Path bifurcation

VRI, the actual reaction path bifurcates into two products before reaching TS7. The rate is determined by TS6, whereas the product ratio is controlled by the shape of the PES near the VRI point and TS7. Thus, a question arises how the product selection occurs when a subtle perturbation, such as isotopic substitution, is introduced and two symmetrical products become asymmetric ... 

The dynamics effect on path bifurcation and nonstatistical product distribution on a slightly perturbed symmetrical PES have also been reported for the C=N isomerization of benzylideneanilines.60... 

Dynamics effects, which were described in previous sections, on reaction pathways, concerted-stepwise mechanistic switching, and path bifurcation have in most cases been examined for isolate systems without medium effects. Since energy distribution among vibrational and rotational modes and moment of inertia of reacting subfragment are likely to be modified by environment, it is intriguing to carry out simulations in solution. The difference or similarity in the effect of dynamics in the gas phase and in solution may be clarified in the near future by using QM/MM-MD method. Such study would provide information that is comparable with solution experiment and help us to understand reaction mechanisms in solution. 

In reactions of mechanistic borderline, the reaction pathway may not follow the minimum energy path, but the reaction proceeds via unstable species on the PES. In other cases, the reacting system remains on the IRC but does not become trapped in the potential energy minimum. In some cases, intermediates are formed in reactions that should be concerted, whereas in other reactions a concerted TS gives an intermediate. Thus, the question of concerted versus stepwise appears too simple and the definition of concerted and stepwise reactions becomes unclear. In some reactions, the post-TS dynamics do not follow IRCs, and path bifurcation gives two types of products through a common TS. 

The potential energy surfaces of the reactions in equations 10 and 11 are more complex than that of equation 14 and involve reaction path bifurcation and low symmetry structures in each channel. The salient features of structures of these complexes and TSs are shown... 

Itoh, S. Yoshimura, N. Sato, M. Yamataka, H. Computational study on the reaction pathway of a-bromoacetophenones with hydroxide ion Possible path bifurcation in the addition/substitution mechanism, 7. Org. Chem. 2011, 76, 8294-8299. 

Distinguished Coordinate and Other Reaction Paths Bifurcations and Other Paths Coordinates and Path-Based Surfaces... 

In Sections IV and V, we discussed these two processes from the viewpoint of chaos. In these discussions, the following points are of importance. As for the barrier crossing, intersection (either homoclinic or heteroclinic) between stable and unstable manifolds offers global information on the reaction paths. It not only includes how transition states are connected with each other, but also reveals how reaction paths bifurcate. Here, the concepts of normally hyperbolic invariant manifolds and crisis are essential. With regard to IVR, the concept of Arnold web is crucial. Then, we suggest that coarse-grained features of the Arnold webs should be studied. In particular, the hierarchy of the web and whether the web is uniformly dense or not would play an important role. 

Beckmann rearrangement of oximes to amides can deviate to fragmentation to form nitriles and carbocations, if the latter possess reasonable stability. Both 1-substituted-phenyl-2-propanones and 3-substituted-phenyl-2-butanones in aqueous solvents give both products, and calculations have been used to probe the mechanisms. In borderline cases, a dynamic path bifurcation from a single transition state is claimed. 

Kinetic evidence for such a dynamic path bifurcation in the mechanism of the Beckmann rearrangement has also been presented for reactions of oxime sulfonates (43, R /R = H/Me). In addition to the rearrangement product (amide), these substrates can also fragment (to alcohols), with the bifurcation in the mechanism apparently occurring after the rate-determining transition state. 

Reaction of a-haloketone with a nucleophile can give carbonyl addition, direct substitution, and proton abstraction. DFT calculations suggest that the reaction of hydroxide with a-bromoacetophenone (PhCOCH2Br) is a borderline case, with an addition/substitution TS, where hydroxide interacts with/bridges the two carbons. Such a TS could serve for two mechanisms, with path bifurcation after the TS, leading to the respective hydrate anion [Ph-C(0H)(0 )CH2Br] and substitution products... 

Katori T, Itoh S, Sato M, Yamataka H (2010) Reaction pathways and possible path bifurcation for the Schmidt reaction. J Am Chem Soc 132(10) 3413-3422. doi 10.1021/ja908899u... 

P. B. Middleton and R. E. Wyatt, Quantum mechanical study of a reaction path bifurcation model, Chem. Phys. Lett. 21 57 (1973). 

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