Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

Abstract

The photodissociation of formaldehyde was studied using quasi-classical trajectories to investigate "roaming," or events involving trajectories that proceed far from the minimum energy pathway. Statistical analysis of trajectories performed over a range of nine excitation energies from 34 500 to 41 010 cm(-1) (including zero-point energy) provides characterization of the roaming phenomenon and insight into the mechanism. The trajectories are described as projections onto three coordinates: the distance from the CO center of mass to the furthest H atom and the azimuthal and polar coordinates of that H atom with respect to the CO axis. The trajectories are used to construct a "minimum energy" potential energy surface showing the potential for any binary combination of these three coordinates that is at a minimum energy with respect to values of the other coordinates encountered during the trajectories. We also construct flux diagrams for roaming, transition-state, and radical pathways, as well as "reaction configuration" plots that show the distribution of reaction geometries for roaming and transition-state pathways. These constructs allow characterization of roaming in formaldehyde as, principally, internal rotation of the roaming H atom around the CO axis at a slowly varying and elongated distance from the CO center of mass. The rotation is nearly uniform, and is sometimes accompanied by rotation in the polar coordinate. The roaming state of formaldehyde can be treated as a separate kinetic entity, much as one might treat an isomer. Rate constants for the formation of and reaction from this roaming state are derived from the trajectory data as a function of excitation energy.

Roaming: A Phase Space Perspective

“…We show that, in phase space, roaming can be precisely characterized via a new dynamical mechanism, which we term shepherding (11). We compare and contrast our results with the recent work of Houston et al (12) in Section 5.3. In Section 6, we consider some additional case studies of the roaming phenomenon, again drawn from our own work (13)(14)(15); these examples again force a consideration of the configuration space point of view versus the phase space point of view.…”

“…Since the seminal work on roaming by Townsend et al, a great deal of further work on the photodissociation of formaldehyde has been carried out (see Section 2.3), culminating in the recent tour de force of QCT analysis by Houston et al (12) (see Section 5.3).…”

“…Houston et al (12) recently carried out classical trajectory calculations for formaldehyde in the fulldimensional space and presented a detailed analysis of the roaming trajectories. In this work, the motion of the remote hydrogen with respect to the CO diatom is studied by projecting the trajectories onto spherical coordinate configuration space (R, θ , φ).…”

The phase space geometry underlying roaming reaction dynamics

“…Huston et al [14], on the other hand, set the criteria such that roaming trajectories have to spend a certain amount of time at a minimum radius, have low average kinetic energy and have on average a certain number of bonds over time.…”

A quantum chemical study on ˙Cl-initiated atmospheric degradation of acrylonitrile

“…30,31 The related technique of ab initio molecular dynamics gives calculations of quasi-classical trajectory. 32,33 In this paper, ab initio calculations were performed on the reaction of cCl with acrylonitrile, and kinetic calculations were carried out using variational transition-state theory. Comparisons between theoretical and experimental results and atmospheric implications are discussed.…”