A graphic representation of an intermediate chemical reaction and the X-rays, laser and detector. Credit: Nina Fujikawa, SLAC National Accelerator Laboratory
X-ray emission spectroscopy is a valuable technique for probing the atomic and electronic structure of materials; leveraging extremely powerful and ultrafast synchrotron X-ray sources, researchers can reveal details of chemical reactions that occur within trillionths of a second. However, many chemical reactions occur in an even shorter timeframe, beginning and ending in less than a picosecond, making them difficult to observe even with advanced X-ray techniques. Researchers at the SLAC National Accelerator Laboratory have now combined two forms of X-ray emission spectroscopy, enabling them to capture one of the fastest reactions in a molecule called ferricyanide for the first time.
The researchers performed their experiments using the X-ray Correlation Spectroscopy Instrument at the Linac Coherent Light Source (LCLS). They combined two different emission lines – a femtosecond Kβ main line and valence-to-core line – to simultaneously probe both spectroscopic regions while exciting a mixture of ferricyanide and water with ultraviolet light. The ultrafast Kβ main line is advantageous for probing short-lived reactions, but is not as useful as the valence-to-core line for capturing the dynamics of metal-ligand bonds, such as the bonds between cyanide ligands and ferric iron in ferricyanide. The valence-to-core line excels at probing metal-ligand structures, but its low intensity compared to the Kβ line limits its time resolution. Combining the two techniques provided the team with a greater level of data and insight into the short-lived transitional states of the excited molecule.The analysis also included the recording of X-ray solution scattering signals.
The experiments captured evidence of a ligand exchange reaction in aqueous ferricyanide. This reaction occurs during a light-induced, ligand-to-metal charge transfer state that lasts only 0.5 picoseconds after excitation. The valence-to-core data revealed the loss of one cyanide ligand during this period, which is then replaced with either the same ligand or a water molecule, in a process more specifically known as photo-aquation. The X-ray solution scattering difference curves provided complementary evidence of the loss of a cyanide ligand, which results in a lower total electron density and the presence of a previously unobserved intermediate ferric penta-coordinate complex. This study was published in Nature Communications.
“This ligand exchange is a basic chemical reaction that was thought to occur in ferricyanide, but there was no direct experimental evidence of the individual steps in this process,” said first author Marco Reinhard. “With only a Kβ main emission line analysis approach, we wouldn’t really be able to see what the molecule looks like when it is changing from one state to the next; we’d only obtain a clear picture of the beginning of the process.”
The researchers believe this combined X-ray spectroscopy technique can be used to study more complex molecules and reactions, such artificial photosynthesis or the transportation of oxygen by hemeproteins in blood.