What mistakes do students make with Nuclear magnetic resonance spectroscopy (A-level only)?

Reading an integration value as the actual number of protons rather than as a ratio. Counting the protons on the signal's own carbon when applying the n+1 rule, instead of those on the neighbouring carbons. Splitting a signal with protons that are equivalent to it, which do not split each other.

AQA · A-Level · Chemistry

Course version: 7405 · Version from Sept 2016

Students should be able to: explain why TMS is a suitable substance to use…

Full specification wording

Nuclear magnetic resonance spectroscopy (A-level only): Students should be able to: explain why TMS is a suitable substance to use as a standard 1 13 use H NMR and C NMR spectra and chemical shift data from the Chemistry Data Booklet to suggest possible structures or part structures for molecules use integration data from H NMR spectra to determine the relative numbers of equivalent protons in the molecule use the n+1 rule to deduce the spin–spin splitting patterns of adjacent, non-equivalent protons, limited to doublet, triplet and quartet formation in aliphatic compounds.

Tetramethylsilane works as the standard because its twelve protons are all equivalent and give one sharp peak; it is unreactive, non-toxic and soluble in organic solvents, it boils at about 27 °C…

Checked against the AQA 7405 specification (opens in a new tab) on 20 September 2026.

Point 4 of 4
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Tetramethylsilane giving a single peak because all of its protons are equivalent.
A further property of tetramethylsilane, such as being unreactive, non-toxic or easily removed because it is volatile.
Using an integration ratio to give the relative numbers of equivalent protons.
Applying the n+1 rule, for example a quartet because the neighbouring carbon carries three protons.
A part structure consistent with the shift, the integration and the splitting taken together.

Common mistakes and how to put them right

Reading an integration value as the actual number of protons rather than as a ratio.
Counting the protons on the signal's own carbon when applying the n+1 rule, instead of those on the neighbouring carbons.
Splitting a signal with protons that are equivalent to it, which do not split each other.
Expecting an O-H proton to split its neighbours, when it normally appears as a singlet.
Saying tetramethylsilane is used because it is cheap, which is not a chemical reason the scheme credits.

Exam tips

Write the reasoning the mark scheme uses, quartet because the neighbouring carbon has three hydrogens, rather than just quartet.
Take each peak in the order shift, then integration, then splitting, and assemble the fragments at the end.
If a spectrum alone cannot settle the structure, say what is missing, for example that overlapping peaks hide the splitting.

Try the lesson

The start of the Nuclear magnetic resonance spectroscopy (A-level only): Students should be able to: explain why TMS is a suitable substance to use as a standard 1 13 use H NMR and C NMR spectra and chemical shift data from the Chemistry Data Booklet to suggest possible structures or part structures for molecules use integration data from H NMR spectra to determine the relative numbers of equivalent protons in the molecule use the n+1 rule to deduce the spin–spin splitting patterns of adjacent, non-equivalent protons, limited to doublet, triplet and quartet formation in aliphatic compounds. lesson, made from checked teaching notes for this specification point.

Have a go first

Why is TMS a suitable standard for ¹H NMR spectroscopy?

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Listen

Tetramethylsilane works as the standard because its twelve protons are all equivalent and give one sharp peak; it is unreactive, non-toxic and soluble in organic solvents, it boils at about 27 °C so it is easily removed, and its peak lies to the right of nearly every other signal so nothing overlaps it. Integration gives the ratio of protons in each environment, never the absolute number, so a ratio of three to two to three needs the molecular formula before it becomes a structure. The n+1 rule says a signal is split into one more line than the number of non-equivalent protons on the neighbouring carbons: a quartet means three neighbours, a triplet two, a doublet one. An ethyl group therefore always shows a triplet and a quartet. A strong answer earns marks for these points. tetramethylsilane giving a single peak because all of its protons are equivalent. a further property of tetramethylsilane, such as being unreactive, non-toxic or easily removed because it is volatile. using an integration ratio to give the relative numbers of equivalent protons. applying the n+1 rule, for example a quartet because the neighbouring carbon carries three protons. Exam tip: Write the reasoning the mark scheme uses, quartet because the neighbouring carbon has three hydrogens, rather than just quartet.

See it

A top-down flowchart. The node TMS standard branches to four properties: all twelve protons equivalent, which leads to one sharp peak; unreactive, non-toxic, soluble in organic solvents; boils at about 27 °C so easily removed; and peak to the right of nearly every other signal, which leads to nothing overlaps it. A separate node, n+1 rule, branches to three outcomes: quartet for three neighbouring protons, triplet for two neighbouring protons, and doublet for one neighbouring proton. — It shows the properties that make TMS a suitable standard and the n+1 rule splitting patterns for doublet, triplet and quartet.

Key words

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integration ratios

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the n+1 rule

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See every definition

integration ratios: Give the relative numbers of equivalent protons in the molecule, never the absolute number, so a ratio needs the molecular formula before it becomes a structure.
the n+1 rule: A signal is split into one more line than the number of non-equivalent protons on the neighbouring carbons, limited to doublet, triplet and quartet formation in aliphatic compounds.

Label the diagram

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The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The node TMS standard branches to four properties: all twelve protons equivalent, which leads to one sharp peak; unreactive, non-toxic, soluble in organic solvents; boils at about 27 °C so easily removed; and peak to the right of nearly every other signal, which leads to nothing overlaps it. A separate node, n+1 rule, branches to three outcomes: quartet for three neighbouring protons, triplet for two neighbouring protons, and doublet for one neighbouring proton.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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Test your understanding of Nuclear magnetic resonance spectroscopy (A-level only): Students should be able to: explain why TMS is a suitable substance to use as a standard 1 13 use H NMR and C NMR spectra and chemical shift data from the Chemistry Data Booklet to suggest possible structures or part structures for molecules use integration data from H NMR spectra to determine the relative numbers of equivalent protons in the molecule use the n+1 rule to deduce the spin–spin splitting patterns of adjacent, non-equivalent protons, limited to doublet, triplet and quartet formation in aliphatic compounds.

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