Delocalisation takes the concept of resonance and says that if the structure interchanges so rapidly that the individual structures cannot be seen, it is as if there are electrons which are associated with more than one nuclear centre.
The idea of molecules resonating between different forms provides a good model for approximating the electronic structure to the actual shape and bond characteristics of molecules. However, there is no evidence that these different forms ever really exist and all attempts to isolate one form or another have been unsuccessful.
A more successful theory is that of the delocalisation model, in which it is accepted that electrons in pi systems can be delocalised throughout a structure, providing there are suitably oriented 'p' orbitals. This approach more closely fits the observations and ties in nicely with the more sophisticated molecular orbital theory (section 2.27).
Delocalisation is similar to resonance only the existance of the different resonating forms give way to a permanently delocalised (i.e. having no specific location between two atoms) molecular orbital.
A good example of delocalisation is ozone.
The ozone layer is a region of the atmosphere where there is an equilibrium between molecular oxygen and ozone gases.
The energy required to promote one of the double bond electrons in oxygen, making a diradical, is considerably higher that the corresponding energy required to promote an electron in ozone.
Ozone is much more reactive than oxygen (lower activation energy) and hence the equilibrium for the above equation lies well to the side of oxygen.
|bond energy||wavelength of light|
As may be appreciated from the data, the energy required to dissociate ozone is in the ultraviolet regions of the spectrum. This absorbs harmful UV radiation preventing much of it from reaching the surface of the earth, where it could be very damaging to health (skin cancers, cell damage etc).
It is thought that until recent times the concentration of ozone has remained fairly constant, however pollution during the course of the 20th century introduced chlorofluorocarbons (CFCs) into the air which break down in the upper atmosphere forming chlorine free radicals.
These chlorine radicals are then able to react with the ozone, removing it faster from the equilibrium with oxygen then can be sustained by the reverse reaction.
Delocalisation stabilises molecules and ions by spreading out the regions of negative charge. This is a far more stable situation than keeping the electrons between two nuclear centres only. The extra energy saved is called the delocalisation stabilisation energy.
The greater the delocalisation the more stable the molecule, or ion. This is reflected in the stability of the polyatomic anions, such as sulfate, nitrate and carbonate.