Real-Time Observation of Carbonic Acid Formation in Aqueous Solution

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Science  12 Nov 2009:
DOI: 10.1126/science.1180060


Despite the widespread importance of aqueous bicarbonate chemistry, its conjugate acid carbonic acid (H2CO3) has remained uncharacterised in solution. Here we report the generation of deuterated carbonic acid in D2O solution by ultrafast protonation of bicarbonate and its persistence for nanoseconds. We follow the reaction dynamics upon photoexcitation of a photoacid by monitoring infrared-active marker modes with femtosecond time resolution. By fitting a kinetic model to the experimental data, we directly obtain the on-contact proton transfer rate to bicarbonate, previously inaccessible using indirect methods. A Marcus free energy correlation supports an associated pKa of 3.45 ± 0.15, substantially lower than the value of 6.35 commonly assumed on the basis of the overall CO2 to bicarbonate equilibrium. This result should spur further exploration of acid-base reactivity in CO2-rich aqueous environments such as those anticipated under sequestration schemes.