Bromoform Molecules Like to Rearrange Their Atoms

The Science

Bromoform (CHBr₃) is a natural compound that breaks down ozone in the Earth’s atmosphere. When bromoform molecules interact with light, they can undergo two different processes. They either undergo “bond dissociation,” where one bromine atom breaks off from the rest of the molecule, or “isomerization,” where the molecule’s atoms rearrange, but the molecule stays intact. Despite extensive research, scientists do not have a full understanding of these processes in bromoform. This is in part because the reactions happen at ultrafast speeds. This research presents unambiguous experimental evidence that approximately 60% of bromoform molecules exposed to ultraviolet light undergo isomerization rather than direct bond dissociation. This indicates that the isomerization process plays a key role in the interaction of bromoform molecules with light.

The Impact

For the first time, scientists can distinguish the proportion of bromoform molecules that directly break bonds (dissociate) vs. those that rearrange (isomerize). This is an important step toward understanding the formation of bromoform isomers. These isomers have long been predicted, but researchers lacked experimental data that confirmed their rate of formation or production relative to other chemical configurations. The new experimental results challenge existing theories, which can predict some of the observed trends, but not all of them simultaneously. Thus, the experiments set the stage for new theoretical efforts.

Summary

The interaction of bromoform molecules with UV light is a challenging problem that has been studied in physical chemistry for decades. One of the outstanding questions in the field is how bromoform molecules react in the first few picoseconds (trillionths of a second) after being excited by a UV photon. The relativistic ultrafast electron diffraction (UED) instrument at SLAC National Accelerator Laboratory provides the necessary resolution to observe these ultrafast motions, providing long-sought answers to these questions. The instrument is part of the Linac Coherent Light Source at SLAC, a Department of Energy Office of Science user facility. Theoretical studies have predicted that some proportion of bromoform molecules isomerize, breaking a carbon-bromine bond and forming a new bromine-bromine bond, while others undergo direct carbon-bromine bond dissociation. However, the rate of isomer formation and its importance relative to other reaction pathways were not clear and needed to be measured experimentally.

In this research, scientists used UED to directly measure the ratio of bromoform molecules undergoing isomerization vs. direct bond dissociation in the gas phase. The results showed that approximately 60% of molecules undergo isomerization, agreeing with some predictions but disproving others. Furthermore, the isomers formed in the first fifth of a picosecond and persisted for the duration of the experiment (1.1 picoseconds). No theory currently predicts the combination of these effects. These results push the boundaries of understanding for bromoform photochemistry and reveal a need for greater theoretical understanding of this reaction.

Funding

This work was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used resources of the Linac Coherent Light Source, a DOE Office of Science user facility.