Proton NMR (¹H NMR) is the most widely used form of Nuclear Magnetic Resonance spectroscopy. It exploits the magnetic properties of hydrogen nuclei (protons) to provide detailed information about the structure of organic molecules — specifically the number, type, and environment of hydrogen atoms present.
When placed in a strong external magnetic field and irradiated with radiofrequency radiation, protons absorb energy and resonate at frequencies that depend on their chemical environment. This produces a spectrum that can be used to identify and determine the structure of unknown compounds.
A ¹H NMR spectrum provides four types of information:
| Feature | What it tells you |
|---|---|
| Number of signals | Number of sets of chemically non-equivalent protons |
| Chemical shift () | Electronic environment of each proton |
| Integration (peak area) | Relative number of protons in each environment |
| Splitting pattern | Number of equivalent protons on adjacent carbons |
Each distinct chemical environment produces a separate signal. Protons that are chemically equivalent (same environment by symmetry) give one combined signal.
Example — Ethanol ():
Ethanol has three signals:
Example — Acetone ():
Acetone has one signal — both methyl groups are equivalent by symmetry, giving a single peak.
This directly answers the FBISE past-paper question: ethanol shows 3 signals (3 environments) while acetone shows 1 signal (all 6 protons equivalent).
Chemical shift is the position of a signal on the NMR spectrum, measured in parts per million (ppm) relative to the reference compound TMS (tetramethylsilane, ) at ppm.
Typical chemical shift values:
| Proton type | (ppm) |
|---|---|
| (alkyl) | 0.7–1.3 |
| 1.2–1.4 | |
| (ether) | 3.3–3.5 |
| (alcohol) | 1–5 (variable) |
| Aromatic | 6.5–8.0 |
| (aldehyde) | 9–10 |
| (carboxylic acid) | 10–12 |
The area under each peak (integration) is directly proportional to the number of protons producing that signal.
Example — Ethyl ethanoate ():
The three signals have integration ratios of 3 : 2 : 3, corresponding to (3H), (2H), and (3H).
Integration does not give absolute numbers — only relative ratios.
Protons on adjacent carbon atoms interact magnetically with each other, causing splitting of signals into multiplets.
The Rule: A proton with equivalent neighbouring protons on adjacent carbon(s) will have its signal split into peaks.
| (neighbouring protons) | Splitting pattern | Name |
|---|---|---|
| 0 | 1 peak | Singlet |
| 1 | 2 peaks | Doublet |
| 2 | 3 peaks | Triplet |
| 3 | 4 peaks | Quartet |
Example — Ethanol ():
Example — Ethane ():
All 6 protons are equivalent. Equivalent protons do not split each other → single singlet.
Compound: Propan-2-ol,
| Signal | (ppm) | Integration ratio | Splitting | Assignment |
|---|---|---|---|---|
| A | ~1.2 | 6 | Doublet | (6H, adjacent to 1H) |
| B | ~3.9 | 1 | Septet | (1H, adjacent to 6H) |
| C | ~2.5 | 1 | Singlet | (1H) |
The proton has 6 equivalent neighbouring protons → split into peaks (septet).
¹H NMR spectroscopy is a powerful tool for structural identification: