Nitrogen localized electron model

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

The pK values have been determined for several pyridine-like heterocycles.162,162° 1626 Their relation to the difference (AW) in -electronic energies of models of the conjugated acids (1FN+) and of the free bases (FFN) has been investigated.7 Values for 1FN+, Wx, AW, 7r-electron densities at the nitrogen atom in the free base (q), and localization energies (A,) of the position in the parent hydrocarbon in which there is a nitrogen atom in the heterocycle are given in Table VIII. 

In conclusion, the barriers for the rearrangements of the cyano derivatives of 1 are completely rationalized by the model of singlet phenylnitrene being described as a diradical one electron localized to nitrogen and the second electron delocalized about the phenyl ring. The predictions of this qualitative model are neatly confirmed by this combined computational and experimental study. 

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