Fluorinated nitrobenzene anion radicals

The interpretation of the experimental a constants for the fluoroalkyl and nitroxide radicals is complicated because the stereochemistry at the radical center is not well defined. This complication does not exist for radicals derived from aromatic compounds. Consequently, several research groups have investigated aromatic radicals to characterize the factors governing spin delocalization to /3-fluorine atoms. One aspect of this work concerns the epr spectra of nitrobenzene anion radicals. Another concerns the contact chemical shifts of paramagnetic transition metal complexes. The latter approach was initiated by Eaton, Josey, and Sheppard who examined stable bis(phenylaminotroponiminato)-nickel(II) complexes (55a). More recently, we have examined the contact chemical shifts in the nmr spectra of nickel acetylacetonate complexes of aniline derivatives (556). 

The observations made in the work on the fluoroalkyl and nitroxide radicals suggests that the deloealization of spin density to /3-hydrogen and /3-fluorine atoms depends importantly on the electronegativity of the other atoms in the alkyl group. This aspect of epr spectroscopy has also been investigated by the study of the constants for the 4-substituted nitrobenzene anion radicals (Table 21). 

Early work established that for semiquinone LVn was modest compared to the constants for the corresponding hydrogen and carbon nuclei in LV and LVI (123). These results infer that the 8-hydrogen atom Is orbital spin population and the 3-carbon atom 25 orbital spin populations are about an order of magnitude greater than the j3-fluorine atom spin population of 0.002. The and constants observed for fluoroalkyl radicals and nitrobenzene anion radicals also indicate, in general, that p" is much greater than p2s-... 

For a-substitution, we can compare, for instance, a for the p-fluoro-nitrobenzene radical-anion with a. for the nitrobenzene radical-anion in aqueous solution the ratio is 7-95 3-40, i.e. 2-3 1 (Kolker and Waters, 1964) for the o-fluoro and p-fluoro-substituted phenoxy radicals the corresponding ratios are 2-6 (Stone and Waters, 1964a). There are probably two contributions to the observed fluorine-splitting (Carrington et al., 1965) a spin-polarization term, as for C— H, giving negative spin density at the fluorine nucleus, and a spin-polarization of the fluorine Is and 2s electrons which arises from spin density in a 2p orbital on fluorine (C—— F) and results in a positive spin density at the... 

---