Capturing roaming molecular fragments in real time
Roaming dynamics in real time
Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo et al. report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D2CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D2 + CO) product channel.
Science, this issue p. 1072
Abstract
Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (H2CO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel–resolved measurements, but not in real-time observations of the roaming fragment itself. Using time-resolved Coulomb explosion imaging (CEI), we directly imaged individual “roamers” on ultrafast time scales in the prototypical formaldehyde dissociation reaction. Using high-level first-principles simulations of all critical experimental steps, distinctive roaming signatures were identified. These were rendered observable by extracting rare stochastic events out of an overwhelming background using the highly sensitive CEI method.
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Supplementary Material
Summary
Materials and Methods
Supplementary Text
Figs. S1 to S16
Tables S1 to S10
Equations S1 to S20
MCTDH Operator File and PES Cuts
Resources
References and Notes
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Science
Volume 370 | Issue 6520
27 November 2020
27 November 2020
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Copyright © 2020, American Association for the Advancement of Science.
This is an article distributed under the terms of the Science Journals Default License.
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Received: 17 April 2020
Accepted: 23 October 2020
Published in print: 27 November 2020
Acknowledgments
We thank A. Laramée and G. Lebrun for technical support; B. Wales and J. Sanderson for providing the analysis software for the CEI measurement files; A. Stolow for discussions; S. Gräfe for simulations on the ionization potential; and the reviewers for comments that truly helped to improve the reliability of our analysis. Funding: This work was supported by the Canada Foundation for Innovation, NSERC, FRQNT, the JSPS Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers (grant no. S2601 to T.E. and A.H.), and the World Research Unit (B-1) of Reaction Infography (R-ing) at Nagoya University, Japan (A.H. and F.L.). V.W. and S.B. acknowledge financial support from the NSERC-Vanier fellowships program. J.M.B acknowledges financial support from NASA (grant no. 80NSSC20K0360). Author contributions: T.E. performed all experiments and data analysis. V.W., S.B., J.D., P.L., and B.E.S. assisted with CEI experiments in ALLS. F.L. was responsible for the ALLS facility at INRS in Canada. H.F., M.F., and A.H. assisted with the photoelectron experiments and analysis at Nagoya University, Japan. S.P.N. and M.S.S. provided the quantum dynamics simulations. P.L.H. supervised and performed trajectory calculations and analysis. J.M.B. directed the construction of PESs and trajectory code. C.Q. and T.E. calculated the propagation on the D2CO3+ PES. A.H. and F.L. discussed results and provided classical interpretation. H.I. conceived and directed the project, supervised experiments, and analyzed data. T.E., P.L.H., J.M.B., and H.I. wrote the manuscript with assistance from all other authors. Competing interests: The authors declare no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the main text or the supplementary materials. All data have been uploaded to Zenodo (30).
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