Predicting and experimentally determining many chemical reaction rates and branching ratios is difficult, especially for those involving short-lived intermediates requiring ultrafast observation techniques. The UV photochemistry of bromoform (CHBr3) is a well-studied reaction, but fully understanding the chemical pathways leading to the production of atomic bromine (Br) and molecular bromine (Br2) remains challenging.
Bromoform is a prominent model system for answering these questions. Chemists have studied the UV photochemistry of bromoform for decades, as it plays a role in ozone depletion in Earth’s atmosphere and is naturally produced by phytoplankton and seaweeds. When exposed to UV light, bromoform is theorized to undergo two processes: dissociation, where one bromine atom separates from the molecule, and isomerization, where the atoms rearrange into a different isomer.
While some researchers claim to have observed evidence of the isomer, its short lifespan has made it difficult to confirm. Additionally, various theories predict different proportions of bromoform that follow each pathway, leading to significant uncertainty in understanding the reaction’s mechanisms.
For the first time, researchers have directly observed how bromoform rearranges its atoms in less than a trillionth of a second after being exposed to an ultraviolet (UV) pulse. Using advanced imaging techniques, they captured a long-predicted pathway in which the ozone-depleting molecule transforms its structure upon interacting with light. This observation provides new insight into the rapid isomerization process of bromoform.
Scientists take a direct look at how light excites electrons
The research team developed an experiment that confirmed the formation of bromoform isomers and determined the proportions of bromoform molecules that undergo dissociation versus those that form isomers. To do this, they first exposed bromoform gas molecules to an ultrafast UV pulse (267 nm wavelength) and then used ultrashort electron pulses to image the excited molecules.
The imaging was conducted with the relativistic ultrafast electron diffraction instrument at the SLAC National Accelerator Laboratory, part of the Linac Coherent Light Source, a facility supported by the U.S. Department of Energy’s Office of Science.
Using the electron images, the researchers measured the distances between atoms in the bromoform molecules and tracked how these distances changed over time. Their analysis revealed that about 60% of bromoform molecules underwent isomerization within the first 200 femtoseconds (1 femtosecond = 1 trillionth of a second) after excitation, and this rearranged structure persisted throughout the 1.1-picosecond (1 picosecond = 1,000 femtoseconds) duration of the experiment.
For the first time, scientists captured atomic motion in 4D
Oliver Gessner, a senior scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), said, “It was fascinating to see exactly the configuration that some people had predicted for this isomer. The other 40% of the bromoform underwent direct dissociation.”
“The result is an important step toward understanding bromoform photochemistry and UV-induced photochemistry in general. The sequence of chemical pathways impacts the final chemical products. The benchmark measurement for a long-debated isomer formation rate makes it possible to refine theories that predict these reactions and their products. Moreover, the study demonstrates that the ultrafast technique is good for providing clear-cut answers to questions about how fast isomers populate and how long they live. That is a very powerful tool.”
Journal Reference:
- Lars Hoffmann, Benjamin W. Toulson et al. UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction. Journal of the American Chemical Society. DOI: 10.1021/jacs.4c07165