PATHWAY APPROACH TO ULTRAFAST PHOTOCHEMISTRY - POTENTIAL SURFACES, CONICAL INTERSECTIONS AND ISOMERIZATIONS OF SMALL POLYENES

The dynamics of molecules, attention to which is being stimulated by m odem ultrafast spectroscopy, as well as progress in quantum chemistry are giving new impetus to the approach which follows the detailed path way of the molecules along the potential energy surfaces from the exci ted to the ground state. Local properties such as slopes, saddle point s, barriers and funnels with conical intersections as exits are taken into account. We show the fruitfulness of this approach by means of ne w and published experimental results on photochemical pericyclic react ions. The experimental data support the importance of conical intersec tions and, in some cases, yield a very detailed picture of the dynamic s on the potential energy surfaces. We emphasize the relation of the p ericyclic potential surfaces to those of cis-trans isomerization and s how that, in this case, the geometries of the conical intersections ar e interrelated and can be intuitively understood. The conical intersec tions of stilbene and polyenes can be deduced from the prototype of et hylene, and their geometries are supported by photochemical evidence. (C) 1998 Elsevier Science B.V. All rights reserved.