Intrinsic reaction coordinate , potential

MEP (IRC, intrinsic reaction coordinate, minimum-energy path) the lowest-energy route from reactants to products in a chemical process MIM (molecules-in-molecules) a semiempirical method used for representing potential energy surfaces... 

Yoshizawa, K., Shiota, Y., Yamabe, T., 1999, Intrinsic Reaction Coordinate Analysis of the Conversion of Methane to Methanol by an Iron-Oxo Species A Study of Crossing Seams of Potential Energy Surfaces , J. Chem. Phys., Ill, 538. 

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... 

Figure 10. Potential energy diagram for the dehydrocyclization of 18 obtained from an Intrinsic Reaction Coordinate (IRC) calculation at B3LYP/6-31G. Note that the IRC starts at the TS but does not quite reach either ground-state structure (reactant-products).

An alternative and more costly approach is to actually follow the reaction from transition state to both the reactants and (independently) the products. In practice, this involves optimization subject to a fixed position along the reaction coordinate. A number of schemes for doing this have been proposed, and these are collectively termed Intrinsic Reaction Coordinate methods. Note, that no scheme is unique while the reactants, products and transition state are well defined points on the overall potential energy surface, there are an infinite number of pathways linking them together, just like there are an infinite number of pathways leading over a mountain pass. 

Potential, N-H Bond Length, and Inverse Vibration Frequencies as Functions of Intrinsic Reaction Coordinate for Inversion of the Ammonia Molecule... 

Fig. 2.7 The ozone/isoozone potential energy surface (calculated by the AMI method Chapter 6), a 2D surface in a 3D diagram. The dashed line on the surface is the reaction coordinate (intrinsic reaction coordinate, IRC). A slice through the reaction coordinate gives a ID surface in a 2D diagram. The diagram is not meant to be quantitatively accurate...

Fig. 5.29 Possible potential energy surfaces for two reactions with the same calculated free energy of activation. Reaction 1 is nevertheless faster than reaction 2 because its transition state region is flatter. As a result, in a given time more molecules can stray from the intrinsic reaction coordinate and pass through the transition state region to the product...

Deng L, Exploring Potential Energy Surfaces using Density Functional Theory and Intrinsic Reaction Coordinate Methods, PhD Thesis (University of Calgary, Calgary, 1996)... 

---