ELECTRONIC SUBSTITUENT EFFECTS IN RADICAL CHEMISTRY - THE DELTA-D SCALE FOR BENZYL-TYPE RADICALS

The D parameter of the localized triplet 1,3-diradicals 3, readily det ermined by EPR spectroscopy, reflects accurately electronic effects of meta and para substituents on the a spin density of the corresponding benzyl-type monoradicals 4. This spectroscopic method is not subject to polar effects in radicals and, thus, offers the opportunity to inve stigate strongly spin-delocalizing substituents such as NO2 and NR2 or even the charged groups NH3+ and O-. Indeed, as shown herein, the nov el Delta D scale provides a sensitive mechanistic tool to probe electr onic effects in radicals through changes in the alpha spin density whi ch derive from spin delocalization effects. This concept has been exte nded to heteroaromatic pi systems, i.e. pyridines, furans and thiophen es, which demonstrate the general applicability of the model presented herein. The elucidation of electronic effects in triplet diradicals f or the necessary basis in the use of paramagnetic species as building blocks for persistent, high-spin polyradicals and, for future perspect ives, the design of organic ferromagnetic materials.