Distinguished reaction coordinate

The second use of Equations (2.36) is to eliminate some of the composition variables from rate expressions. For example, 0i-A(a,b) can be converted to i A a) if Equation (2.36) can be applied to each and every point in the reactor. Reactors for which this is possible are said to preserve local stoichiometry. This does not apply to real reactors if there are internal mixing or separation processes, such as molecular diffusion, that distinguish between types of molecules. Neither does it apply to multiple reactions, although this restriction can be relaxed through use of the reaction coordinate method described in the next section. 

The reaction coordinate is the vibration between the molecule and the surface, and is not included in the vibrational partition function of the transition state. Again we can distinguish two extremes. 

In the consideration of chemical transformations, we shall distinguish elementary and combined (stepwise) stoichiometric transformations. The elemen tary chemical transformations are those that run through the formation of only one transition state. The transition state is not thermalized, or at least not thermalized at the reaction coordinate. The stepwise transformations comprise the formation of some intermediate products, which we shall always consider as thermalized. 

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). 

Fig. 6. Free energy landscape in which during a transition from A to B both the variable q and q change systematically left hand side). For this case, the variable q is a good order parameter and suffices to distinguish the equilibrium fluctuations in the stable states A and B. By following g as a function of time t we can detect transitions between the stables states right hand. side). However, this variable does not capture all essential aspects of the transition and is therefore not a good reaction coordinate. In the definition of such of a good reaction coordinate capable of describing the complete course of the reaction the variable q can not be neglected...

The term saddle point is synonymous with transition structure. The saddle point is distinguished by its possessing convex curvature along and concave curvature perpendicular to the reaction coordinate. Discussions of reaction surfaces often use the term saddle point interchangeably with transition structure. 

The two mechanisms have very dilferent transition states, and so should be distinguished by structural correlations if suitable molecules can be found to map the reaction coordinate. 

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