Proton NMR
Proton (¹H) NMR
- Proton NMR looks at the hydrogen atoms in a molecule.
- The spectrum gives three kinds of information.
- A few practical tricks make it readable.
What you read off
- chemical environments: protons in different environments appear at different chemical shifts (number of peak groups = number of environments).
- relative numbers: the peak areas give the ratio of each type of proton.
- splitting: a peak is split by the protons on the neighbouring carbon — the $n+1$ rule.
| Neighbours ($n$) | Pattern |
|---|---|
| 0 | singlet |
| 1 | doublet |
| 2 | triplet |
| 3 | quartet |
Practice
In proton NMR, the number of groups of peaks tells you:
Protons in different environments give peaks at different chemical shifts.
Practice
The relative areas of the peaks give:
Peak area is proportional to how many protons of that type there are (e.g. 3:2:1 for ethanol).
Practice
By the n+1 rule, a CH₃ group next to a CH₂ group appears as a:
The CH₃ has 2 neighbouring protons (on the CH₂), so n+1 = 3 lines — a triplet.
Practical points
- TMS (tetramethylsilane) is the standard, set at chemical shift 0.
- a deuterated solvent (e.g. $\text{CDCl}_3$) gives no proton signal of its own.
- shaking with D₂O makes O–H and N–H peaks disappear (their H is swapped for D), identifying those protons.
Practice
Shaking a sample with D₂O makes which peaks disappear?
The exchangeable O–H and N–H hydrogens swap for deuterium, so their peaks vanish — identifying them.
You've got it
Key idea
- ¹H NMR: number of peak groups = proton environments; areas = ratio of protons
- splitting: the $n+1$ rule ($n$ neighbours → $n+1$ lines): 0→singlet, 1→doublet, 2→triplet, 3→quartet
- TMS = reference (shift 0); a deuterated solvent is silent; D₂O removes O–H/N–H peaks