Valley ridge inflection points, reaction

B3.5.7.3 BIFURCATION OF THE REACTION PATH AND VALLEY-RIDGE INFLECTION POINTS... 

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 general shape of the PES for this reaction is given in Figure 8.17. Note that the MEP proceeds from TS 63 to 62, and the MEP does not bifurcate. There is, however, a ridge that some trajectories may cross, allowing for trajectories to go from 63 to 64. This behavior is similar to that of a valley ridge inflection point, but there is no VRl on the MEP in this case. 

Figure 1 A model potential energy surface illustrating minima, transition structures, second-order saddle points, a valley-ridge inflection point, and reaction paths (Reproduced from Ref, 2a, with permission from World Scientific Publishing)...

Let s consider what happens after the molecule passes over TSl. The molecule continues to follow the reaction path downhill until it reaches the valley ridge inflection (VRI) point. At this point, the gradient in one direction perpendicular to the reaction path becomes zero, and after this point, the PES actually falls downhill faster in the direction off of the reaction path than continuing on the reaction path to TS2. It is important to note that the VRI is not a critical point the gradients are not all zero at a VRI point. So at the VRI, the pathway diverges off of the ridge that leads to TS2, in one direction toward Product 1 and in a second direction toward Product 2. What is so unusual about this type of surface is that the reaction paths that lead from the reactant over TSl then proceed onward to two products (Product 1 and Product 2), without crossing any more transition states. In other words, this surface has a TS that leads to two products ... 

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