Photochemistry by conical intersections: A practical guide for experimentalists

Many photochemical reactions are believed to proceed through conical intersections. The properties of conical intersections leading to the ground state of a given system are discussed using the phase-change rule: the ground-state total electronic wave function changes its sign when the system is transported along a complete loop around a conical intersection. It is shown that this property may be used to find the conical intersections present in the system, to predict possible products and even the energy disposal. An important corollary is that in a photochemical reaction involving a conical intersection, more than one product is necessarily formed. One of the products is always a 'photochemically allowed' one (Woodward-Hoffmann nomenclature), the second may be a thermally allowed one. A method to qualitatively predict the geometry of a conical intersection is presented and compared with previous calculations. For the 1,4-hexadiene system, the method was shown to help in locating computationally a conical intersection that can lead to the formation of benzene and H₂, accounting for the 'helicopter-type' motion observed by Lee and coworkers [J. Chem. Phys. 95 (1991) 297].