Conical Intersections Leading to Chemical Reactions in the Gas and Liquid Phases

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.

Topology of Conical Intersection Seams and the Geometric Phase

“…7,12,16−26 Since a molecule can be endowed with a large excess of vibrational energy after electronic excitation, it has become clear that extended portions of a CS can be relevant for nonadiabatic dynamics. 12 Therefore, methods have been proposed that allow to systematically track a CS, or to find points of particular interest in it. [17][18][19][20][21][22]24 In this work, we want to highlight two topological properties of the most typical and relevant CSs, those with dimension N − 2, where N is the number of internal coordinates of the molecule.…”

“…The crossings of potential energy surfaces deeply affect the excited state dynamics of many molecular systems. − For instance, easily accessible crossings lead to ultrafast decay of excited states, avoiding photodamage of many biological compounds. On the contrary, a crossing placed along the reaction coordinate can greatly enhance the yield of a photochemical reaction.…”

“…Therefore, the search and characterization of crossing seams are important items in the computational investigation and interpretation of excited state dynamics. Specific algorithms have been devised to locate the energy minima with the degeneracy constraint, that is, the most accessible points of a CS from the energetic point of view. ,,− Since a molecule can be endowed with a large excess of vibrational energy after electronic excitation, it has become clear that extended portions of a CS can be relevant for nonadiabatic dynamics . Therefore, methods have been proposed that allow to systematically track a CS, or to find points of particular interest in it. − , …”