%0 Journal Article
%T Conical Intersections Leading to Chemical Reactions in the Gas and Liquid Phases
%A Yehuda Haas
%J Advances in Chemistry
%D 2014
%R 10.1155/2014/419102
%X 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¡±¡ªvalence bond structures that represent stable molecules and other stationary points on the potential energy surface¡ªis 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. 1. Introduction Analysis of chemical reactions is usually based on the concept of potential energy surfaces (PESs), which are derived from the Born-Oppenheimer (BO) approximation. Reactions usually start in the ground electronic state, where the reactant is found in a local minimum. Energy must be supplied to the system in order to initiate the reaction and move on to the product. In thermal reactions the energy source is heat. The reaction proceeds along a trajectory that leads adiabatically to a transition state, which is a local maximum along the reaction coordinate (RC). Subsequently, the system goes down to the product. The entire route occurs on one potential surface (PES) only, along a single coordinate; this is thus a one-dimensional (1D) process. The RC is a combination of internal degrees of freedom, usually expressed in normal coordinates. This scenario, introduced in the 1930¡¯s, appears to account well for the vast majority of thermal reactions. There are exceptions, of course: some reactions lead to electronically excited states, creating the phenomenon of chemiluminescence [1]. Recently, a new mechanism termed roaming reactions has been introduced [2]. According to this approach, 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
%U http://www.hindawi.com/journals/ac/2014/419102/