Year: 2024
Type: article
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 3 ) 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 2 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 3 (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 3 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.
DOI: 10.1021/jacs.4c07165
Authors: Lars Hoffmann, Benjamin W. Toulson, Jie Yang, Catherine A. Saladrigas, Alfred Zong, Sri Bhavya Muvva, Joao Pedro Figueira Nunes, Alexander H. Reid, Andrew R. Attar, Duan Luo, Fuhao Ji, Ming-Fu Lin, Qingyuan Fan, Stephen P. Weathersby, Xiaozhe Shen, Xijie Wang, Thomas J. A. Wolf, Daniel M. Neumark, Stephen R. Leone, Michael W. Zuerch, Martin Centurion, Oliver Gessner
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G1229526393, G1286236842, G1286251239, G1451874870, G2037563265, G2371548206, G262532246, G303061748, G30714459, G3083819904, G3957843039, G419881185, G4235786860, G42916606, G4501827968, G495301720, G498139845, G5218879237, G5437197419, G5570207115, ULTRAFAST ELECTRON DIFFRACTION FROM ALIGNED MOLECULES, G650090426, G6550344761, G7168589519, G7283024673, G7331630580, G8382000481, G911061179, G969889393
OpenAlex ID: W4403179225