Delocalized electronic system, definition

The usage of the molecular orbital version of the Lewis definitions allows to discuss donor and acceptor interactions, involving delocalized electron systems and locahzed but multicentered bonds. Moreover, one can take into account all degrees of electron donation ranging from nearly zero in the case of weak intermolecular attractions and idealized ion association, to complete transfer of one or more electrons (redox). 

First, it is not necessary that the donor and acceptor orbitals be localizable on a single atom or between two atoms, as implied by Lewis dot structures. That is, the orbitals may be multi-centered even in a relatively localized representation. Thus donor-acceptor interactions involving delocalized electron systems (jt-ring/ are naturally subsumed by the definitions. 

Definitions. Define and illustrate the following terms (a) hybridization, (b) trigonal bipyramidal shape, (c) octahedral shape, (d) resonance, (e) delocalized 7T system, (f) p-d tt bond, (g) principle of charge dispersal, (h) electron... 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

Homoconjugation can lead to a bond (electron) delocalization energy, which reveals an excess stability of the system when compared to suitable reference compounds. The selection of appropriate reference compounds is essential for the definition of homoconjugation. 

If you examine this structure you will see that there is definitely a pyrrole ring but that the pyridine ring is not all there. Of course, the lone pair and the tt electrons are all delocalized but this system, unlike indole and quinoline, is much better regarded as a ten-electron outer ring than as two six-electron rings joined together,... 

When part of a delocalized aromatic molecule, the anion, after excited state electron transfer, gives rise to a not too reactive radical so that if one plays with the sequence of hydrogen abstraction from solvent followed by proton abstraction from the base, the initial anion may be restored and the solvent usefully replaces the sacrificial donor. This is the case with naphtholate as anion, certainly an interesting anionic sensitizer to test in other systems. Since the solvent is by definition at high concentration, its recovering role is played with a maximum efficiency reaction between the solvent and one of the radicals arising from the electron transfer is a good way to circumvent the back electron transfer. 

There are several fundamental reasons why the GMH and adiabatic formulations are to be preferred over the traditionally employed diabatic formulation. The definition of the diabatic basis set is straightforward for intermolecular ET reactions when the donor and acceptor units are separated before the reaction and form a donor-acceptor complex in the course of diffusion in a liquid solvent. The diabatic states are then defined as those of separate donor and acceptor units. The current trend in experimental design of donor-acceptor systems, however, has focused more attention on intramolecular reactions where the donor and acceptor units are coupled in one molecule by a bridge.The direct donor-acceptor overlap and the mixing to bridge states both lead to electronic delocalization, with the result that the centers of electronic localization and localized diabatic states are ill-defined. It is then more appropriate to use either the GMH or adiabatic formulation. 

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