Page 358 - Instant notes

I8
MAGNETIC RESONANCE
SPECTROSCOPY

Key Notes
Nuclear magnetic resonance (NMR) spectroscopy is the resonant
absorption of radiofrequency radiation when nuclei with non-zero
spin angular momentum convert between spin states separated in
energy by an applied magnetic field. NMR provides information
on the different chemical environments of nuclei in a molecule.
The magnitude of the absorption is proportional to the number of
equivalent nuclei in the same environment.
The frequency of nuclear magnetic resonance absorption depends
on the local magnetic environment of the nuclei in the molecule
and is reported as a chemical shift, δ, relative to the resonance
frequency of a reference standard.
The NMR resonance absorption due to a group of equivalent
nuclei is split into fine structure if coupling occurs between their
magnetic moment and those of neighboring nuclei in the
molecule. The magnitude of the splitting is measured by a spin-
spin coupling constant. A group of N equivalent protons splits the
absorption line of a nearby group into N+1 lines with intensities
given by the coefficients of the (N+1) th binomial expansion.
Electron spin resonance (ESR) is the resonance absorption of
microwave radiation by unpaired electrons in a magnetic field.
The technique provides information on the electronic structure of
radicals, triplet states and d-metal complexes with unpaired
electrons. An ESR absorption shows hyperfine structure if the
electron couples with a neighboring magnetic nucleus.
Related topic General features of spectroscopy (I1)

Principles of nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy provides information on the
chemical environment of the nuclei in a molecule. Many atomic nuclei possess spin
angular momentum which is quantified by a nuclear spin quantum number, I. The
nuclear spin angular momentum may take 2I+1 different orientations relative to an
arbitrary axis in space, each of which is distinguished by values of the quantum number,
m I,