Exploring Multiple Reaction Paths to a Single Product Channel

Osborn, David L.

doi:10.1002/9780470259474.ch4

Cited by 13 publications

(9 citation statements)

References 71 publications

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“…An early, confirmed example of this behavior, often called “roaming,” was stimulated by and speculated about in experimental work on formaldehyde dissociation in the research groups of Moore − and then confirmed by the research groups of Suits − and Bowman. ,− Reviews of this research − and further studies of formaldehyde dissociation and the H + HCO reaction ensued. − Similar roaming behavior has subsequently been found in other unimolecular dissociations, including those of acetaldehyde, − larger aldehydes, methyl formate, − acetone, alkanes, NO 3 , ,, nitromethane, − methyl nitrite, carbon dioxide, and Criegee intermediates . Roaming in bimolecular reactions has also been observed and discussed. − Several reviews of this active area of research have recently appeared. − Consequently, it is now well-established that many systems react via trajectories that deviate strongly from the minimum energy path. The consequence of this behavior for transition-state theory is still not completely understood.…”

Section: Introductionmentioning

confidence: 83%

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

2016

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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.

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Section: Introductionmentioning

confidence: 83%

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

2016

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“…Transition-state theory (TST) and its variants play a central role in predicting elementary chemical reaction rates. , In conventional TST, the reaction proceeds via a well-defined (tight) transition state (TS) connecting the reactants and the products. In recent years, an intriguing non-TS mechanism, roaming, has attracted much attention. − In this mechanism, the reactive flux or trajectory bypasses the conventional TS, and the incipient radicals, rather than dissociating into the radical products, roam on a flat region of potential energy surface (PES) and eventually form molecular products via a radical–radical abstraction reaction. − ,− …”

Section: Introductionmentioning

confidence: 99%

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

2021

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Photodissociation of acetaldehyde (CH 3 CHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond timeresolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.; et al. Chem. Sci. 2020, 11, 6423−6430). Here, we complement the pair-correlated product speed distribution of CO(v = 0) at the high-j side of the CO rotational state distribution in the CO + CH 4 channel and detail the twodimensional data analysis of the time-resolved images. As a result, extensive comparisons with other studies can be made and the branching fractions of the previously assigned TS cc (S 0 ), non-TS cc (S 0 ), and CI(S 1 /S 0 ) pathways for the CO(v = 0) + CH 4 molecular channel are evaluated to be 0.74 ± 0.08, 0.15 ± 0.02, and 0.11 ± 0.02, respectively. Together with the macroscopic branching ratio between the molecular (CO + CH 4 ) and radical (CH 3 + HCO) channels at 267 nm from the literature, a global view of the microscopic pathways can then be delineated, which provides invaluable insights and should pave the way for further studies of this interesting system.

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“…In several recent combined theory and experiment papers, it has been demonstrated that under certain conditions, the two radicals may roam until they reach an attractive portion of the PES that leads to internal abstraction and then to molecular products. According to these studies, roaming-type mechanisms should, in fact, be quite general in molecules for which an initially frustrated dissociation can be followed by a barrierless internal abstraction. − While most often roaming accounts for only a minor fraction of the total products, under favorable conditions, roaming may even be the dominant mechanism and certainly needs to be taken into account for a complete picture of molecular dissociation.…”

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confidence: 99%

Roaming Pathway Leading to Unexpected Water + Vinyl Products in C₂H₄OH Dissociation

Kamarchik

Koziol

Reisler

et al. 2010

J. Phys. Chem. Lett.

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We report molecular dynamics simulations of the unimolecular dissociation of energetic C2H4OH radicals using a full-dimensional potential energy surface (PES) fit to density functional theory (DFT) and coupled cluster (CCSD) calculations. Quasiclassical trajectories are initiated on the surface using microcanonical sampling at a total energy of 85.0 kcal/mol (44.1 kcal/mol relative to the zero-point energy). The trajectories reveal a roaming channel leading to the formation of water. The transition state (TS) corresponding to direct water production is energetically inaccessible. However, the roaming pathway finds a lower-energy path via frustrated dissociation to hydroxyl that makes water + vinyl production feasible. The trajectory calculations suggest that the roaming pathway constitutes a minor (a few percent) channel of the overall C2H4OH dissociation. The mechanism of the roaming reaction is analyzed in terms of the geomteric proximity of the TSs leading to OH loss and internal abstraction as well as the partitioning of vibrational energy into different modes.

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Exploring Multiple Reaction Paths to a Single Product Channel

Cited by 13 publications

References 71 publications

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

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

Roaming Pathway Leading to Unexpected Water + Vinyl Products in C₂H₄OH Dissociation

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Exploring Multiple Reaction Paths to a Single Product Channel

Cited by 13 publications

References 71 publications

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

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

Roaming Pathway Leading to Unexpected Water + Vinyl Products in C2H4OH Dissociation

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Product

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Roaming Pathway Leading to Unexpected Water + Vinyl Products in C₂H₄OH Dissociation