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Why do NMR experiments use radio-frequency waves?

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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 nuclear magnetic resonance (NMR), radio-frequency waves are used because they correspond to the energy required to induce transitions between the discrete magnetic energy states of atomic nuclei.12

When nuclei with a non-zero magnetic moment are placed in a strong, stationary magnetic field, they precess at a specific rate known as the Larmor frequency. To observe resonance, a weak oscillating field—typically in the radio-frequency range—is applied perpendicular to the stationary field. When the frequency of this oscillating field matches the Larmor frequency of the nuclei, the system absorbs energy from the radio waves, causing the nuclei to change their orientation and energy state.12

Radio-frequency waves are specifically utilized because the energy intervals between these nuclear magnetic states are relatively small, placing the required resonance frequencies within the radio-frequency spectrum. This selective absorption allows scientists to measure nuclear magnetic moments and gain detailed information about molecular structures.132