Roaming: A Phase Space Perspective

Abstract

In this review we discuss the recently described roaming mechanism for chemical reactions from the point of view of nonlinear dynamical systems in phase space. The recognition of the roaming phenomenon shows the need for further developments in our fundamental understanding of basic reaction dynamics, as is made clear by considering some questions that cut across most studies of roaming: Is the dynamics statistical? Can transition state theory be applied to estimate roaming reaction rates? What role do saddle points on the potential energy surface play in explaining the behavior of roaming trajectories? How do we construct a dividing surface that is appropriate for describing the transformation from reactants to products for roaming trajectories? How should we define the roaming region? We show that the phase space perspective on reaction dynamics provides the setting in which these questions can be properly framed and answered. We illustrate these ideas by considering photodissociation of formaldehyde. The phase-space formulation allows an unambiguous description of all possible reactive events, which also allows us to uncover the phase space mechanism that explains which trajectories roam, as opposed to evolving toward a different reactive event.

Re‐Evaluating the Transition State for Reactions in Solution

“…Because the rate is calculated as the flux through the TS, any non‐reactive trajectory that crosses the TS dividing surface, or reactive trajectory that crosses it more than once will increase the flux through the dividing surface, thus leading to an overestimate of the rate constant. This means that TST gives us an upper limit on the true rate constant, and that if we found a dividing surface without any recrossing then TST would give the exact value of the rate constant (subject to certain caveats).…”

Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions

“…This behaviour is also seen in the intermediates formed in ion-molecule reactions, where ion-dipole forces hold the reaction complex together 48 . As a result of these common longrange attractive features, roaming reaction pathways are expected to play a role in ion-molecule reactions 49 . A signature of roaming reaction processes can be found in the way that energy is distributed into the product degrees of freedom.…”

UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization

“…The effect of roaming dynamics in the photochemistry of ions is an interesting question that has been little explored to date. Roaming in ion-molecule reactions has been discussed recently by Mauguière et al [58][59][60] from the perspective of a phase space interpretation of the dynamics. An example of roaming, albeit before the term roaming was coined, in an ionic system highlighted by Mauguière et al 58 (and earlier by Klippenstein et al 61 ) is the decomposition of metastable protonated propylamine to form propyl radicals and ammonium cations studied by Audier and Morton.…”

“…Roaming in ion-molecule reactions has been discussed recently by Mauguière et al [58][59][60] from the perspective of a phase space interpretation of the dynamics. An example of roaming, albeit before the term roaming was coined, in an ionic system highlighted by Mauguière et al 58 (and earlier by Klippenstein et al 61 ) is the decomposition of metastable protonated propylamine to form propyl radicals and ammonium cations studied by Audier and Morton. 62 It is quite possible that the presence of long-range attractive ion-dipole or ion-induced dipole interactions between the charged and neutral 'radical' products means roaming may be a general phenomenon in the fragmentation of ionic species.…”