Model calculations of photochemical reactivity

Abstract

The first part of this paper is a review of the author’s recent work on theoretical aspects of organic photochemical reactions. Basic photochemical processes are summarized briefly in terms of Born-Oppenheimer hypersurfaces. The importance of the location of minima, funnels and barriers in the S1and T1 surfaces for determining the outcome of a reaction is pointed out. Various types of minima and funnels through which return to S0occurs are listed, discussed in simple MO and VB terms, and used for a simple classification of photochemical reactions, and for a discussion of singlet-triplet differences. Correlation diagrams are used to estimate the location of barriers in S1and T1. This review of previous work is followed by presentation of new results of model calculations for pericyclic reactions, in particular, for photocycloadditions. These illuminate the role of excimers in such processes, point out their relation to the process of triplet-triplet annihilation, and provide new information about the nature of excited electronic states at pericyclic biradicaloid geometries. It is shown that in addition to the well-known tendency to rapidly return to the ground state, the lowest excited singlet state should also exhibit an inclination for cross-bonding in the cyclic array. © 1975, Walter de Gruyter. All rights reserved.