MODELING PHOTOCHEMICAL REACTIVITY OF ORGANIC-SYSTEMS - A NEW CHALLENGE TO QUANTUM COMPUTATIONAL CHEMISTRY

The applicability of modern methods of quantum computational chemistry to the problem of modelling photochemical mechanisms is discussed. It is demonstrated that one of the central mechanistic features is the s urface-crossing region which takes the form of a conical intersection. It is shown that this mechanistic feature can yield a new rationaliza tion of common experimental observations such as the simultaneous prod uction of different photoproducts, the occurrence of rigid stereochemi stry, and the occurrence of very low fluorescence quantum yields. In a ddition, a simple VB model can often lead to accurate predictions abou t the nature of the surface-crossing region and the nature of the reco upling pathways on the ground-state surface.