Inorganic free radicals, electron spin

Electron spin resonance (ESR) spectroscopy is also known as electron paramagnetic resonance (EPR). spectroscopy or electron magnetic resonance (EMR) spectroscopy. The main requirement for observation of an ESR response is the presence of unpaired electrons. Organic and inorganic free radicals and many transition metal compounds fulfil this condition, as do electronic triplet state molecules and biradicals, semicon-ductor impurities, electrons in unfilled conduction bands, and electrons trapped in radiation-damaged sites and crystal defect sites. 

First consider the special case of isotropic hyperfine interaction in which the hyperfine interaction becomes a scalar and can be written in front of the dot product of the nuclear and electron spin angular momentum vectors. For simplicity the electron g factor will also be considered to be isotropic and to be a scalar. This simplification typically applies to most organic and some inorganic free radicals in liquids and also to a few cases in solids. This occurs because rapid tumbling of the molecular species averages out the anisotropic interactions. Also, since the quadrupole interaction is typically small and can only be experimentally resolved in special cases, it will be left out of the simplified spin Hamiltonian. The resulting simplified spin Hamiltonian becomes... 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

For polyatomic radicals in the gaseous phase the above spin-Hamiltonian does not apply. Here, the presence of unquenchend orbital and rotational angular momenta necessitates the introduction of several magnetic hyperfine coupling constants to describe the interaction between one nucleus and the free electron. These are defined and explained in the introduction to the tables on inorganic radicals. 

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