¹H NMR: Shifts, Integration & Splitting (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about proton NMR including chemical shift, integration and spin-spin splitting.

The three go together because spin-spin splitting provides structural information in addition to chemical shift and integration data, so one spectrum gives you three separate pieces of information on the same molecule.

A proton NMR spectrum shows signal intensity plotted against chemical shift, measured in parts per million. Three things can be read off it. The chemical shift of each peak tells you which environment the protons are in. The area under each peak tells you how many protons are in that environment. And in a high-resolution spectrum, the splitting pattern tells you how many protons sit on the neighbouring carbon atom. Used together, they let you suggest a structure for the molecule.

We'll take chemical shifts first, then integration, then spin-spin splitting.

Protons in different chemical environments absorb radiofrequency radiation at slightly different magnetic field strengths. That difference in resonance position is the chemical shift, and it is measured in parts per million. A single sharp peak appears at the far right of the spectrum at zero parts per million. That is the reference signal from tetramethylsilane, used as an internal standard, and every other shift is measured from it.

Different types of proton appear within characteristic chemical shift ranges, which helps identify functional groups. Ethanol has three different proton environments, CH₃, CH₂ and OH, so its low-resolution spectrum shows three peaks, each at its own shift. Protons in the same environment are chemically equivalent, for example in 1,2-dichloroethane all four hydrogens are exactly equivalent, so it produces one single peak.

The area under each peak is the integration, and it indicates the relative number of protons in each chemical environment. Peaks vary in height, but it is the area under each peak, not its height, that is used to determine the proton ratio. The spectrometer produces an integration trace which measures the area under each absorption peak which gives you the ratio of protons in each environment.

This spectrum is methyl chloroethanoate, which has two proton environments. The CH₂ group contributes two protons and the CH₃ group contributes three, so the integration ratio of the peaks is two to three. For ethanol, the integration ratio of the CH₃, CH₂ and OH peaks is three to two to one. Where the chemical shift says which environment a peak belongs to, the integration says how many protons are in it.

High-resolution proton NMR provides additional structural information by showing splitting patterns. A signal may be split into several smaller peaks, known as a multiplet. The splitting is caused by spin-spin coupling between a proton and nonequivalent protons on adjacent carbon atoms, and it lets you determine how many hydrogen atoms are attached to the neighbouring carbon.

The number of peaks follows the n plus one rule, where n is the number of equivalent protons on the neighbouring carbon atoms. You should be able to recognise singlets, doublets, triplets and quartets. A doublet has an intensity ratio of one to one, a triplet one to two to one, and a quartet one to three to three to one. These ratios follow the pattern of Pascal's triangle. In ethanol, the CH₃ signal is a triplet because it has two neighbours, and the CH₂ signal is a quartet because it has three. A quartet and a triplet in the same spectrum usually indicate an ethyl group.

The chemical shift of a peak shows which environment its protons are in, measured in parts per million from the tetramethylsilane reference at zero. The area under the peak shows how many protons are in that environment. The splitting pattern, from the n plus one rule, shows how many protons are on the neighbouring carbon. Taken together, those three readings are what you use to suggest a structure for the molecule.