Coulomb repulsion, lone pairs

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... 

Many azamacrocycles, like cyclam [2] for instance, contain more than one nitrogen atom and therefore possess more than one pK. Therefore, in the discussion of the basicity of these compounds, it is not only the in- and OMt-orientation of the lone pair or the proton, respectively, that has to be considered. There is also an influence of the first on the second, third, etc. protonation. The developing Coulomb repulsions are very sensitive to the charge separation. In 15-membered rings this phenomenon has been investigated in detail (Arnaud-Neu et al., 1979). 

This difference between these two carbenes can be attributed to the presence of the lone pair of electrons on nitrogen in 4b. Two-center, three-electron bonding in the molecular plane of 4b, involving the lone pair, produces some unpaired a spin density on nitrogen. Coulombic repulsion between the a and tt electrons of... 

These approximations assume that contributions in V arising from coulombic attraction to nuclei other than the one on which Xii is located, and repulsions from the core, lone-pair, and valence electron clouds surrounding these other nuclei cancel to an extent that... 

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