In
nuclear magnetic resonance (NMR), the
frequency at which a particular
nucleus absorbs
photons. This is called the
resonance frequency and it is a function of both the
elemental identity of the nucleus (e.g.
nitrogen,
carbon,
hydrogen) and the local chemical environment around the nucleus. The local chemical environment (things like which other atoms the nucleus is
covalently bound to, what kind of bonds are between them, and the nearby presence of
charged groups either bonded or not bonded to the nucleus) effects the electron density around the nucleus, and in the NMR's
magnetic field these electrons create their own magnetic field opposite in direction to the external magenetic field. The nucleus 'feels' a variable net magnetic field depending on the local electron density, and thus has a variable resonance frequency (see
Zeeman effect.)
The term 'chemical shift' was originally intended as an insult. Two different groups of physicists, one at
MIT and one at
Caltech, were trying to measure an intrinsic property of the hydrogen nucleus by measuring it's resonant frequency in a homemade NMR spectrometer. When they looked at
ethanol, they saw three different resonance frequencies, one for each type of
proton in the molecule. They wrote a paper describing their data and stated that 'local chemical interactions' obscured the true spin properties of the hydrogen nucleus and this method therefore was 'not promising'.
These 'unpromising' chemical shift differences now form the basis for NMR's use in
medicine,
chemistry, and
biology.