NMR van labiele protonen: temporele resolutie

Protons like the –OH in ethyl alcohol exhibit a range of chemical shift values due to the varying intermolecular hydrogen bonding between the proton and the heteroatom of the other molecules.

The –OH proton undergoes rapid proton exchange with other molecules and is known as a labile proton. The exact δ value and the spin-spin coupling depend on the proton exchange speed, influenced by the purity of the alcohol sample, solvent, and temperature.

For example, in the proton NMR spectrum of ethanol, the very slow proton exchange in a pure dry sample shows a triplet due to the spin-spin coupling with adjacent methylene protons. The methylene protons are seen as a multiplet, and the methyl protons are presented as a triplet.

Trace amounts of an acid or base increase the proton exchange rate making the –OH proton exhibit a broad singlet. Accordingly, the protons of the adjacent methylene group give a quartet due to the splitting caused only by the adjoining methyl group.