Electron paramagnetic resonance isotropic spectra

Electron Paramagnetic Resonance Spectra. Only two of these complexes exhibit well-resolved EPR spectra. A narrow, isotropic signal observed at g = 2.005 for the trinuclear complex 12 at low temperatures is consistent with an S = 1/2 ground state169), but a detailed description of the electronic properties of the complex remains to be developed. The [Fe(MoS4)2]3 ion shows a rhombic S = 3/2 EPR spectrum that is very solvent dependent and, under certain conditions, is somewhat similar in apperance to that of FeMo-com). For example, in frozen aqueous solution, the apparent g values are 5.3,2.6, and 1.7181). If complex 14 also proves to have an S = 3/2 ground state, a somewhat similar EPR spectrum at low temperature would be expected as well. 

A paramagnetic atom with Td symmetry should give only one resonance line, but when this atom has a nuclear spin, the electron and nuclear spins can couple by hyperfine interaction, and for a nuclear spin I, each electronic spin component splits into 21+1 components giving the same number of Am/ = 0 resonances. For instance, the ESR spectrum of tetrahedral interstitial Al I = 5/2) produced by electron irradiation of AZ-doped silicon is an isotropic sextuplet due to transitions between the six nuclear sublevels of each electronic-spin component ([54], and references therein). The electron spin of a centre can also interact with the nuclear spins of neighbouring atoms to give additional structures and this is clearly shown for 29 Si atoms (I = 1/2) in Fig. 4 of [54]. The ESR spectrum can thus also determine the atomic structure of the centre. This can also occur for non-cubic centres and the hyperfine structure is superimposed on the orientational structure. 

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