Proton NMREdexcel A-Level Chemistry: Revision notes

High-resolution 1H NMR spectroscopy detects hydrogen-1 nuclei (protons). It gives the positions of the hydrogen atoms in a molecule, by showing one group of peaks for each proton environment.

A spectrum gives three kinds of information:

• Chemical shift (δ, ppm): the type of proton
• Relative peak area: the number of protons in each environment
• Splitting pattern: the number of protons on adjacent carbons

Shifts are measured relative to TMS at 0 ppm. Protons are in the same environment if they are bonded in the same way, usually by symmetry.

Electronegative atoms and π bonds nearby deshield protons and raise the shift. Typical ranges (ppm):

• CH₃ in an alkyl chain: 0.7–1.2
• CH₂ in an alkyl chain: 1.2–1.4
• CH₃ or CH₂ next to C=O: 2.0–2.7
• CH₂ or CH₃ bonded to O (alcohol, ester): 3.3–4.3
• Aromatic ring H: 6.0–9.0
• Aldehyde CHO: 9.4–10.0
• Carboxylic acid COOH: 10.0–12.0
• Alcohol O–H: 0.5–5.0, variable position

Always use the data booklet and name the proton type when assigning.

The area under each peak (or group of peaks) is proportional to the number of protons in that environment. Areas are given as a ratio, often as integration numbers.

A ratio 3 : 2 : 3 means groups of 3, 2 and 3 protons, or multiples of these. Match the total to the molecular formula: if the formula has 8 hydrogens and the ratio is 3 : 2 : 3, the numbers are exactly 3, 2 and 3.

The area is for the whole split pattern, not for one line of a triplet.

A proton is split by non-equivalent protons on the adjacent carbon atom. If there are n such protons, the peak splits into n + 1 lines.

• n = 0: singlet
• n = 1: doublet (1 : 1)
• n = 2: triplet (1 : 2 : 1)
• n = 3: quartet (1 : 3 : 3 : 1)

Protons in the same environment do not split each other. A CH₃ group next to a CH₂ gives a triplet, and the CH₂ gives a quartet. An ethyl group CH₃CH₂– is a triplet plus a quartet. O–H protons usually appear as singlets.

To predict the spectrum of propanoic acid, CH₃CH₂COOH:

1. Identify the environments: CH₃, CH₂, COOH (3 peaks)
2. Give the shift of each: CH₃ 0.7–1.2, CH₂ 2.0–2.7 (next to C=O), COOH 10.0–12.0
3. Give the area: 3 : 2 : 1
4. Give the splitting: CH₃ next to CH₂ is a triplet, CH₂ next to CH₃ is a quartet, COOH is a singlet

For butanone, CH₃COCH₂CH₃: CH₃CH₂ gives a triplet (3) and a quartet (2), and the CH₃CO gives a singlet (3).

Work through the data in order:

1. Count the peaks for the number of environments
2. Use the area ratio and the molecular formula for the number of protons in each
3. Use the shifts to identify the type of proton
4. Use splitting to find which groups are neighbours
5. Assemble the fragments and check against the formula and any rejected isomers

Example: C₄H₈O with a triplet (3), a singlet (3) and a quartet (2), and no peak near 9.5, is butanone. A peak at 9.4–10.0 would indicate an aldehyde.