What is NMR spectroscopy?

The nuclear magnetic resonance principle

Certain atomic nuclei, such as hydrogen-1 and carbon-13, behave like tiny magnets because of their spin. In a strong external magnetic field these nuclei align with or against the field, separated by a small energy gap, and precess at a characteristic frequency. A radiofrequency pulse tuned to this frequency is absorbed, flipping nuclei to the higher state — the resonance condition. As the nuclei relax, they emit a detectable radio signal. This is the same nuclear magnetic resonance physics that underlies MRI, here used analytically rather than for imaging.

Chemical shift and coupling

The power of NMR comes from the chemical shift: electrons around each nucleus slightly shield it from the external field, so nuclei in different chemical environments resonate at slightly different frequencies. The pattern of shifts maps the distinct positions of atoms in a molecule.

Neighbouring nuclei also influence one another, splitting signals into characteristic multiplets (spin–spin coupling). Together, shift and coupling let chemists deduce how atoms are bonded.

Uses in research

NMR spectroscopy is a primary tool for determining the structure of organic and biological molecules without destroying the sample. Chemists use it to confirm the identity and purity of compounds, while structural biologists use multidimensional NMR to solve the three-dimensional shapes of proteins in solution. It complements mass spectrometry, which gives mass and formula. Reproducible NMR depends on referencing chemical shifts to a standard and reporting acquisition conditions so spectra can be compared.