Triatomic complex potential energy surface

In an indirect reaction [2] A + BC —t B-A-C —t AB + C or AC + B. In a first step, the A atom inserts into the BC diatom forming an ABC complex. Two new bonds (AB and AC) are formed while the BC bond is broken. Then the complex dissociates with a breaking of one of these two bonds. This reaction mechanism is called insertion. In contrast with abstraction reactions, all three bonds in the triatomic molecule ABC participate actively in the reaction. Two bonds are formed teni] )orarily while only one exists for the reactants and products. Thus, the potential energy surface involves a very deep well (several eV) which correspond to a stable ABC molecnle or radical. When the lifetime of the ABC molecule is larger than its rotational period, angular distributions of the products are symetric with a backward/forward peak and the population of rovibrational states of the products presents a statistical character. 

The vibrations of the activated complex resemble those of an ordinary triatomic molecule except for the asymmetric stretch. A stable linear triatomic molecule GHI would have a potential energy surface as shown by the contours of equal potential energy in Figure 26.2, with a relative minimum at point b instead of a saddle point. It has four normal modes two bends, a symmetric stretch, and an asymmetric stretch. Motion along the path labeled abc corresponds to the asymmetric stretch, and motion along the path labeled gbh corresponds to the symmetric stretch. The activated complex has three vibrational modes two bends and a symmetric stretch, which is like that of GHI because the saddle in the potential energy surface corresponds to a minimum in the gbh direction. The asymmetric stretch of the activated complex corresponds to... 

In the last decade or so a number of efficient numerical methods for solving the quantum mechanical equations of motion have been developed. On the other hand, the calculation of accurate potential energy surfaces required for realistic dynamics studies has also made significant progress. Both developments have led to a number of very fine applications, mostly for triatomic molecules, which can be considered as prototypes for more complex systems. 

---