Friday, July 25, 2014

When the buffalo roam, do they go over the pass or across the plain?

In one of our papers from last year (recapped in an earlier post), we found that in at least one chemical reaction (the ketene isomerization), the reaction could involve rather distinct pathways. On the one hand, the system could go across the break between the two energetic mountains separating the reactant and product. On the other hand, it could find a flat plain in which it could meander slowly across. The first of these two cases involves a narrow pass that is difficult for it to get through. The other is a wide plain but it costs a lot of energy to get up to it. Chemical reaction rate theory is built on the notion that the reaction always goes across the narrow passage as long as it’s the easiest one to get over. However, in the last decade there has been a lot of work observing that roaming over the flat plain has its privileges.

My postdoctoral student, Inga Ulusoy, and I wondered whether the ketene reaction gave rise to both possible classes of paths. It did! We also wondered the degree to which each path affected the rate in which the molecule reacted. We found earlier that the traditional pathway (over the break between the barriers) was the most important one in a model of the reaction with only two degrees of freedom. This led us and others to question whether our result was an accident of the simplicity of our model. In our recent paper, we extended the model to a larger number of degrees of freedom. Interestingly, the main result was the same. Namely, the reacting partners still have the possibility of roaming, but the direct path over the break between the barriers is still the most important one.

The article, "Revisiting roaming trajectories in ketene isomerization at higher dimensionality,” was recently published at Theoretical Chemistry Accounts 133, 1528 (2014). (doi:10.1007/s00214-014-1528-z) The work was supported by the AFOSR. Equally, importantly, it was a real treat to include our work in an issue published in honor of Greg Ezra's 60th birthday. I have followed his work since I was a graduate student, and have learned much from it. While science is immutable, it’s the fact that people are involved in the discovery that makes it humane. And for this reason, it’s particularly fun to be able to contribute to issues that honor the people involved in advancing science.

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