What is NMR?

NMR stands for "nuclear magnetic resonance."

It is an analytical technique used to understand the structure and dynamics of molecules. For nearly each element in the periodic table there is an isotope (usually non-radioactive) that has a non-zero quantum mechanical property called "spin." These nuclear spins are observable by NMR. A familiar example would be the nucleus of a hydrogen atom, otherwise known as a proton, and found in many molecules.

NMR can be used to study nuclei in solids, semi-solids, solutions or gases. The NMR instrumentation consists of a computer, electronics, and a magnetic field. The computer controls radio-frequency pulses generated by the electronics and transmitted to a coil. The sample, in an appropriate glass tube or rotor, is placed into the coil at the center of the magnet where the field inhomogeneity is smallest. When the rf-pulse is in resonance with the nuclear spins of the sample, the spins are perturbed from equilibrium. Both the resonance frequency ("chemical shift") of the spins and the time to return to equilibrium (relaxation time constant) are fundamental NMR measurements. The chemical shift depends on the local electronic environment of the nucleus and is sensitive to the molecular structure. The relaxation time constant depends on dynamic fluctuations in the molecule. Connectivities and proximities of nuclei within a molecule are measured by NMR through spin-spin coupling constants, such as J-couplings (through-bond) and dipolar couplings (through-space).

From the 2003 Nobel Prize in Physiology or Medicine, October 2003 press release:
Atomic nuclei in a strong magnetic field rotate with a frequency that is dependent on the strength of the magnetic field. Their energy can be increased if they absorb radio waves with the same frequency (resonance). When the atomic nuclei return to their previous energy level, radio waves are emitted.

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