Despite the widespread importance of aqueous bicarbonate chemistry,its conjugate acid carbonic acid (H2CO3) has remained uncharacterisedin solution. Here we report the generation of deuterated carbonicacid in D2O solution by ultrafast protonation of bicarbonateand its persistence for nanoseconds. We follow the reactiondynamics upon photoexcitation of a photoacid by monitoring infrared-activemarker modes with femtosecond time resolution.
- By fitting akinetic model to the experimental data
- , we directly obtain theon-contact proton transfer rate
- to bicarbonate, previously inaccessibleusing indirect m
- ethods. A Marcus free energy correlation supportsan
associated pKa of 3.45 ± 0.15, substantially lowerthan the value of 6.35 commonly assumed on the basis of theoverall CO2 to bicarbonate equilibrium. This result should spurfurther exploration of acid-base reactivity in CO2-rich aqueousenvironments such as those anticipated under sequestration schemes