Variable Power and Temperature

As stated earlier, the amount of information available about a paramagnetic species from a powder EPR pattern is largely determined by the resolution of the spectra. If information on the gand A anisotropy can be extracted from the spectrum, then details on the electronic structure of the paramagnet can be derived. If variable temperature measurements are performed, then information on the dynamics of the species can also be derived. This can be illustrated through an example, based on the EPR spectrum of adsorbed NO2 on an oxide surface [19]. 

For NO2, the unpaired electron is mainly associated with the nonbonding orbital  

For a pure dipole interaction, the anisotropic term in the above equation should have the form -a, -a, la. This implies that the unpaired electron is not purely p based, but also there must be some occupancy of the orthogonal p orbitals. In other words, the anisotropic tensor is actually the result of two dipolar interactions with two radius vectors, each of which is along a coordinate axis of the molecule. Therefore the anisotropic tensor is the sum of two dipolar coupling tensors and can be decomposed into two traceless components (-a, -a, la) and (-h, Ih, -h) as follows  

Using the reported atomic value of the dipolar N constant as 

The fractional occupancy of the nitrogen s orbital, may be determined from the isotropic coupling constant, which is equal to (where 

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