Abstract
The current status of the role of conical intersections (CoIns) in molecular photochemistry is reviewed with a special emphasis on the procedures used to locate them. Due to space limitations, the extensive literature of the subject is given by referring the reader to representative references, whereas the author group’s work is described in detail. The basic properties of CoIns are outlined and contrasted with those of transition states in thermal reactions. Location of CoIns using the method of Longuet-Higgins sign-inverting loops is described in detail. The concept of “anchors”—valence bond structures that represent stable molecules and other stationary points on the potential energy surface—is introduced and its use in constructing loops is described. The authors’ work in the field is outlined by discussing some specific examples in detail. Mathematical aspects and details are left out. The main significance of the method is that it explains a large body of photochemical reactions (for instance, ultrafast ones) and is particularly suitable for practicing chemists, using concepts such as reaction coordinates and transition states in the search.
Highlights
Analysis of chemical reactions is usually based on the concept of potential energy surfaces (PESs), which are derived from the Born-Oppenheimer (BO) approximation
Current quantum chemical calculations are largely performed by advanced methods of the molecular orbital (MO) theory originally developed by Hund, Slater, and Mulliken, such as Hartree-Fock, complete active space (CAS), and multireference configuration interaction (MRCI)
As the theory of thermal reactions is well-developed, an attractive option is the development of a model on equal footing for photochemical ones
Summary
Analysis of chemical reactions is usually based on the concept of potential energy surfaces (PESs), which are derived from the Born-Oppenheimer (BO) approximation. Some reactions start out on a certain route (for instance to bond cleavage) but at a certain point begin a roaming motion resulting in a different reaction pattern Such developments are important but are rather limited compared to the classical transition state theory and are ignored in this work. Light absorption promotes the molecule to an electronic excited state; the reaction is nonadiabatic, involving more than one PES. The first is nonsymmetric and leads, in a 1D space, to avoided crossing; the second tunes the energy difference to achieve crossing and is a symmetric one Their combined effect violates the BO approximation—leading to a nonadiabatic process. In view of the popularity and relatively easy and cheap use of DFT, these developments may become a key factor in the field
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