Every proton in the sample precesses around the external field B₀ (the vertical axis) at its Larmor frequency ω = γB₀(1−σ), where σ is the local electronic shielding. Chemically distinct protons (CH₃, CH₂, OH, aromatic ring…) sit in different electronic environments, so they shield the nucleus by a different amount and precess at a slightly different frequency. Reported as chemical shift δ (ppm, field-independent), each group appears as its own platform along the sample axis — its ring of spins spins visibly faster the further its shift is from the reference.
δ (ppm) = (ν − ν_ref) / ν_spectrometer × 10⁶
peaks split into n+1 lines when coupled to n equivalent neighbouring protons
J (Hz) = spacing between adjacent lines, independent of field strength
- Field strength — raises the spectrometer's absolute operating frequency (MHz). Chemical shifts stay fixed in ppm, but everything reported in Hz — including the spin-spin splitting — spreads out, which is why higher-field instruments resolve overlapping multiplets better.
- Line broadening — shorter T2 (faster spin dephasing/decoherence) widens every line in the spectrum and visibly fans the precessing spin vectors out of phase in the 3D scene.
- J-coupling — toggles the n+1 splitting rule so you can compare a coupled multiplet against the same signal collapsed to a single decoupled line.
The same physics extends to 2D experiments: COSY correlates coupled protons through-bond (off-diagonal peaks reveal which multiplets share a J), while NOESY correlates protons through-space proximity instead — both build directly on the shift/coupling picture shown here.