Using an X-ray free-electron laser and time-resolved X-ray solution scattering, researchers have watched, on a femtosecond timescale, the detailed steps that occur when covalent bonds form in solution.
This is the first time chemists have used the X-ray scattering technique to observe bond formation in solution. The method is more sensitive to molecular structure and thus more capable of revealing reaction steps than the ultraviolet-visible and infrared spectroscopy methods typically used in ultrafast reaction analysis. The work could lead to a better understanding of the chemistry of bimolecular reactions by providing an unprecedented level of detail on the course of reactions.
Hyotcherl Ihee of the Institute for Basic Science and the Korea Advanced Institute of Science & Technology, both in Daejeon, South Korea; Shin-ichi Adachi of the High Energy Accelerator Research Organization, in Tsukuba, Japan; and coworkers studied the femtosecond chemical and structural changes of bond formation in a noncovalent complex, [Au(CN)2–]3 (Nature 2015, DOI: 10.1038/nature14163). Upon excitation by a laser, the complex forms internal covalent bonds and then adds another gold group to produce [Au(CN)2–]4.
Bond formation has been more difficult to analyze than bond breaking because timing the meet-up of two reactants is harder than watching a single molecule break up. Researchers in the past have made the process easier with a well-known work-around—ensuring that reactants are bound noncovalently to begin with, enabling them to find each other quickly. For example, Richard B. Bernstein and Ahmed H. Zewail of California Institute of Technology used this approach in 1987 to monitor a bimolecular gas-phase reaction using spectroscopy with picosecond time resolution (J. Chem. Phys., DOI: 10.1063/1.453280).
Ihee, Adachi, and coworkers used a similar work-around. But they did it in solution, a more challenging medium than the gas phase for studies of ultrafast reaction dynamics, and they used X-ray scattering to probe the reaction.
They watched the reaction at the SACLA facility, in Sayo, Japan, which houses an X-ray free-electron laser. These types of lasers emit X-rays by accelerating electrons to high energies. In the experiments, the researchers initiated the reaction with a laser pulse and then analyzed the diffraction of an additional X-ray pulse as it interacted with the reacting molecules. The chemists observed as the trimeric complex went from a bent to linear conformation, formed and shortened two Au–Au bonds, and then formed a third bond to produce a tetramer. They also obtained three-dimensional structures of reaction intermediates with sub-angstrom spatial resolution.
“The ability to visualize the dynamics of chemical reactions—that is, atomic motion—is a kind of holy grail, and this work represents an important step in this direction,” says Eric Vauthey of the University of Geneva, who is a specialist in ultrafast photochemistry.
The study’s novelty is its use of X-ray scattering to follow bond formation in a weakly bound complex in solution, notes Erik T. J. Nibbering of the Max Born Institute, in Berlin, who is a femtosecond chemistry expert. Chemists’ ultimate goal is to explain how atoms move in a structure during the course of a reaction. Spectroscopy struggles to show this, and the use of time-resolved X-ray scattering helps overcome this limitation, he says.