Proton (1H) NMR spectroscopy

Chemistry A2 · Analytical techniques. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A proton NMR spectrum gives four pieces of information. The number of peaks is the number of different proton environments. The chemical shift, δ, shows the type of proton. The relative peak area shows how many protons are in each environment. The splitting shows how many protons are on the neighbouring carbon atoms.

Shifts are measured from tetramethylsilane, TMS, which is given δ = 0. The sample is dissolved in a deuterated solvent such as CDCl₃, which has no ¹H atoms and so adds no peak.

Protons in O–H and N–H groups swap with deuterium when D₂O is added, so their peak disappears.

Rules to remember

n + 1 rule: a proton with n equivalent protons on adjacent carbon atoms gives n + 1 lines
0 neighbours: singlet; 1: doublet; 2: triplet; 3: quartet
Equivalent protons do not split each other; O–H and N–H protons usually give a singlet and do not split their neighbours
TMS, Si(CH₃)₄: 12 equivalent protons give one sharp peak at δ = 0; it is unreactive and easily removed
Approximate δ: alkyl H 0.9–1.7; H–C–C=O 2.2–3.0; H–C–aryl 2.3–3.0; H–C–O 3.2–4.0
Approximate δ: aryl H 6.0–9.0; aldehyde H 9.3–10.5; –COOH 9.0–13.0
A peak that disappears with D₂O is an O–H or N–H proton

Common mistake

Students count the protons in the group itself to find the splitting. Splitting comes from the protons on the adjacent carbon: a CH₃ next to a CH₂ is a triplet (2 + 1), not a quartet.

Worked example

Predict the proton NMR spectrum of ethanol, CH₃CH₂OH, and say what changes when D₂O is added.

There are three proton environments (CH₃, CH₂, OH), so three peaks with areas 3 : 2 : 1.
CH₃ has 2 protons on the next carbon: triplet (2 + 1), at about δ 1.2.
CH₂ has 3 protons on the next carbon and is next to oxygen: quartet (3 + 1), at about δ 3.7.
OH gives a singlet; its proton exchanges with deuterium, so this peak disappears with D₂O.

Answer: Triplet (area 3), quartet (area 2) and singlet (area 1); the O–H singlet disappears with D₂O

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