chemical shift
/ KEM-ih-kul shift /
Chemical shift is how NMR tells one hydrogen atom from another by listening to its slightly different pitch. Imagine a choir where each singer's voice is colored a little by the room they stand in; even singing the same note, a singer in a soft, carpeted corner sounds a touch different from one in a bare stone hall. In a molecule, the electrons around each nucleus form its "room," subtly tuning its NMR frequency.
More precisely, the chemical shift is the small difference between a nucleus's resonance frequency and that of a reference compound, reported on a scale called delta (δ) in parts per million so it does not depend on the magnet's strength. Electrons shield a nucleus from the field; more shielding gives a lower shift, less shielding a higher one.
It matters because the chemical shift reveals the chemical environment of each atom: a hydrogen next to oxygen resonates at a predictably different position than one buried in a hydrocarbon chain. The honest caveat is that shift alone gives the type of environment, not the full connectivity — you read it alongside peak areas and spin-spin coupling to build the whole picture.
In ethanol's proton NMR, the CH₃ peak appears near δ 1.2, the CH₂ near δ 3.7, and the OH near δ 2.6. The oxygen pulls electron density away from nearby hydrogens, pushing their signals to higher chemical shift.
Each peak's position reports the chemical neighborhood of its nucleus.
Reporting chemical shift in parts per million (the δ scale) rather than in raw frequency means the same value holds on magnets of any strength, so spectra from different instruments stay directly comparable.