Biradical localized

Forbes M D E 1993 The effect of localized unsaturation on the scalar exchange coupling in flexible biradicals J. Phys. Chem. 97 3390-5... 

The combination is in this case an out-of-phase one (Section I). This biradical was calculated to be at an energy of 39.6 kcal/mol above CHDN (Table ni), and to lie in a real local minimum on the So potential energy surface. A normal mode analysis showed that all frequencies were real. (Compare with the prebenzvalene intermediate, discussed above. The computational finding that these species are bound moieties is difficult to confimi experimentally, as they are highly reactive.)... 

Thus, these results indicated the involvement of heavy atom tunneling in the localized biradicals. The rates of decay for 19,20, and 9 could be fitted with Bell s simple model of tunneling through a parabolic barrier. Assuming log A (s ) = 8.0, and... 

A second class of molecules with two unpaired electrons has the two electrons localized, to a first approximation, in different parts of the molecule. We refer to such molecules as biradicals (or, with three or more unpaired electrons, triradicals, etc.). Examples are the dinitroxides (e.g. 1), and certain binuclear vanadium(iv) and copper(n) complexes. 

In metalloproteins two paramagnets can be much farther apart, and so the dipolar interaction can be correspondingly weaker. Furthermore, the centers will usually each have significant g-anisotropy, and their local structures will differ and will have a complex mutual geometrical relationship. We therefore use the symmetric biradical as a simple model to obtain a first impression of the type of spectral patterns to be encountered. 

Localization of an unpaired electron in the framework of a definite molecular fragment can sometimes lead to the formation of ion-radicals with spatially separated charge and radical sites. They can be considered free radicals with an appended, remote charge. These species form a particular class of distonic ion-radicals. Distonic is from the Greek word diestos and the Latin word distans, both meaning separate. Yates et al. (1984) introduced this term for ions that formally arise by the removal of an electron from a zwitterion or a biradical. 

If a chemical reaction passes through a biradical intermediate, intersystem crossover may occur because of the localization and consequent near degeneracy. An example is provided by the reaction of ground state... 

Fig. 7. Possible potential curves for the reaction (10-1). (The splitting of the perturbed curves I and II may be exaggerated in the region of the localized biradical intermediate.)...

The calculations showed an apparent preference for the inversion of configuration in the reaction, for which the PES had a very distinct local minimum corresponding to a biradical. It was suggested that an experimental observation of a preference for inversion of configuration at the migrating carbon should not be taken as definitive evidence against a biradical mechanism. The dynamics effect may make mechanistic discussion by means of product analyses less straightforward. 

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