ABSTRACT
Molecular Mechanical Approaches................. 204 4.4 Examples..................................................................... 205
4.4.1 Ethylene ........................................................... 205 4.4.2 Butadiene ......................................................... 210 4.4.3 Stilbene............................................................. 218
4.5 Conclusions ................................................................. 221 Acknowledgments................................................................ 224 References............................................................................ 224
4.1. INTRODUCTION
As amply demonstrated by other contributions in this volume, current thinking about photochemical mechanisms is strongly influenced by conical intersections.1-5 The lowest-lying of these points of exact degeneracy between two or more6 electronic states can be thought of as photochemical analogs of the transition state in a thermal reaction.5 Hence, the widespread efforts to develop efficient algorithms for locating these points7,8 and the efforts to elucidate them for photochemical reactions are not surprising. However, one must not forget
that there are some crucial differences between intersections and transition states. Transition states in thermal reactions are generally reached from below by rare trajectories with sufficient energy to scale the barrier. In contrast, intersections in photochemical reactions are usually reached from above by trajectories resembling downhill skiers careening out of control. One can easily imagine that it will therefore be very dangerous to apply thermal reasoning to photochemical reactions, and, in fact, the lowest energy point along an intersection seam could easily have little or no relevance to the photochemistry.
