The hydration structure of aqueous carbonic acid from X-ray absorption spectroscopy

Despite much effort, aqueous carbonic acid (H2CO3) remains poorly characterized because it is very short-lived. We describe the detection and characterization of aqueous H2CO3 by X-ray absorption spectroscopy, wherein protonation of a bicarbonate solution continuously generates the acid under ambient conditions. Accompanying first principles calculations of the carbon K-edge transitions facilitate spectral assignment and interpretation in terms of the H2CO3 π* orbital, which exhibits a small (0.2 eV), systematic blueshift relative to that of bicarbonate. These results establish the detailed hydration properties of this short-lived molecule and will thereby facilitate future studies of carbonate chemistry in biological and geological system.