UV-Induced Reaction\nPathways in Bromoform Probed with\nUltrafast Electron Diffraction

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