Intrinsic reaction coordinate , direct

Intraanchor reactions, conical intersection, two-state systems, 437-438 Intramolecular electron transfer, electron nuclear dynamics (END), 349-351 Intrinsic reaction coordinate (IRC), direct molecular dynamics, theoretical background, 358-361... 

Figure 21. la is a representative plot of the variation of the potential energy V(RC) along the intrinsic reaction coordinate Rc for a one-step process A —> B. Note that Rc is treated as a vector, always in the direction from reactants to products. For the reverse process, therefore, Rc would increase in the opposite direction. V(RC) goes through a maximum at the point RC =RC /3, which is the position of the... 

The top of the profile is maximum (saddle point) and is referred as the transition state in the conventional transition state theory. It is called a saddle point because it is maximum along the orthogonal direction (MEP) while it is minimum along diagonal direction of Fig. 9.12. The minimum energy path can be located by starting at the saddle point and mapping out the path of the deepest descent towards the reactants and products. This is called the reaction path or intrinsic reaction coordinate. 

The geometrical properties of the PES in the vicinity of a transition state mean that the steepest descent path down from the transition state (also generally calculated in mass-weighted coordinates, and called the intrinsic reaction coordinate or IRC) will usually lead only to a single reactant in one direction and a single product (or intermediate) in the other. However, a transition state can sometimes be shared by more than one reactant and/or product. One of these cases arises when the PES possesses a so-called valley-ridge inflection point (VRI). °... 

The key idea of mass weighting, for instance in Fukui s term the intrinsic reaction coordinate (IRC) as a mass weighted steepest descent, is the transformation of the (x,y) coordinate system such that the transformed gradient direction coincides with a real distortion under conservation of the center of mass. 

The relative energies obtained for the optimized structures of reactants, transition states, and products provide the reaction energy profile. Transition states are theoretically determined as a saddle point on the potential energy surface, and are confirmed by frequency analysis as well as IRC (intrinsic reaction coordinate) search then kinetics and thermochemistry of a reaction can be obtained. Since direct experimental evidence of elementary reactions is limited, the theoretical infortnation provides insight for improving the current properties of the catalyst. Studies of many catalytically important reactions have been reviewed recently. " ... 

Once a transition state is located, it is essential to determine the corresponding reactant and product. The best means of doing this is to construct an intrinsic reaction coordinate (IRC) (see Reaction Path Following). Starting with the Hessian (force constant matrix) at the transition state, a minimum-energy path is located (usually in mass-weighted coordinates) to reactants in one direction and to products in the other direction. A less reliable (but less expensive) method is to simply make a distortion in the direct of steepest descent and to optimize to a minimum. Of course, a combination of the... 

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