Electron delocalization resonance approach

Generalized to any type of carbon-centered radical, this approach has led to use of C—H bond dissociation energies for estimating the unpaired electron delocalization energies of these species. As shown in Tables XXXIII, XXXIV, and XXXV, bond dissociation energies which, except for a constant, are equal to heats of reaction do not provide satisfactory resonance (or stabilization) energies of free radicals. Indeed, as stated before, a heat of reaction can never be used for determining any property of one of the species involved in that reaction. 

The two MO approaches to polyatomic systems, localized and delocalized, are useful in different circumstances. When localized descriptions are possible, they correspond more closely to the simple chemical pictures of electron-pairbonds provided by the Lewis and VSEPR models. Such descriptions are not always possible, and 3c or other delocalized models provide an alternative to the resonance approach (see below and Topic Cl). Delocalized MO theory is also more useful for interpreting electronic spectra of molecules. 

Spontaneous Raman scattering always occurs when the laser excitation frequency is less than the frequency associated with an allowed electronic transition of the molecule. As the probe laser frequency approaches that of an electronic transition in the molecule, certain vibrational modes that couple strongly to the transition increase in intensity (pre-resonance) with respect to other Raman allowed modes of the molecule. When the excitation frequency coincides with the electronic transition frequency (resonance), a dramatic increase in vibrational band intensities is observed. This effect has been observed in many molecules and especially in polymer films, such as polydiacetylene, that consist of extended regions of electron delocalization owing to the presence of conjugated double and triple carbon-carbon bonds in the linear network (40)(41). 

Thus, in the present approach, the major focus is on the question of how we can influence the external parameters like solvent and counterion and the intrinsic structural parameters within the systems A-l-A to force the electron-hopping process into the timescale of the experiment, or at least to establish clearly the borderline cases. That we are still looking at an electron-hopping process in the case of effective charge delocalization over the entire molecule and not at a pure resonance phenomenon may be reassured by VIS/NIR spectroscopy of the neutral and charged species the absorption of a single chromophore should be detected unless a very fast process > 1012 Hz is taking place. 

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

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