Ultrafast electron imaging reveals never-before-seen nuclear motions in hydrocarbon molecules triggered by light.
The interaction between hydrocarbon molecules and light can influence the formation of nitrous acid in the atmosphere, a compound that plays a significant role in air pollution. In this study, researchers investigated how proton transfer contributes to these light-induced molecular processes.
Intramolecular proton transfer occurs when a proton moves from one part of a molecule to another within the same molecule. To observe these rapid movements, the team used an ultrafast electron camera capable of capturing molecular motion on a scale more than 10,000 times smaller than the width of a human hair.
This high-resolution, time-resolved imaging technique, combined with advanced computational modeling, revealed that a proton transfer event is followed by an out-of-plane twisting motion—both critical steps in the molecule’s energy relaxation pathway.
Understanding Molecular Relaxation
Relaxation is the process by which the molecule moves from an excited, high-energy state to a lower energy ground state after absorbing light.
Previous studies have proposed various ways that hydrocarbon molecules may relax after interacting with light. However, scientists lacked experimental data to verify which process occurs. This study identified a key relaxation pathway involving proton transfer and molecular “twisting.” This result lays the groundwork for studies of more complex molecules that scientists believe undergo similar interactions. It will also help researchers better understand how pollution forms.
Observing Molecular Dynamics in Real Time
The interactions between light and nitroaromatic hydrocarbon molecules have important implications for chemical processes in our atmosphere that can lead to smog and pollution. However, changes in molecular geometry due to interactions with light can be very difficult to measure because they occur at sub-Angstrom length scales (less than a tenth of a billionth of a meter) and femtosecond time scales (one millionth of a billionth of a second).
The relativistic ultrafast electron diffraction (UED) instrument at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory provides the necessary spatial and time resolution to observe these ultrasmall and ultrafast motions. The LCLS is a Department of Energy (DOE) Office of Science light source user facility.
In this research, scientists used UED to observe the relaxation of photoexcited o-nitrophenol. Then, they used a genetic structure fitting algorithm to extract new information about small changes in the molecular shape from the UED data that were imperceptible in previous studies. Specifically, the experiment resolved the key processes in the relaxation of o-nitrophenol: proton transfer and deplanarization (i.e., a rotation of part of the molecule out of the molecular plane). Ab-initio multiple spawning simulations confirmed the experimental findings. The results provide new insights into proton transfer-mediated relaxation and pave the way for studies of proton transfer in more complex systems.
Reference: “Photo-induced structural dynamics of o-nitrophenol by ultrafast electron diffraction” by J. P. F. Nunes, M. Williams, J. Yang, T. J. A. Wolf, C. D. Rankine, R. Parrish, B. Moore, K. Wilkin, X. Shen, Ming-Fu Lin, K. Hegazy, R. Li, S. Weathersby, T. J. Martinez, X. J. Wang and M. Centurion, 16 May 2024, Physical Chemistry Chemical Physics.
DOI: 10.1039/D3CP06253H
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