Classical and quantum dynamical manifestations of index-2 saddles: concerted versus sequential reaction mechanisms

TL;DR

This study investigates how higher index saddles influence chemical reaction mechanisms, revealing that the shift from concerted to stepwise pathways is driven by classical dynamics and phase space structure, with implications for understanding reaction behavior.

Contribution

It provides a detailed classical and quantum dynamical analysis of index-2 saddles, highlighting the classical origin of reaction mechanism shifts and phase space complexity.

Findings

Classical dynamics cause the shift from concerted to stepwise mechanisms.

Phase space exhibits mixed regular and chaotic behavior affecting reaction pathways.

Rich dynamical behaviors, including Murrell-Laidler mechanisms, occur above saddle energies.

Abstract

The presence of higher index saddles on a multidimensional potential energy surface is usually assumed to be of little significance in chemical reaction dynamics. Such a viewpoint requires careful reconsideration, thanks to elegant experiments and novel theoretical approaches that have come about in recent years. In this work, we perform a detailed classical and quantum dynamical study of a model two degree of freedom Hamiltonian, which captures the essence of the debate regarding the dominance of a concerted or a stepwise reaction mechanism. We show that the ultrafast shift of the mechanism from a concerted to a stepwise one is essentially a classical dynamical effect. In addition, due to the classical phase space being a mixture of regular and chaotic dynamics, it is possible to have a rich variety of dynamical behaviour, including a Murrell-Laidler type of mechanism, even at energies sufficiently above that of the index-2 saddle. We rationalize the dynamical results using an explicit construction of the classical invariant manifolds in the phase space.