A level Chemistry · Edexcel 9CH0 · Topic 19A
NMR spectroscopy
Key points
- Nuclei with spin (¹H, ¹³C) absorb radio waves in a strong magnetic field; the energy absorbed depends on the chemical environment.
- TMS (tetramethylsilane) is the reference at δ = 0: it is inert, volatile and gives one strong peak. Samples are dissolved in deuterated solvents such as CDCl₃.
- ¹³C NMR: the number of peaks = number of different carbon environments; use chemical shift tables (e.g. C=O at 160–220 ppm).
- ¹H NMR: number of peaks = number of H environments; relative peak areas (integration) give the ratio of H atoms.
- n + 1 rule: a signal is split into n + 1 peaks by n hydrogens on adjacent carbons (singlet, doublet, triplet, quartet). O–H and N–H usually give singlets.
- Symmetry reduces the number of environments; use it to tell isomers apart.
- MRI uses the same principle on protons in body tissues.
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Nuclear magnetic resonance (NMR) spectroscopy is the chemist's best tool for working out how the atoms in a molecule are connected. You need to interpret ¹³C and ¹H spectra and use them, with other data, to deduce structures.
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