The rate at which a process takes place is determined by the flux through any dividing surface that separates initial from final configurations. Transition state theory (TST) greatly simplifies the description of a dynamical process by the assumption that with a correctly chosen dividing surface, the long-time dynamics of the system can be neglected. In TST, only local information is used to determine the rate of a process, and in many cases, the TST approximation is sufficiently accurate. With the recent discovery of roaming pathways in reaction dynamics, a challenge is posed to TST. In roaming pathways, an atom or fragment of the molecule moves far out during the reaction, leading to large separation lengths and a slow conversion of reactants to products. In these roaming trajectories, the conventional bottleneck of the reaction is circumvented, and they do not cross the col of the potential energy landscape. We investigate the challenge of roaming to TST by using a simple model of ketene isomerization, for which we obtain the classically exact and TST reaction rates in good agreement with experiment. We show that in this model, there exists two pathways which carry the signature of roaming. In the simple model system employed here, the presence of these roaming trajectories does not unseat the existence of a single dividing surface which can be used to calculate reaction rates.

Roaming trajectories in a reduced-dimensional model of ketene isomerization

247th National Meeting (2014)

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