Roaming pathways and rates: A case study on ketene(#ACS #JPC-A #justpublished)

It's no surprise that chemists care about chemical reactions. We want to know both how the reactions take place—mechanism or pathway—and how much time it takes to happen—rates. One of the most successful, though approximate, theories has been transition state theory (TST), in part, because it provides an answer to both questions. You simply need to find the saddle (or col) on the potential energy landscape between reactants and products. That bottleneck, which can be described with varying levels of fanciness, gives you a sense of how the atoms in the reacting system have to distort so as to proceed to products. The energy of the bottleneck can be used in a well-known formula to obtain the rate. Recently, however, Joel Bowman and others have discovered the possibility that the reactants could avoid the bottleneck entirely. These roaming trajectories thus pose a challenge to TST, and have generated a lot of well-deserved buzz.

In those cases when roaming trajectories wander so far away from the transition state (bottleneck) that new product channels (such as radical molecules) become accessible, there is no doubt that everything goes topsy turvy. However, we were curious as to whether roaming trajectories would turn TST upside down even when such channels are not available. In recent work, we studied the ketene isomerization reaction—that is when it interconverts from one form to another—and found that it gave rise to roaming trajectories (such as the one pictured here.) Unfortunately, TST remains reasonable for this system as long as one is careful to generalize the dividing surface associated with the bottleneck so as to appropriately include roaming trajectories. So perhaps all remains good with TST after all?!

The title of the article is "Effects of Roaming Trajectories on the Transition State Theory Rates of a Reduced-Dimensional Model of Ketene Isomerization" and the work was funded by the AFOSR. It was released recently at J. Phys. Chem. A, ASAP (2013). (doi:10.1021/jp402322h) 

Click on http://dx.doi.org/10.1021/jp402322h to access the article.