UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction

For many chemical reactions, it remains notoriously difficult to predict and experimentally determine the rates and branching ratios between different reaction channels. This is particularly the case for reactions involving short-lived intermediates, whose observation requires ultrafast methods. The UV photochemistry of bromoform (CHBr<sub>3</sub>) is among the most intensely studied photoreactions. Yet, a detailed understanding of the chemical pathways leading to the production of atomic Br and molecular Br<sub>2</sub> fragments has proven challenging. In particular, the role of isomerization and/or roaming and their competition with direct C-Br bond scission has been a matter of continued debate. Here, gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) is used to directly study structural dynamics in bromoform after single 267 nm photon excitation with femtosecond temporal resolution. The results show unambiguously that isomerization contributes significantly to the early stages of the UV photochemistry of bromoform. In addition to direct C-Br bond breaking within <200 fs, formation of iso-CHBr<sub>3</sub> (Br-CH-Br-Br) is observed on the same time scale and with an isomer lifetime of >1.1 ps. The branching ratio between direct dissociation and isomerization is determined to be 0.4 ± 0.2:0.6 ± 0.2, i.e., approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The structure and time of formation of iso-CHBr<sub>3</sub> compare favorably with the results of an ab initio molecular dynamics simulation. The lifetime and interatomic distances of the isomer are consistent with the involvement of a roaming reaction mechanism.

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Publication Details

Journal
Journal of the American Chemical Society
Published
2024-10-07
DOI
https://doi.org/10.1021/jacs.4c07165
Citations
3
Primary Topic
Advanced Chemical Physics Studies
Type
article
Field-Weighted Citation Impact
1.01

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article

UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction

Stephen Weathersby, Daniel M. Neumark, Fuhao Ji, Martin Centurion et al.
3 citations
Journal of the American Chemical Society
Advanced Chemical Physics Studies
1.01
article

UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction

Stephen Weathersby, Daniel M. Neumark, Fuhao Ji, Martin Centurion, Sri Bhavya Muvva, Ming‐Fu Lin, Michael Zuerch, Xiaozhe Shen, Benjamin W. Toulson, Duan Luo, Qingyuan Fan, Andrew Attar, Catherine A. Saladrigas, Stephen R. Leone, Alexander H. Reid, Alfred Zong, Thomas Wolf, Xijie Wang, Lars Hoffmann, Oliver Geßner, Jie Yang, J. Pedro F. Nunes
article en
3 citations

Abstract

For many chemical reactions, it remains notoriously difficult to predict and experimentally determine the rates and branching ratios between different reaction channels. This is particularly the case for reactions involving short-lived intermediates, whose observation requires ultrafast methods. The UV photochemistry of bromoform (CHBr<sub>3</sub>) is among the most intensely studied photoreactions. Yet, a detailed understanding of the chemical pathways leading to the production of atomic Br and molecular Br<sub>2</sub> fragments has proven challenging. In particular, the role of isomerization and/or roaming and their competition with direct C-Br bond scission has been a matter of continued debate. Here, gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) is used to directly study structural dynamics in bromoform after single 267 nm photon excitation with femtosecond temporal resolution. The results show unambiguously that isomerization contributes significantly to the early stages of the UV photochemistry of bromoform. In addition to direct C-Br bond breaking within <200 fs, formation of iso-CHBr<sub>3</sub> (Br-CH-Br-Br) is observed on the same time scale and with an isomer lifetime of >1.1 ps. The branching ratio between direct dissociation and isomerization is determined to be 0.4 ± 0.2:0.6 ± 0.2, i.e., approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The structure and time of formation of iso-CHBr<sub>3</sub> compare favorably with the results of an ab initio molecular dynamics simulation. The lifetime and interatomic distances of the isomer are consistent with the involvement of a roaming reaction mechanism.

Journal of the American Chemical Society
University of Nebraska–Lincoln (US), Lawrence Berkeley National Laboratory (US), SLAC National Accelerator Laboratory (US), University of California, Berkeley (US)
U.S. Department of Energy, Adolph C. and Mary Sprague Miller Institute for Basic Research in Science, University of California Berkeley, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, SLAC National Accelerator Laboratory
Openalex Percentile: Top 24%
Advanced Chemical Physics Studies
1.01
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