CaSTL Seminar with Richard J. Saykally

Aqueous carbonic acid(H₂CO₃) is the centerpiece of both the global carbon cycle and physiological buffer systems, yet it remains poorly characterized despite enormous effort.  This reflects the fact that carbonic acid is intrinsically unstable  upon contact with even a single water molecule, reacting in via a proton chain mechanism to ultimately form aqueous bicarbonate and carbonate anions and hydrated protons, which comprises the reversible mechanism of dissolution of CO₂ gas.  While solid and gaseous carbonic acid have been studied in some detail, only two spectroscopic studies of the aqueous form have been successful.  We report the detection and characterization of aqueous H₂CO₃ by X-ray absorption spectroscopy in a liquid microjet designed to effect rapid mixing of reactant solutions, 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 carbonic acid pi*carbonyl orbital, which exhibits a small(0.1 ev) but systematic redshift relative to that of bicarbonate.