The n+1 rule
A set of equivalent hydrogens with n hydrogens on adjacent carbons is split into n + 1 peaks.
| Neighbouring H (n) | Peaks | Name | Relative heights |
|---|---|---|---|
| 0 | 1 | singlet (s) | 1 |
| 1 | 2 | doublet (d) | 1 : 1 |
| 2 | 3 | triplet (t) | 1 : 2 : 1 |
| 3 | 4 | quartet (q) | 1 : 3 : 3 : 1 |
| 4 | 5 | quintet | 1 : 4 : 6 : 4 : 1 |
| 6 | 7 | septet | 1 : 6 : 15 : 20 : 15 : 6 : 1 |
The heights come from Pascal's triangle.
Three things that don't split
- Equivalent hydrogens don't split each other. The six H in ethane give one singlet.
- OH and NH hydrogens usually appear as broad singlets and don't split their neighbours, because they swap quickly between molecules. Shaking with D₂O makes them disappear.
- Hydrogens more than three bonds apart (H–C–C–H is three bonds) usually don't split each other.
Patterns worth recognising
| You see | It suggests |
|---|---|
| Quartet (2H) + triplet (3H) | An ethyl group, CH₂CH₃ |
| Septet (1H) + doublet (6H) | An isopropyl group, CH(CH₃)₂ |
| Singlet (9H) | A tert-butyl group, C(CH₃)₃ |
| Singlet (3H) near 2.1 ppm | CH₃ next to C=O, with no neighbours |
| Singlet (3H) near 3.3–3.9 ppm | OCH₃ |
Worked example: ethanol
- CH₃ at about 1.2 ppm: triplet, 3H. Its neighbour, the CH₂, has 2 H → 2 + 1 = 3 peaks.
- CH₂ at about 3.7 ppm: quartet, 2H. Its neighbour, the CH₃, has 3 H → 4 peaks. (The OH doesn't split it.) It's further downfield because it's next to oxygen.
- OH, about 2–5 ppm: broad singlet, 1H. Its position depends on concentration.
Worked example: propan-2-ol
The two methyl groups are equivalent: one doublet (6H) near 1.2 ppm, split by the single CH. The CH is split by six H, giving a septet (1H) near 4.0 ppm. The OH is a broad singlet.
Where signals appear (chemical shift)
| Type of H | δ / ppm |
|---|---|
| R–CH₃, R–CH₂–R | 0.9–1.7 |
| H–C–C=O | 2.0–2.7 |
| H–C–O (alcohols, ethers, esters) | 3.3–4.3 |
| C=C–H (alkene) | 4.5–6.5 |
| Ar–H (benzene ring) | 6.5–8.0 |
| CHO (aldehyde) | 9.0–10.0 |
| COOH | 10–13 (broad) |
Exact values vary with the molecule; check your course's data sheet. For functional groups, IR is often quicker: see the IR spectroscopy table.
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