chemical shift

In an NMR spectrum, each kind of hydrogen does not sit at a single universal frequency; it sits at a position that depends on its chemical surroundings. That position along the horizontal axis is the chemical shift, and it is the first thing a chemist reads. A hydrogen next to an electron-hungry oxygen shows up in one place; a hydrogen on a plain alkane chain shows up in another. The chart is, in effect, a map of electronic environments.

Chemical shift is measured in parts per million (ppm), written delta or the Greek letter δ, on a scale running roughly 0 to 12 for protons, with the reference compound TMS set at exactly 0. Using ppm rather than raw frequency is deliberate: it makes the number independent of the particular magnet's strength, so a shift reported on one instrument means the same on any other. The reason different hydrogens land at different shifts is shielding: electrons circulating around a nucleus partly screen it from the external field, so an electron-rich hydrogen feels a weaker field and resonates at low ppm (it is shielded, upfield), while a hydrogen near an electron-withdrawing group is exposed to more of the field and resonates at high ppm (deshielded, downfield).

Because the shift reports the local electron density, it is a direct readout of what a hydrogen is attached to. Typical zones help enormously: alkane C-H around 0.9–1.5 ppm, hydrogens next to a carbonyl or oxygen around 2–4 ppm, vinyl hydrogens on a double bond around 5–6, aromatic-ring hydrogens around 7–8, an aldehyde C-H near 9–10, and a carboxylic-acid O-H far downfield near 11–12. Knowing these ranges turns a row of peaks into a description of the molecule's environments.