Transition state theory diabatic

The traditional theory for the rate of chemical reactions is the transition-state theory [21] (abbreviated as TST). In fact, all the rate constants given so far in previous sections were formulated, in general terms, within the framework of the TST. It is tacitly assumed in this theory that fluctuations in the reactant state are so rapid that all the substates comprising the reactant state are always thermally equilibrated in the course of reaction. According to this assumption, the reactant population in the transition state is always maintained in thermal equilibrium with the population in the reactant state since both states are located on the reactant-state adiabatic (or diabatic) potential. Therefore, calculation of the rate constant is greatly simplified... 

In chemical dynamics, one can distinguish two qualitatively different types of processes electron transfer and reactions involving bond rearrangement the latter involve heavy-particle (proton or heavier) motion in the formal reaction coordinate. The zero-order model for the electron transfer case is pre-organization of the nuclear coordinates (often predominantly the solvent nuclear coordinates) followed by pure electronic motion corresponding to a transition between diabatic electronic states. The zero-order model for the second type of process is transition state theory (or, preferably, variational transition state theory ) in the lowest adiabatic electronic state (i.e., on the lowest-energy Bom-Oppenheimer potential energy surface). 

While chemical reactivity is normally discussed in terms of transition state theory, corresponding to motion on an adiabatic potential energy surface, a complementary theory has been formulated (see the review by Metiu et in which the passage from reactants to products is formulated in terms of a transition between two diabatic surfaces. We shall refer to this as diabatic transition structure theory. This approach is a very natural one to use in semiclassical trajectory calculations (see, for example. Refs. 40) and, as we shaU presently discuss, enables a simple interpretation of transition structure. 

The Type 3 SN2 reaction between Cl- + CH3SHf is interesting because it represents a formal anion-cation recombination through substitution. Because charges are annihilated in forming the transition state, polar solvents will significantly destabilize product formation. Fortunately, the loss in solvation of the two ions is compensated for by electrostatic attractions in bringing the two reactant species into contact. Therefore, the outcome of an SN2 reaction of Type 3 depends on the balance of Coulomb stabilization and solvent destabilization. The reactant and product diabatic states are defined as follows in MOVB theory ... 

BJ theory replaces the classically derived energy surfaces and transition-state model of the MH treatment (Figs. 1 and 2) with a quantum-mechanical tunneling model. The BJ diabatic energy surfaces are shown in Fig. 990 Only the solvent is treated classically, and the X-axis now only represents solvent reorganization. Both energy... 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

In this article we will review the theoretical as well as the experimental work devoted to these processes. In the first Section a resume will be given of the theory of diabatic transitions at crossings between molecular states. The interaction between two particles is treated comprehensively and is followed by a brief discussion of crossings of multidimensional potential surfaces. Section II reviews the experimental work. Because the instrumental techniques are not essentially different from those applied to beam research on electronic excitation we will refer to the article on this subject in this volume. 

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