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I was working with nudged elastic band (NEB) to model some reactions, and someone asked what would happen if there are multiple pathways between the reactant and product (minima on the PES). This led me to wonder, if there are possibly reactions where there can be multiple transition states (or multiple pathways) between the same two minima on the PES, and I could not think of an example.

This is different than bifurcating reactions (or VRI), where one TS connects to three minima. I am also not looking for the trivial case of a conformational rearrangement of the reactant and product, which then gives a conformer of the TS.

Are there any non-trivial examples of such reactions where there are multiple pathways between the reactant and product (or two minima on the same PES), with multiple TSs connecting the two minima? (I am looking mainly for cases which have been computationally modelled)

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  • $\begingroup$ to me that sounds like conical intersections of two PES, is that what you are referring to? $\endgroup$ Dec 20, 2023 at 14:09
  • $\begingroup$ @JaafarMehrez No, I am asking about two different pathways between two minima (discounting trivial cases of conformers) on the same PES $\endgroup$
    – S R Maiti
    Dec 20, 2023 at 16:43

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Perhaps the simplest example is the transition between cis and trans diazene, N2H2. Both are planar structures, with the two hydrogens either on the same or opposite sides of the central N=N. There are two different TSs connecting these two minima, one corresponding to a rotation around the N=N with C2 symmetry, and another corresponding to an inversion of one of the nitrogen with Cs symmetry. For large flexible systems, there will be many TSs corresponding to different conformations of the reactant and product, and thus different conformational TSs, but that is perhaps not what you are looking for? For enzymatic reactions, there will be a large number of TSs corresponding to different configurations of the combined substrate and enzyme.

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  • $\begingroup$ Thanks! yes, the trans-diazene inversion is exactly the type of reaction i was looking for. Do you have any more examples of similar reactions? And are there examples of such enzymatic reactions with multiple TSs where the PES has been computationally modelled? I would be interested to read if you have any references. $\endgroup$
    – S R Maiti
    Dec 21, 2023 at 21:58
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    $\begingroup$ Another example could be the E2/E1/E1cb tribe, same reactant and product, but different reactions paths, possibly with intermediates. Enzymatic reactions are typically studied by MD methods with enhanced sampling methods. $\endgroup$ Dec 22, 2023 at 19:20

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