What is special about how roaming chemical reactions traverse their potential surfaces? Differences in geodesic paths between roaming and non-roaming events

Abstract

With the notable exception of some illustrative two-degree-of-freedom models whose surprising classical dynamics has been worked out in detail, theories of roaming have largely bypassed the issue of when and why the counterintuitive phenomenon of roaming occurs. We propose that a useful way to begin to address these issues is to look for the geodesic (most efficient) pathways through the potential surfaces of candidate systems. Although roaming manifests itself in an unusual behavior at asymptotic geometries, we found in the case of formaldehyde dissociation that it was the pathways traversing the parts of the potential surface corresponding to highly vibrationally excited reactants that were the most revealing. An examination of the geodesics for roaming pathways in this region finds that they are much less tightly defined than the geodesics in that same region that lead directly to dissociation (whether into closed-shell products or into radical products). Thus, the broader set of options available to the roaming channel gives it an entropic advantage over more conventional reaction channels. These observations suggest that what leads to roaming in other systems may be less the presence of a localized "roaming transition state," than the existence of an entire region of the potential surface conducive to multiple equivalent pathways.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning

“…They observed that the onset of roaming occurs on the hundreds of femtoseconds (rather than the nanoseconds time scale as commonly assumed), and both the onset and duration of roaming are in a strongly stochastic character. This observation seems in line with the conclusion drawn from an earlier theoretical approach based on the geodesic paths (not classical trajectories) approach . That theoretical study scrutinized the onset of the paths that will eventually roam and suggested that their stochastic character in the early stage sets the destination.…”

“…This observation seems in line with the conclusion drawn from an earlier theoretical approach based on the geodesic paths (not classical trajectories) approach. 63 That theoretical study scrutinized the onset of the paths that will eventually roam and suggested that their stochastic character in the early stage sets the destination. Then, what are the deciding factors, if any, that guide the trajectory toward the roaming region or to follow the tight TS path?…”

Real-time tracking of the entangled pathways in the multichannel photodissociation of acetaldehyde

“…A promising theoretical approach based on the geodesic paths (not classical trajectories) has recently been formulated and applied to the formaldehyde benchmark. 41 That study scrutinized the onset of the paths that will eventually roam and suggested that their stochastic character in the early stage sets the destination. A nonlinear dynamics theory from the phase space perspective, albeit with reduced dimensionality, has also been developed to frame and to analyze the roaming phenomenon and dynamical characteristics.…”

Simulation of the electron ionization mass spectra of the Novichok nerve agent

“…Several dissociation mechanisms such as McLafferty rearrangements 32 and two‐step fragmentations were found and discussed. We also found pathways involving roaming hydrogen atoms, 33,34 an unconventional fragmentation mechanism that bypasses the typical saddle point transition state 35 …”