Carbon Rings Under Stress

Ultrafast X-ray experiments provide direct evidence that interaction of light with a hydrocarbon molecule produces strained molecular rings.

A gas-phase X-Ray scattering experiment captures cyclopentadiene rapidly transforming into the strained bicyclo[2.1.0]pentene. This structure change is triggered by a pump pulse (blue) and detected through X-ray scattering (yellow). — Image courtesy of Greg Stewart, SLAC National Accelerator Laboratory

The Science

When molecules interact with ultraviolet (UV) light, they can change shape quickly, producing strain—stress in a molecule’s chemical structure due to an increase in the molecule’s internal energy. These processes typically take just tens of picoseconds (one millionth of a millionth of a second). Advanced capabilities at X-ray free electron laser (XFEL) facilities now enable scientists to create images of these ultrafast structural changes. In this work, researchers found structural evidence of a strained bicyclic molecule (a molecule consisting of two joined rings) that emerges from the chemical reaction that occurs when a cyclopentadiene molecule absorbs UV light. Cyclopentadiene is a good sample chemical for studying a range of reactions, and these findings have broad implications for chemistry.

The Impact

Highly strained molecules have a variety of interesting applications in solar energy and pharmaceuticals. However, strain doesn’t typically occur naturally—energy must be added to a molecular system to create the strain. Identifying processes that produce molecules with strained rings is a challenge of broad interest in physical chemistry. This research confirms a prediction that controlled interactions between light and cyclopentadiene molecules produce strained structures. The team was also able to differentiate between types of ring strain, which could inform future methods for synthesizing molecules. The results have practical applications and are important to chemistry theory.

Summary

When molecules interact with UV light, the shape of the molecules can quickly change, typically on timescales in the range of tens of picoseconds. Advanced capabilities at XFEL facilities now enable imaging the ultrafast structural changes. In this work, researchers exploited ultrafast X-ray pulses at the Linac Coherent Light Source (LCLS), a Department of Energy Office of Science light source user facility at SLAC National Accelerator Laboratory.

The team used LCLS’s X-ray pulses to study the structural dynamics of cyclopentadiene molecules (i.e., an organic molecule C5H6, where the carbon atoms from a symmetric ring), observing the transition to a highly strained state after photoexcitation. The team excited the molecules with UV light pulses at 243 nanometers and compared time-dependent X-ray scattering data to theoretical models of scattering from different strained molecules that exist during the course of the reaction. The data are consistent with the direct formation of bicylo[2.1.0]pentene, as hypothesized but not directly observed previously. On the other hand, the experiments did not observe tricyclo[2.1.0.0]pentane, which was also hypothesized as a potential product. These results pave the way toward a greater understanding of how molecular strain plays a role in photoexcited hydrocarbon molecules.

Contact

Peter M. Weber
Brown University
[email protected]

Funding

This work was primarily 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 at SLAC National Accelerator Laboratory. Additional funding was provided by the National Science Foundation, the Engineering and Physical Sciences Research Council in the United Kingdom, the Leverhulme Trust, and the European Union.