Abstract

This chapter describes the strategy and operational approach to the practical computational investigation of reaction mechanisms in organic photochemistry. The focus is to show the way this task can be achieved through high-level ab initio quantum chemical computations and ad hoc optimization tools, using either real or model (that is, simplified) systems. Another purpose of the chapter is to show that, nowadays, a computational chemist can adapt his/her “instruments” (the method, approach, and level of accuracy) to the problem under investigation, as every other scientist does when there is a problem to study and a methodology to be chosen. In particular, different and often complementary computational tools may be used. The chapter describes extensive use of a combined ab initio CASPT2(10,11)//CASSCF(12–15) methodology as it has been proven to reproduce data with nearly experimental accuracy. The operational procedure for approaching a photochemical problem are then described and discussed. The applications of such a procedure to the intriguing problems of determining the mechanism of the photoinduced cis-trans isomerization of a retinal protonated Schiff base (RPSBs) model and of azobenzene (Ab) are discussed. The complex network of reaction paths underlying the photochemical reactivity of cyclooctatetraene is discussed to illustrate both general and subtle aspects of photochemical organic reaction mechanisms.

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