chemical shift

Meaning: Electrons around a nucleus shield it slightly from the applied magnetic field, so nuclei in different environments resonate at different frequencies. Electronegative atoms and some pi systems pull electron density away, deshielding nearby nuclei, so they appear at higher δ (downfield, left). Electron-rich, alkyl-like environments are more shielded and appear at lower δ (upfield, right). Typical ¹H values: alkyl C–H about 0.9–1.5, H on a carbon bonded to O or a halogen about 3–4, vinyl 4.5–6.5, aromatic 6.5–8, aldehyde 9–10, carboxylic acid 10–12.

Equation: δ = (νsample − νTMS) / νspectrometer × 10⁶, where ν is frequency in Hz, νspectrometer is the operating frequency of the instrument in Hz, and δ is in ppm (a dimensionless ratio, so no SI unit).

On the MCAT: A passage gives a structure or a spectrum, and you rank hydrogens from most to least downfield, or assign peaks to groups. Chemical shift is usually read together with integration (relative number of H) and splitting (neighboring H). ¹³C NMR uses the same idea on a wider scale.

Common confusion: Students read "downfield" as a lower number. Downfield means higher δ and more deshielding. Also, δ in ppm stays the same at different field strengths, while the separation in Hz changes. O–H and N–H shifts are variable because of hydrogen bonding, so do not rely on a fixed value for them.