Metal-centered radicals, electron paramagnetic

Seventeen-electron species have also been found to form complexes with noble gases. For example, the two paramagnetic radicals RrMn(00)5 and [KrFe(00)5]+ have been detected by EPR spectroscopy by Morton, Perutz, and co-workers following the y-radiolysis of HMn(00)5 and Fe(00)5 in laypton matrices at 77 and 20 K, respectively (37). Evidence for the interaction of Kr with the unpaired electron on the metal center came from the observation of hyperfine couphng with a single Kr nucleus in the EPR spectra of these species. As an example, the EPR spectrum obtained from y-radiolysis of HMn(CO)5 in a matrix of krypton enriched to 42% in the isotope Kr (I = ) is shown in Fig. 5. The spectrum shows the resonances of the Mn(CO)5 radical with characteristic decets of satellites due to hyperfine interaction between the unpaired spin on Mn and a Kr nucleus. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

Electrons and holes that are generated in particulate semiconductors are localized at different defect sites on the surface and in the lattice of the particles. Electron paramagnetic resonance (EPR) results have shown that electrons are trapped as two reduced metal centers—Ti(III) sites—eoordinated either [38, 39] 1) with anatase lattice oxygen atoms only, or 2) with OH or H2O the holes are trapped as oxygen-centered radicals covalently linked to surface titanium atoms [40] (Figure 7). This is summarized by Eqs. (7)-(9). 

Magnetic interactions of a paramagnetic metal center and a free radical ligand are of interest for the development of new types of molecular magnetic materials. Complexes of this type (such as (83)) change their magnetic properties upon irradiation by visible light and can serve as a basis for development of novel photo-activated memory units for electronic devices.346,347... 

The electron-donor centers on metal oxides can be measured by adsorbing certain organic molecules on the surface of the oxide. The transfer of an electron from the donor site of the oxide to the adsorbed molecule creates a paramagnetic ion detectable using electron paramagnetic resonance (EPR) spectroscopy. Che et al. (1972) studied the adsorption of tetracya-noethylene (TCNE) on MgO that had been pretreated between 100 and 800°C. Using EPR methods they identified the presence of adsorbed TCNE radical anion. As the pretreatment temperature increased from 100 to 800°C, the concentration of the radical anion passed through two maxima, one at 200°C and the other at 700°C. The electron-donor centers were found to be associated with OH and O ions with a low coordination number. 

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