Counting signals in ¹H NMR
Every set of chemically equivalent hydrogens gives one signal. If you see the symmetry, you know how many signals to expect before looking at the spectrum.
Rules and values
- H are equivalent when symmetry or free rotation about single bonds turns one into the other
- The three H of a CH₃ group are always equivalent
- Symmetric groups (the two CH₃ of isopropyl, the three CH₃ of tert-butyl) give one signal
- OH, NH and COOH protons give their own (usually broad) signal
- The integral tells you how many H belong to a signal
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task
Dipropyl ether is symmetric. How many signals do you expect in its ¹H NMR spectrum?
Number of signals
Type whole numbers, e.g. 3. NMR shifts are typical values (±0.3 ppm), splitting follows the n+1 rule.
Worked example
How many signals does the ¹H NMR spectrum of ethyl propanoate show? Count the chemically different kinds of hydrogen.
Solution
- 1.H are equivalent when symmetry (mirror, rotation, free rotation about single bonds) turns one into the other.
- 2.δ ≈ 4.1 ppm, q, 2 H: CH₂ on the ester oxygen (–O–C=O), 3 neighbouring H
- 3.δ ≈ 2.3 ppm, q, 2 H: CH₂ next to the COO group, 3 neighbouring H
- 4.δ ≈ 1.3 ppm, t, 3 H: CH₃ with ester O one bond further (shifted a little), 2 neighbouring H
- 5.δ ≈ 1.2 ppm, t, 3 H: CH₃ with COO one bond further (shifted a little), 2 neighbouring H
- 6.4 signals in total.
Typical mistake: Counting equivalent groups twice (e.g. the two CH₃ of an isopropyl group) or forgetting OH/NH. Exchangeable H give a signal too.
3-methylbutan-2-one is symmetric. How many signals do you expect in its ¹H NMR spectrum?
Solution
- 1.Equivalent groups: 2 × CH₃ → one signal with 6 H.
- 2.δ ≈ 2.6 ppm, sept, 1 H: CH next to C=O, 6 neighbouring H
- 3.δ ≈ 2.1 ppm, s, 3 H: CH₃ next to C=O, 0 neighbouring H
- 4.δ ≈ 1.2 ppm, d, 6 H: CH₃ (2 equivalent) with C=O one bond further (shifted a little), 1 neighbouring H
- 5.3 signals in total.
Typical mistake: Counting every CH₃ or CH₂ separately. Symmetric groups give one shared signal; its integral then shows more H.
Common questions
Does the OH proton count as a signal?
Yes. It usually shows as a broad singlet whose position depends on solvent and concentration. In D₂O it exchanges and disappears.