Splitting in ¹H NMR: the n+1 rule
A signal is split by the H on the neighbouring carbons. With n neighbouring H you get n + 1 lines, and their heights follow Pascal’s triangle.
Rules and values
- n neighbouring H (on directly bonded carbons) → n + 1 lines
- 1 singlet, 2 doublet, 3 triplet, 4 quartet, 5 quintet, 6 sextet, 7 septet
- Equivalent H do not show coupling with each other
- No visible coupling across O or N; OH and NH usually do not couple (fast exchange)
- Ethyl group: quartet (2 H) + triplet (3 H); isopropyl: septet (1 H) + doublet (6 H)
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task
Which splitting and which integral does the signal of the marked H atoms in 2-methylpropan-1-ol (isobutanol) have?
Splitting
Integral (number of H)
Type whole numbers, e.g. 3. NMR shifts are typical values (±0.3 ppm), splitting follows the n+1 rule.
Worked example
Look at the marked H atoms in butan-1-amine (butylamine). Which splitting does their ¹H NMR signal show, and what is the integral?
Solution
- 1.Marked: CH₃ → integral 3 H.
- 2.H on the directly neighbouring C atoms: n = 2 (equivalent H of the group itself, OH/NH and H across O or N do not count).
- 3.n + 1 = 3 lines → triplet.
- 4.δ ≈ 0.9 ppm, t, 3 H: CH₃ in the alkyl region, 2 neighbouring H
Typical mistake: Counting the group’s own H (CH₃ next to CH₂ is a triplet, not a quartet) or coupling across an oxygen (the OCH₃ of an ester is a singlet).
Which splitting and which integral does the signal of the marked H atoms in methyl 2-methylpropanoate have?
Solution
- 1.Marked: CH₃ → integral 3 H.
- 2.There is no visible coupling across O or N; OH and NH protons exchange quickly and do not couple.
- 3.H on neighbouring C atoms: n = 0 → 1 lines, singlet.
- 4.δ ≈ 3.7 ppm, s, 3 H: CH₃ on the ester oxygen (–O–C=O), 0 neighbouring H
Typical mistake: Counting the H on the other side of the oxygen or nitrogen. Here coupling only runs through C–C.
Common questions
Why is the OCH₃ of an ester a singlet?
The nearest carbon with H is on the other side of the oxygen. Coupling across the oxygen is too small to see.