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7.10Advanced Organic Chemistry

Nuclear Magnetic Resonance Spectroscopy — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 10 of 12

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Nuclear Magnetic Resonance Spectroscopy: 1H and 13C NMR — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Explain the principles of NMR and use chemical shift, integration and splitting patterns to deduce structures.Nuclei with an odd number of nucleons, such as 1H and 13C, behave like tiny magnets.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Deuterated solvent
  • Number of peaks: the number of different proton environments

Part 1 of 5Principles of NMR

Nuclei with an odd number of nucleons, such as 1H and 13C, behave like tiny magnets.

1
In a strong external magnetic field they can align with or against the field.
2
Radio-frequency radiation of the right energy flips the nuclei to the higher-energy alignment; this resonance is detected.
3
The exact frequency depends on the electron environment: electrons shield the nucleus,
  • so nuclei in different environments absorb at different positions.
4
Positions are measured as chemical shift (δ, ppm) relative to the standard tetramethylsilane (TMS, Si(CH3)4), set at δ = 0.
  • TMS is used because it is inert, volatile, gives one sharp signal from 12 equivalent hydrogens and is at the end of the scale.
5
Samples are dissolved in deuterated solvents (CDCl3) or CCl4 to avoid solvent 1H signals.

Part 2 of 5Carbon-13 NMR

The number of peaks equals the number of different carbon environments.

Carbon-13 NMR
Peaks are single lines (no splitting is shown in the usual spectrum).
Chemical shift ranges (approximately): alkyl C 0–50; C-Cl/C-Br 30–70; C-O 50–90; C=C and aromatic 110–160; C=O in esters and acids 160–185; C=O in aldehydes and ketones 190–220.
  • Symmetry reduces the number of signalsfor example, propanone has two (CH3 and C=O), and benzene has one.

Part 3 of 5Proton (1H) NMR

Four pieces of information are used:

1
Number of peaks: the number of different proton environments.
2
Chemical shift: indicates the type of environment (about 0.9–1.7 alkyl; 2.0–2.7 next to C=O; 3.3–4.3 H-C-O or C-halogen; 4.5–6.0 alkene; 6.5–8.0 benzene ring; 9.5–10 aldehyde; 10–12 carboxylic acid; 4–12 variable for OH).
3
Integration: the ratio of peak areas equals the ratio of the numbers of hydrogens in each environment.
4
Splitting: the n+1 rule — a peak from H on a carbon with n equivalent H on adjacent carbons splits into n+1 lines: singlet (0 neighbours), doublet (1), triplet (2), quartet (3).
  • Hydrogens on O (OH) and N usually appear as singlets because of rapid exchange.

Adding D2O removes OH and NH signals, since the H is replaced by D, helping to identify those groups.

Part 4 of 5Putting it together

Start with the molecular formula (from mass spectrometry) and the number of peaks; use integration to assign numbers of hydrogens; use shifts to identify groups; use splitting to see which groups are neighbours. Example: ethyl ethanoate shows three 1H peaks: a triplet (CH3CH2, 3H), a singlet (CH3CO, 3H) and a quartet (OCH2, 2H).

Part 5 of 5Worked examples

  1. How many peaks are in the 13C NMR of propan-2-ol? [2]2 marks

  2. A compound C4H8O2 has a 1H spectrum: singlet 3H at δ 2.0, quartet 2H at δ 4.1, triplet 3H at δ 1.2. Give its structure. [3]3 marks

  3. Give the splitting of the CH2 peak in CH3CH2Br. [1]1 mark

  4. Why is TMS used as the reference compound? [2]2 marks

Key terms

TermMeaning
Chemical shiftPosition of a peak relative to TMS in ppm.
TMSTetramethylsilane, the NMR reference compound.
IntegrationPeak area, proportional to number of H.
SplittingPeak splitting by neighbouring H following the n+1 rule.
Deuterated solventSolvent in which H is replaced by D so it gives no 1H signal.

Avoid these mistakes

1
Confusing the number of peaks with the number of hydrogens.
2
Forgetting that equivalent H on the same carbon or by symmetry do not split each other.
3
Applying the n+1 rule to OH protons.
4
Reading integration as the exact number rather than the ratio.

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