Photochemistry by conical intersections: a practical guide for experimentalists

Many photochemical reactions are believed to proceed through conical inters ections. The properties of conical intersections leading to the ground stat e of a given system are discussed using the phase-change rule: the ground-s tate total electronic wave function changes its sign when the system is tra nsported along a complete loop around a conical intersection. It is shown t hat this property may be used to find the conical intersections present in the system, to predict possible products and even the energy disposal. An i mportant corollary is that in a photochemical reaction involving a conical intersection, more than one product is necessarily formed. One of the produ cts is always a 'photochemically allowed' one (Woodward-Hoffmann nomenclatu re), the second may be a thermally allowed one. A method to qualitatively p redict the geometry of a conical intersection is presented and compared wit h previous calculations. For the 1,4-hexadiene system, the method was shown to help in locating computationally a conical intersection that can lead t o the formation of benzene and H-2, accounting for the 'helicopter-type' mo tion observed by Lee and coworkers [J. Chem. Phys. 95 (1991) 297]. (C) 2001 Elsevier Science B.V. All rights reserved.