How do conduction electrons cause metallic shifts in NMR spectra?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

In metals, metallic shifts in NMR spectra occur due to an interaction between the spins of conduction electrons and the applied magnetic field.1

Conduction electrons—which are not bound to specific atoms and move freely throughout the metal—interact with the external field, causing the resonant frequency of the nuclei to shift from the value that would be observed for the same nucleus in an insulator. These shifts are significant because they provide valuable data regarding the magnetic susceptibility, the density of states of the conduction electrons, and the quantum mechanical wave functions that describe their energy states.1