Nuclear magnetic resonance

Some nuclei have non-zero spin and an associated magnetic moment. In an external magnetic field, their allowed energy states separate; the energy difference depends on field strength and the nucleus. Radiofrequency energy at the corresponding resonance frequency can induce transitions. The frequency is commonly described through the Larmor relation and is proportional to the applied field for a given nuclear species. Nuclear spin is a quantum property, not literal rotation of a tiny object.

In chemical spectroscopy, a sample is placed in a magnetic field and excited with radiofrequency energy. The measured response is converted into a spectrum, often by Fourier transformation of a time-domain signal. The position of a resonance relative to a reference is expressed as chemical shift, usually in parts per million; local electron distributions alter the effective field experienced by nuclei and thus contribute to different shifts. Signal areas and spin–spin splitting provide additional structural clues. Interpreting the spectrum combines these observations rather than relying on one peak alone.

NMR spectroscopy is a non-destructive analytical technique used to help determine molecular structure and study chemical or biological samples. MRI uses the same broad magnetic-resonance basis but adds spatial encoding so that signals can be reconstructed into images. In clinical MRI, hydrogen nuclei are especially useful because they are abundant in the body. Differences in signal relaxation and other measured properties contribute to image contrast; MRI is not simply a photograph of a single resonance frequency.

NMR signals can be weak because the energy differences between nuclear spin states are small, so instruments use strong, stable magnetic fields and sensitive detection. A spectrum depends on the nucleus observed, the sample's molecular environment, measurement conditions, and interactions among spins. NMR is therefore a powerful source of evidence about a sample, but interpretation depends on experimental design and complementary chemical knowledge.