Molecular systems conical intersection pairing

The central mechanistic feature in most photochemical mechanisms is the conical intersection. Thus we hope to present some thoughts about how to predict and rationalize the molecular and electronic structure of such mechanistic features using VB ideas. It turns out that one can derive analytical results for n orbitals with n electrons so we shall develop the main ideas with reference to the photochemistry of some simple model systems such as the cycloaddition of two ethylene molecules and the radiationless decay of benzene. Once one allows zwitterionic systems, lone pairs and heteroatoms, the same principles apply but analytical results are not available so easily and one must be content with a qualitative analysis at the moment. 

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