Why do nuclei in different molecular environments absorb different radio frequencies in NMR?

In nuclear magnetic resonance (NMR), nuclei of the same species absorb different radio frequencies depending on their molecular environment due to a phenomenon known as the chemical shift.12

The physical origin of this effect lies in the interaction between an external magnetic field and the electrons surrounding the nucleus. When an external magnetic field is applied, it polarizes the closed electron shells of the atoms. This polarization generates a small, secondary magnetic field that opposes or reinforces the external field at the site of the nucleus.1

Because atoms in different molecular sites—such as hydrogen atoms bonded to different elements like carbon or oxygen—have different local electron distributions, the resulting secondary magnetic fields vary. This causes the effective magnetic field experienced by each nucleus to differ slightly from the external field. Consequently, the resonant frequency required to induce a spin transition shifts, allowing scientists to distinguish between these distinct chemical environments and determine complex molecular structures.12