Molecular photochemistry: A general method for localizing conical intersections using the phase-change rule

A photochemical reaction in which a conical intersection is involved is sho wn to lead to several different products. In particular, thermally allowed products are produced in many cases in addition to photochemically allowed ones. This is a consequence of the electronic wave-function phase-change ru le [H. C. Longuet-Higgins, Proc. R. Soc. London Ser. A. 1975, 344, 147], wh ich is a necessary condition for the existence of conical intersections. Th e rule is used to define the two coordinates along which the conical inters ection is formed, and hence its approximate geometry. These two coordinates are defined by the use of the structures of three chemical species on the ground-state surface, termed anchors. Two of the anchors can be chosen as t he reactant and the desired product; the third is another possible product. The phase-change rule requires that either one or all the transition state s between the three anchors must be phase-inverting, for instance antiaroma tic, When only one of the transition states is phase-inverting, a "thermall y allowed" product is always involved. The well-known importance of antiaro matic transition states in photochemical pericyclic reactions is explained by their essential role in forming conical intersections. The model provide s a rationalization for the properties of many of the recently calculated c onical intersections. The phase-change rule provides a simple, chemically o riented method for both the prediction of the course and stereoselectivity of photochemical reactions. It can also be used to reject structures propos ed for conical intersections by showing that the conditions necessary for i ts presence are not fulfilled.