Experiments provide evidence that interaction of light with a hydrocarbon molecule produces strained molecular rings

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 work appearing in The Journal of Physical Chemistry A, 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.

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.

The study 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 team used the Linac Coherent Light Source’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.