48th CEMS Colloquium

Title

Visualizing chemical reactions in solution with ultrashort X-ray pulses

Abstract

X-ray is one of the most powerful and key experimental tools to visualize molecules. Structure, elasticity, and dynamics of molecules under external stimuli (pressure, temperature, shear stress, photoexcitation, etc.) have been successfully unveiled by X-ray techniques. Here I show an example to capture the structural dynamics of molecule in the course of chemical reaction by using ultrashort X-ray pulses in picosecond to femtosecond time domain.

In this particular case, the chemical reaction is bond formation between Au atoms. Capturing transient molecular structure in the bond formation processes has been elusive, because it is difficult to initiate and follow such diffusion-limited bimolecular processes with ultrafast time resolution. In this regard, Au oligomer complex, [Au(CN)₂^–]_n, offers a good model system to study the dynamics of bond formation in solution [1,2]. Using femtosecond time-resolved X-ray scattering, we successfully visualized in real time the birth of a gold trimer complex, [Au(CN)₂^–]₃, that occurs via photo-induced formation of Au-Au covalent bonds [3]. This work showcases the possibility of tracking detailed structural changes in solution with sub-ps temporal and sub-angstrom spatial resolutions, thanks to the advent of X-ray free electron lasers and the advance of data analysis of time-resolved solution scattering data.

[1] Rawashdeh-Omary, M. A., Omary, M. A., Patterson, H. H. & Fackler, J. P., J. Am. Chem. Soc. 2001, 123, 11237-11247.
[2] Iwamura, M., Nozaki, K., Takeuchi, S. & Tahara, T., J. Am. Chem. Soc. 2013, 135, 538-541.
[3] Kim, K. H., Kim, J. G., Nozawa, S., Sato, T., Oang, K. Y., Kim, T. W., Ki, H., Jo, J., Park, S., Song, C., Sato, T., Ogawa, K., Togashi, T., Tono, K., Yabashi, M., Ishikawa, T., Kim, J., Ryoo, R., Kim, J., Ihee, H. & Adachi, S., Nature, 2015, 518, 385-389.