Covalency paramagnetic resonance

The initial product of the inhibition step is not known in this case and may be a molecular complex.8 The direct reaction of the ethane with the peroxy radical is an example of a covalent compound giving a reaction resembling that of a related free radical. The molecular weight determination by Gomberg was therefore a necessary part of the proof that he was dealing with radicals and not merely an unusually reactive hydrocarbon. The presence of free radicals has since been confirmed by measurements of the paramagnetic susceptibility and the paramagnetic resonance absorption.9-10 The latter evidence also rules out an alter-... 

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

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). 

For example, McIntosh et al. [8] have detected a charge-separated species by electron paramagnetic resonance spectroscopy of a number of covalently linked P-Q compounds. 

In this review we shall emphasize the ionic viewpoint, but not because we are unaware of the importance of covalent bonding. We believe that only after the irrelevance of much that has been presented as qualitative stereochemical evidence for eovalency has been appreciated, does the importance of quantitative studies, for example of unpaired electron distributions by paramagnetic resonance experiments, become clear. Our program, then, is to interpret as many features as possible of the stereochemistry of metal compounds from the point of view of an ionic theory and only incidentally to draw attention to the modifications that are required if the covalent effects (which, to a greater or lesser extent, influence the properties of all compounds) are to be taken into account. 

In every case we have emphasized that the distortions are not without effect on the covalent character of the bonds involved. We have tried to show that the tendency to distortion could often have an electrostatic origin, and hence that other than stereochemical arguments must be used in determining the degree of covalency both in distorted and in undistorted complexes. Such methods include the study of antiferromagnetic interaction, nuclear magnetic resonance spectra, and paramagnetic resonance spectra. 

Of these five key fields, electron paramagnetic resonance (EPR) spectroscopy as a local, sensitive, and highly selective technique can naturally add important information to fields (2) and (3), i.e., formation of complex polymer systems through non-covalent interactions and characterization and properties thereof, respectively. Polymeric systems have been under investigation with EPR spectroscopy for more... 

Complex 146 (entry 0-3 of Table 3) forms a free radical in THF solution and the electron paramagnetic resonance (EPR) spectrum shows a weak triplet signal with hyperflne splitting by coupling with the P ligands. The triplet disappears when Ag(0) precipitates. The presence of covalent radicals [Ag—CH(R)OH]"+, R = H, Me, in Ag(l)-containing molecular sieves loaded with MeOH or EtOH was detected by EPR and electron spin echo envelope modulation spectroscopies. ... 

In the case of paramagnetic ions, the effect of the solvent on the dissolved ion may be followed by studying the effect on the unpaired electrons by means of electron paramagnetic resonance (EPR) spectroscopy [Sy 69]. In concentrated solutions, solution X-ray examinations may give information on the immediate environment of the dissolved ion [We 69, Ah 78]. Mossbauer studies on rapidly frozen solutions [Bu 72, Bu 73, Ve 70a, Ve 70b, Ve 79] permit conclusions to be drawn primarily about the symmetry and covalency of the inner coordination sphere in dissolved species containing a Mossbauer atom. 

Regen (1974) described a method of assessing the mobility of the resin sites by covalent attachment of a spin-label probe [2,2,6,6-tetramethyl-4-piperidinyl-l-oxy group (Tempo) (Fig. 3-1. R = (P)—)] to the chlo-romethylated co(polystyrene-DVB). Rotational correlation times (t) were calculated from observed room-temperature electron paramagnetic resonance (epr) spectra. The degree of swelling, q, (swelled volume/ dry volume) was determined from the measured density of the dry resin and the weight of the imbibed solvent. The data indicate that those... 

---