMR and carbon-13 NMRAQA A-Level Chemistry: Flashcards

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What information does NMR give about a molecule?

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What information does NMR give about a molecule?: The position (environment) of ¹³C or ¹H atoms in the molecule.
Why are several analytical techniques used together?: Together they allow the structures of new compounds to be confirmed.
Why is ¹³C NMR simpler than ¹H NMR?: Each peak is a single line with no splitting, and there are fewer peaks.
What is chemical shift?: The position of a peak, in ppm on the delta scale, relative to TMS.
What does the chemical shift depend on?: The molecular environment of the atom, such as nearby electronegative atoms or C=O groups.
What is the standard used in NMR, and what is its δ value?: Tetramethylsilane, TMS, Si(CH₃)₄, at δ = 0.
Why does TMS give one peak?: All four carbon atoms are equivalent.
Give two other reasons why TMS is a suitable standard.: It is unreactive and volatile, so it does not react with the sample and is easily removed.
How many peaks does propanone give in ¹³C NMR?: Two: the two CH₃ carbons are equivalent, and the C=O carbon is different.
What are the ¹³C shift ranges for C=O in esters/acids and in aldehydes/ketones?: Esters and acids 160–185 ppm; aldehydes and ketones 190–220 ppm.
What is the ¹³C shift range for a C–O carbon?: 50–90 ppm.
What is the ¹³C shift range for C–C in an alkyl group?: 5–40 ppm.
What are equivalent carbon atoms?: Carbon atoms in identical environments, which give a single peak.

Exam questions on Principles of NMR and carbon-13 NMR

Nuclear magnetic resonance (NMR) spectroscopy is used together with other analytical techniques to confirm the structures of new compounds. A chemist records ¹³C NMR spectra of samples dissolved in a suitable solvent, with tetramethylsilane (TMS), Si(CH₃)₄, added as a standard.
Explain what is meant by chemical shift and why the chemical shift of a carbon atom depends on its molecular environment.2 marks
Compound Y has the molecular formula C₄H₈O₂. Its ¹³C NMR spectrum has four peaks, at δ = 14, 21, 60 and 171 ppm. Data booklet shift ranges (δ / ppm): C–C 5–40; C–O 50–90; C=C 90–150; C=O (esters, carboxylic acids) 160–185; C=O (aldehydes, ketones) 190–220.
State the type of carbon atom responsible for the peak at δ = 171 ppm and for the peak at δ = 14 ppm in the spectrum of Y, using the shift ranges given.2 marks
Compound Z has the molecular formula C₃H₆O. Its ¹³C NMR spectrum has two peaks, at δ = 30 and δ = 206 ppm. Data booklet shift ranges (δ / ppm): C–C 5–40; C–O 50–90; C=C 90–150; C=O (esters, carboxylic acids) 160–185; C=O (aldehydes, ketones) 190–220.
Deduce the structure of Z. Explain your reasoning using the spectrum.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).