Organic Chemistry: Orbitals: Molecular Orbital Theory

Electron Delocalization

One of the greatest successes of MO theory is that it accounts for electron delocalization in a natural way. We have seen that some molecules require resonance structures to be represented accurately. In all such cases, electrons are delocalized over several bonds/atoms. One main drawback of the VB model is that it assigns electrons to specific atoms/bonds and therefore breaks down when it comes to explaining delocalized electrons. The MO model has no such problem; it offers a clean approach to describing delocalization that is superior to writing a bunch of awkward resonance structures.

Application of MO Theory to Extended Π-systems

Unfortunately, the complexity of the full MO model increases exponentially with the size of the molecule. In order for MO theory to be useful in practice, we limit its application to portions of a molecule that are extensively delocalized. This often occurs when Π electrons and lone pairs overlap over several contiguous atoms.

Let us once again consider benzene, the classic example of resonance. Recall that benzene consists of six identical C-C bonds each with a bond order of 1 1/2. In order to gain a reasonably simple MO treatment of benzene, the key is to consider the Π framework separately from the σ framework. We can assume that the σ bonds are fairly localized and are accurately described by the VB Model. The six Π electrons can be considered in a separate MO scheme without much loss of accuracy and predictive power.