Clarifying the structure of carbonic acid

For many decades, carbonic acid (H2CO3)—formed from CO2 and water— was regarded as a “nonexisting” free molecule that only existed in equilibrium with its deprotonated ions, bicarbonate (HCO3−) and carbonate (CO32−), in solution. Both H2O and CO2 are very stable, and any covalently bound product of these two molecules will be thermodynamically less stable. However, theoretical work in the late 1970s suggested that kinetic barriers could prevent H2CO3 decomposition (1), and in 1987, H2CO3 was shown to have a finite lifetime in gas-phase experiments (2). In the 1990s, two independent syntheses of solid H2CO3 were published, but the two solids had rather different spectroscopic properties (3, 4). In a series of publications (5–7), these two forms of carbonic acid were assigned to polymorphic structures that, upon sublimation, were claimed to produce different structures in the gas phase. A recent, very elegant study by Reisenauer et al. (8) resolves these discrepancies and demonstrates that there is only one form of solid H2CO3. The second “polymorph” was identified as the methyl ester of carbonic acid