One of the main research interests of the Saykally group is using x-ray absorption spectroscopy (XAS) to probe ions in solution. In this technique, synchrotron-generated x-rays are used to excite core-level (typically 1s) electrons to anti-bonding molecular orbitals. The energy of core-level electrons is generally not affected by the structure or environment of the molecule, however the anti-bonding MOs are more sensitive to both of these factors. A molecule can then exhibit different x-ray absorption spectra in different solvation environments. The experimental spectra can then be compared to simulated spectra, to determine the solvation environment of the molecule. Current projects being studied include Li+ ions in typical Li-ion battery solvents, and mixtures of HCl and carbonate/bicarbonate in an attempt to detect aqueous carbonic acid.
One aspect of the XAS project is an attempt to detect aqueous carbonic acid. While it is common knowledge that carbonate/bicarbonate and acids react to form water and carbon dioxide (and vice versa), the mechanism for this reaction is not known. It is generally taught that the intermediate in this reaction is carbonic acid (H2CO3), however this molecule has never been detected in aqueous solutions. This project is working on detecting H2CO3 using XAS and a small dead-volume liquid microjet mixing system.
Above: Isosurfaces for π* or σ* resonances of hydrated carbonate species. Isosurfaces illustrate the localization of π* or σ* resonances along C=O or C–OH bonds for each species. Atom colors are aqua for carbon, red for oxygen, white for hydrogen, and blue for sodium. The σ* resonances are hybridized with the surrounding water to create a large band of states, but some localization along the carbon–oxygen σ* bonds can still be seen in the representative isosurfaces. Right: Calculated NEXAFS carbon K-absorption edge spectra for acidic pH range for the three possible components: CO2 gas (A), dissolved CO2 (B), and carbonic acid (C). Each calculated spectrum represents the average of 100 individual spectra, with ± one standard deviation shown in the shaded areas.