What is reading a proton spectrum?

Three pieces of information come from each set of peaks: - Number of peaks equals the number of proton environments. - Integration (the step height or area) gives the whole-number ratio of protons. - Splitting uses the n+1 rule: a singlet has no adjacent protons, a doublet has one, a triplet two, a quartet three. Typical shifts include R-CH_3 near 0.9, O-CH near 3.5, and O-CHO (aldehyde) near 9.7\ ppm.

Pearson Edexcel Edexcel 2022-style practice: (a) State what carbon-13 NMR tells you about a molecule. (b) Ethanol, CH_3CH_2OH, is run by proton NMR. Predict the number of peaks, their splitting, and the integration ratio.

Count environments for carbon, then apply the n+1 rule for protons. (a) The number of peaks equals the number of different carbon environments in the molecule (1). (b) Ethanol has three proton environments: CH_3, CH_2 and OH, so three peaks (1). The CH_3 is split by the two CH_2 protons into a triplet (n+1 = 3); the CH_2 is split by the three CH_3 protons into a quartet (n+1 = 4); the OH is a singlet (it does not normally couple) (1). Integration ratio CH_3 : CH_2 : OH = 3 : 2 : 1 (1).

EnglandChemistrySyllabus dot point

How do spectroscopy and chromatography let us determine the structure of an unknown compound?

Proton and carbon-13 NMR spectroscopy, chemical shift and splitting, combining spectroscopic data to determine structures, and chromatography (TLC, gas and HPLC) for separation and analysis.

An Edexcel 9CH0 answer covering proton and carbon-13 NMR, chemical shift and splitting, combining spectroscopic data to deduce structure, and chromatography techniques.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-02

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What this topic is asking

Edexcel Topic 19 wants you to interpret proton and carbon-13 NMR spectra (chemical shift, integration and splitting), combine NMR with mass spectrometry and infrared data to determine a full structure, and describe the principles of thin-layer, gas and high-performance liquid chromatography for separation and analysis.

The answer

NMR spectroscopy

How NMR works

Certain nuclei ( $^1\text{H}$ and $^{13}\text{C}$ ) behave like tiny magnets. In a strong applied magnetic field they can align with or against the field; absorbing radio-frequency radiation flips them between these states. Nuclei in different chemical environments experience slightly different shielding from surrounding electrons, so they resonate at slightly different frequencies, giving separate peaks. Samples are dissolved in a solvent with no ordinary protons, such as $\text{CDCl}_3$ , so the solvent does not swamp the spectrum.

Reading a proton spectrum

Three pieces of information come from each set of peaks:

Number of peaks equals the number of proton environments.
Integration (the step height or area) gives the whole-number ratio of protons.
Splitting uses the $n+1$ rule: a singlet has no adjacent protons, a doublet has one, a triplet two, a quartet three. Typical shifts include $\text{R-CH}_3$ near $0.9$ , $\text{O-CH}$ near $3.5$ , and $\text{O-CHO}$ (aldehyde) near $9.7\ \text{ppm}$ .

Combining data to find structure

Use the molecular ion from mass spectrometry for the molecular mass and fragment clues, infrared for functional groups (broad $\text{O-H}$ , sharp $\text{C=O}$ , $\text{N-H}$ ), and NMR for the carbon and hydrogen framework. Together these usually pin down a single structure. A reliable order is: deduce the molecular formula (mass spec), identify functional groups (IR), then build the skeleton from NMR environments and splitting.

Chromatography

Deducing a structure from combined spectra

A compound has $\text{M}^+ = 60$ in its mass spectrum, a very broad IR absorption from $2500$ to $3300\ \text{cm}^{-1}$ plus a sharp peak at $1710\ \text{cm}^{-1}$ , and a proton NMR with a singlet at $11.5\ \text{ppm}$ (integration $1$ ) and a singlet at $2.1\ \text{ppm}$ (integration $3$ ). Deduce the structure.

step 1: Use the molecular mass

$\text{M}^+ = 60$ suggests $\text{C}_2\text{H}_4\text{O}_2$ ( $24 + 4 + 32 = 60$ ).

step 2: Use the IR

The broad band to $2500\ \text{cm}^{-1}$ with a strong $\text{C=O}$ at $1710\ \text{cm}^{-1}$ is the signature of a carboxylic acid.

step 3: Use the NMR

A peak at $11.5\ \text{ppm}$ (integration $1$ ) is the acidic $\text{COOH}$ proton; the singlet at $2.1\ \text{ppm}$ (integration $3$ ) is a $\text{CH}_3$ with no neighbours.

step 4: Assemble

$\text{CH}_3$ plus $\text{COOH}$ gives $\text{CH}_3\text{COOH}$ , ethanoic acid, which matches $\text{C}_2\text{H}_4\text{O}_2$ and a mass of $60$ .

Examples in context

Example 1. MRI scanners. Magnetic resonance imaging is proton NMR applied to the human body: it maps the hydrogen nuclei in water and fat. Different tissues relax at different rates, giving contrast in the image. The technique is the same physics as bench NMR (nuclei flipping in a magnetic field) but tuned to produce a spatial map rather than a chemical-shift spectrum, which is why hospitals avoid the word "nuclear" to reassure patients.

Example 2. Drug testing by GC-MS. Anti-doping laboratories use gas chromatography coupled to mass spectrometry to detect banned substances in athletes' urine. The GC separates the complex mixture by retention time, and as each component leaves the column the mass spectrometer records its fragmentation pattern, which acts as a fingerprint matched against a database. This combination of separation plus identification is exactly the principle Edexcel asks candidates to describe.

Try this

Q1. State what the number of peaks in a carbon-13 NMR spectrum tells you. [1 mark]

Cue. The number of different carbon environments in the molecule.

Q2. A proton NMR peak is split into a quartet. State how many protons are on the neighbouring carbon. [1 mark]

Cue. Three (a quartet means $n+1 = 4$ , so $n = 3$ ).

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20195 marksA compound X has molecular formula

\text{C}_3\text{H}_6\text{O}

. Its proton NMR spectrum shows two peaks: a singlet at

2.1\ \text{ppm}

(integration

6

) and no other peaks. Its IR spectrum has a strong absorption at

1715\ \text{cm}^{-1}

. Deduce the structure of X, explaining how each piece of data is used.

Show worked answer →

Combine IR, integration and splitting to reach a single structure.

The IR peak at $1715\ \text{cm}^{-1}$ shows a $\text{C=O}$ group (1). The formula $\text{C}_3\text{H}_6\text{O}$ with a carbonyl and no $\text{O-H}$ (no broad absorption) means an aldehyde or ketone (1). The proton NMR shows a single peak, so all six H atoms are in one environment (1); a singlet means no neighbouring protons on adjacent carbons (1). Two equivalent $\text{CH}_3$ groups either side of a $\text{C=O}$ fit this: X is propanone, $\text{CH}_3\text{COCH}_3$ (1).

Markers reward using each datum (IR, integration, splitting) explicitly and arriving at propanone rather than propanal (an aldehyde would show a low-integration $\text{CHO}$ peak near $9.7\ \text{ppm}$ ).

Edexcel 20224 marks(a) State what carbon-13 NMR tells you about a molecule. (b) Ethanol,

\text{CH}_3\text{CH}_2\text{OH}

, is run by proton NMR. Predict the number of peaks, their splitting, and the integration ratio.

Show worked answer →

Count environments for carbon, then apply the $n+1$ rule for protons.

(a) The number of peaks equals the number of different carbon environments in the molecule (1).

(b) Ethanol has three proton environments: $\text{CH}_3$ , $\text{CH}_2$ and $\text{OH}$ , so three peaks (1). The $\text{CH}_3$ is split by the two $\text{CH}_2$ protons into a triplet ( $n+1 = 3$ ); the $\text{CH}_2$ is split by the three $\text{CH}_3$ protons into a quartet ( $n+1 = 4$ ); the $\text{OH}$ is a singlet (it does not normally couple) (1). Integration ratio $\text{CH}_3 : \text{CH}_2 : \text{OH} = 3 : 2 : 1$ (1).

Related dot points

Sources & how we know this

Pearson Edexcel A-Level Chemistry (9CH0) specification — Pearson Edexcel (2015)