Surface reactions curve-crossing model

The third situation, figure 3.13(c), is an example of electronic predissociation. That is, the molecule passes from one electronic state, which is bound, to a dissociative state. The reaction rate is then dependent on the strength of the coupling between the two surfaces, a problem that can be understood by curve crossing models... 

Our purpose has been to review the MC-SCF method with particular reference to transforming to a VB model in order to interpret the results. While theoretical computation can now yield accurate and detailed information about chemical reactivity, with present computing technology, we will always be limited to the study of prototype reactions. Thus we must also try to understand why the potential surfaces have the topology they have in terms of simple models. The examination of models that reproduce the computed surface topology is very important because it allows one apply the results obtained obtained for prototype systems in a qualitative way to real systems. In particular it can be seen that the empirical models such as the Evans [25J curve crossing model that has been extensively applied recently by Shaik[26] and co-workers can now be implemented rigorously. 

This simple model shows how linear free energy relationships can arise from reactant and product energy surfaces with a transition state between them that is defined by the curve-crossings. Figure 19.19 shows how shifting the equilibrium to stabilize the products can speed up the reaction. It also illustrates that such stabilization can shift the transition state to the left along the reaction coordinate, to earlier in the reaction. If mil Im l, the transition state will be closer to the reactants than to the products, and if I mi Evans-Polanyi model rationalizes why stabilities should correlate linearly with rates. 

Figure 3.11 Orientation dependence of the cross-section for the reaction H + D2(v= /= 0) HD + D at the two indicated values of the collision energy Ej- The ordinate is dff R/dcos y = 2ctr(cos y). The solid curves were calculated from the angle-dependent line-of-centers model, Eq. (3.34), and the (open and filled) points represent dynamical computations (these are quasi-classical trajectory results that have statistical error bars as discussed in Chapter 5) on the ab initio potential surface referred to in Figure 3.10 [adapted from N. C. Blais, R. B. Bernstein, and R. D. Levine, J. Phys. Chem. 89, 20 (1985)].

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