A proton NMR spectrum answers four questions at once: how many hydrogen environments there are, what kind each one is, how many hydrogens are in each, and how many hydrogens sit on the neighbouring carbons.
In a magnetic field, 1H nuclei absorb radio waves at frequencies that depend on their electron surroundings. The chemical shift, δ in ppm, is measured relative to TMS, Si(CH3)4, set at 0. The number of peaks equals the number of different proton environments, and the relative peak areas (integration) give the ratio of hydrogens in each. Each peak is split by the hydrogens on neighbouring carbons according to the n+1 rule: n neighbours give n+1 lines, so a singlet, doublet, triplet or quartet. Samples are dissolved in a deuterated solvent such as CDCl3, which gives no proton signal, and shaking with D2O makes O–H and N–H peaks disappear.
Electronegative atoms near a hydrogen pull electron density away from it, so its peak moves to higher δ (to the left of the spectrum). Use the Data Booklet ranges to assign each peak.
| Type of proton | Example | δ / ppm |
|---|---|---|
| alkyl, R–CH3, R–CH2–R | CH3 in ethanol | 0.9–1.7 |
| next to a carbonyl, CH3–C=O | propanone | 2.0–2.5 |
| on a carbon joined to oxygen, O–CH3, O–CH2 | CH2 in ethanol | 3.3–4.3 |
| alcohol, R–OH | OH in ethanol | 0.5–6.0 (variable) |
| aromatic, on a benzene ring | benzene | 6.0–9.0 |
| carboxylic acid, –COOH | ethanoic acid | 9.0–13.0 |
| aldehyde, –CHO | ethanal | 9.3–10.5 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
How many peaks are in the low-resolution proton NMR spectrum of propanone, CH3COCH3?
A CH3 group is bonded to a CH2 group, which has no other neighbouring H. How is the CH3 peak split?
Why is TMS added to NMR samples?
Which peak disappears when the spectrum of ethanol is rerun after shaking with D2O?
Hydrogens on the next carbon along nudge the field a proton feels. Count them, add one, and you have the number of lines in its peak.
| H on adjacent carbon(s), n | Lines, n+1 | Name | Line heights |
|---|---|---|---|
| 0 | 1 | singlet | 1 |
| 1 | 2 | doublet | 1 : 1 |
| 2 | 3 | triplet | 1 : 2 : 1 |
| 3 | 4 | quartet | 1 : 3 : 3 : 1 |
Read each row from left to right: shift tells you the type of proton, area the number, splitting the neighbours.
| Compound | Peaks: δ / ppm, H count, splitting, assignment |
|---|---|
| ethanol, CH3CH2OH | 3.7, 2H, quartet (O–CH2); about 2.6, 1H, singlet (OH, lost with D2O); 1.2, 3H, triplet (CH3) |
| ethyl ethanoate, CH3COOCH2CH3 | 4.1, 2H, quartet (O–CH2); 2.0, 3H, singlet (CH3–C=O); 1.3, 3H, triplet (CH3) |
| propanone, CH3COCH3 | 2.1, 6H, singlet: one environment, no neighbouring H |
| 1-chloropropane, CH3CH2CH2Cl | 3.5, 2H, triplet (CH2Cl, shifted by Cl); 1.8, 2H, sextet (middle CH2, 5 neighbours); 1.0, 3H, triplet (CH3) |
Splitting comes from H on the adjacent carbon, not from the H in the group itself: a CH3 next to a CH2 is a triplet, not a quartet. Peak area gives the number of H; the number of lines does not. The O–H peak is usually a singlet and is the one that disappears with D2O. TMS is at 0 and δ increases to the left.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
An ester C4H8O2 gives three peaks: a singlet at 2.0 (3H), a quartet at 4.1 (2H) and a triplet at 1.3 (3H). What is it?
What is the splitting of the CH2Cl peak in the spectrum of 1-chloropropane, CH3CH2CH2Cl?
Butanone, CH3COCH2CH3, is analysed. What are the relative peak areas, and how many peaks are there?
A compound shows a peak at δ 9.8 that is a triplet. Which structural feature does this suggest?
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