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The halomethane compound bromoform (CHBr3) has devastating effects on the ozone layer. In the upper layers of the atmosphere, bromoform reacts with UV radiation, releasing bromine molecules which destroy ozone molecules. This reaction, however, has long puzzled scientists; the molecules involved seem to wander relative to each other in a way that energetically does not make sense. Scientists at European XFEL have now revealed structural evidence for this roaming mechanism for the first time, establishing it as a universal characteristic of photochemical reactions.
The study, published in Nature Communications, provides key insights for the field of atmospheric photochemistry and how halomethane compounds such as bromoform impact the ozone layer.
The ozone layer envelops Earth some 15-30 km above the planet’s surface. Ozone gas absorbs ultraviolet light as it enters the atmosphere, thereby protecting life on Earth from the effects of the harmful radiation. Ozone, however, reacts readily with other compounds also found in the stratosphere, leading to ozone depletion, and ultimately the creation of the ozone hole. One such compound is bromoform (CHBr3). Emitted by marine life such as phytoplankton and algae, this halomethane compound interacts with UV light when it moves up into the upper layers of the atmosphere, suspended in water droplets or aerosols. The interaction with UV light triggers a reaction, which ultimately results in bromine molecules being released. More than 100 times more destructive than other halogen gases such as chlorine, bromine is extremely reactive and rapidly destroys ozone molecules. These types of photo-reactions are what causes the ozone hole. Now, using the European XFEL, scientists have revealed key structural details of this type of reaction, improving our understanding of atmospheric chemistry.
The puzzle of the roaming molecules
Details of the ultrafast light-induced reaction involving bromoform have long puzzled scientists. It is known that following the initial interaction with UV light, bromoform breaks into fragments, some of which realign to form stable chemical compounds. However, curiously, the molecules involved in these reactions do not behave as would be expected. “Stable intermediate complexes are eventually formed after bromoform is fragmented by UV light,” explains Qingyu Kong, scientist at the Soleil synchrotron, and principal scientist on the study. “However, in the initial stages of the reaction these fragments may join together in structural configurations that do not seem to make sense energetically, at least in the classical view. Rather than fully separating or binding together in a way that takes the least amount of energy, fragments slowly migrate relative to each other in a way that bypasses conventional transition states.” To explain these observations, scientists postulated the hypothesis called roaming, indicating the way the ions seem to wander further afield to find a new stable configuration. However, to date no direct structural evidence has been provided that supports this theory in bromoform.
Using the European XFEL, a team of researchers has now resolved this ultrafast roaming mechanism for the first time. Previous studies at synchrotrons have revealed the final products of these reactions, but have not been able to unravel the initial steps. “It was evident that the first crucial step of the reaction, where the roaming mechanism is thought to take place, happens much faster than the X-ray pulses at the synchrotron could detect” says European XFEL scientist and main author of the recent study, Dmitry Khakhulin.
Femtosecond pulses do the trick
For their experiment, the scientists injected bromoform solutions as thin jets into the experiment area at the FXE instrument at European XFEL. A femtosecond optical laser pulse triggered the reaction; a delayed X-ray pulse was then used to capture information about the different stages of the reaction. The method is known as femtosecond time-resolved X-ray solution scattering.
In contrast to previous experiments, the ultrashort X-ray pulses generated by the European XFEL enabled the scientists to now capture all structural steps of the reaction, from breaking of the first bonds and the roaming dynamics of the fragments to the formation of bromine and various recombination products. The results showed that within 150 femtoseconds following the start of the reaction, roaming had taken place and the stable intermediate compounds start gradually forming.
“Thanks to the ultrashort X-rays pulses generated by the European XFEL, we were able to provide structural evidence of roaming in bromoform for the first time,” says Khakhulin.
Influence of the solvent
Previous studies at synchrotrons also indicated that the outcome of the reaction was influenced by the solvent. For their studies at European XFEL, the scientists used two different liquids as solvents - methanol and methylcyclohexane – to determine whether this was the case.
“Our results showed that, although the intermediate was formed in both liquids, what happened after that was highly dependent on the solvent the bromoform was suspended in,” explains Kong. When suspended in methanol, methanol molecules competed to bind with the intermediate compound so that the initial short-lived intermediate decomposed. The less reactive methylcyclohexane, however, meant the intermediate relaxed to a stable product.
“Taken together, these new insights are important steps towards a more comprehensive understanding of atmospheric photochemistry, and offer a new framework to understand the behaviour of compounds such as bromoform within the context of gas-phase environment, aerosols and water droplets,” concludes Kong.
This study was conducted as part of a long-term collaboration at the FXE instrument. The work involved close collaboration between European XFEL, Soleil synchrotron, ShanghaiTech University and ESRF.
About European XFEL
European XFEL in the Hamburg area is an international research facility of superlatives: 27,000 X-ray flashes per second and a brilliance that is a billion times higher than that of the best conventional X-ray sources open up new opportunities for science. Research groups from around the world are able to map the atomic details of matter, decipher the molecular composition of cells or viruses, take three-dimensional “photos” of the nanoworld, “film” chemical reactions, and study processes such as those occurring deep inside planets.
The non-profit company cooperates closely with its main shareholder, the research centre DESY, and other organisations worldwide. European XFEL has a workforce of more than 550 employees. At present, 12 countries have signed the European XFEL convention: Denmark, France, Germany, Hungary, Italy, Poland, Russia, Slovakia, Spain, Sweden, Switzerland, and the United Kingdom. Germany (through the Federal Ministry of Research, Technology and Space and the states of Hamburg and Schleswig-Holstein) covered 57 per cent of the construction costs for the research facility, Russia 26 per cent. The remaining partner countries contributed between one and three per cent. Operating costs are also shared among the partner countries, based on a calculation that reflects both their shares in the company and its facility usage.
Contact:
Bernd Ebeling
+49 40 8998 6921
bernd.ebeling@xfel.eu
https://doi.org/10.1038/s41467-026-69374-4
https://www.xfel.eu/news_and_events/news/index_eng.html?openDirectAnchor=3001&am...
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