Mechanistic Photochemistry and Conical Intersections

The focal point of the mechanism associated with a nonadiabatic transition through a conical intersection is the description of the reaction path from the excited state of the reactant to the product on the ground state. In this chapter, we show that the location of conical intersections may be deduced by simple valence bond (VB) analysis of the elementary reactions between reactant and product/s. The loop, determined by two reaction coordinates, can be phase-preserving or phase inverting, and allows to locate the conical intersection according to Longuet-Higgins phase change theorem.