Quinolines orbital energies

By way of example, we might consider the interaction of a quinoline ligand whose donor nitrogen lone pair is directed exactly toward the metal. The local pseudo-symmetry is C2 and we may label the metal-nitrogen axis as Z, with Cartesian coordinates X and Y lying in, and perpendicular to, the quinoline plane respectively. The central assumption of the AOM is that the d orbital energy matrix is diagonal within this coordinate frame and we write... 

It is difficult to treat the effect of a heteroatom on the localization energies of aromatic systems, but Brown has derived molecular orbital parameters from which he has shown that the rates of attack of the phenyl radical at the three positions of pyridine relatively to benzene agree within 10% with the experimental results. He and his co-workers have shown that the formation of 1-bromoisoquinoline on free-radical bromination of isoquinoline is in agreement with predictions from localization energies for physically reasonable values of the Coulomb parameters, but the observed orientation of the phcnylation of quinoline cannot be correlated with localization ener-... 

A systematic and intensive theoretical study of reactivity has been reported by Brown and his colleagues,8,115,139-142 who discussed the reactivity of pyridine, quinoline, and isoquinoline in terms of localization energies. They investigated the values of these indices, first of all for electrophilic substitution, with regard to the value of the Coulomb integral of the heteroatom orbital and the orbitals adjacent to it (auxiliary inductive parameters). They demonstrated that the course of electrophilic substitution can be estimated from theoretical reactivity indices if 77-electron densities are used for reactions that occur readily and localization energies for those occurring only reluctantly. 

Analogously, although the enhanced reactivity of quinoline derivatives (relative to pyridine counterparts) in nucleophilic additions such as the formation of pseudobases and Reissert compounds probably relates to the relatively lower energy of the LUMO in the quinoline derivatives, the radicals from these heterocycles, in which the corresponding orbitals are singly occupied, are anionic thus, reactivity toward nucleophiles is also unlikely to be important. 

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