A Little Water, and It Goes to Pieces

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Science  03 Mar 2000:
Vol. 287, Issue 5458, pp. 1557
DOI: 10.1126/science.287.5458.1557a

Carbonic acid (H2CO3) decomposes rapidly in solution, and was long believed to also be unstable in the gas phase. However, recent experimental studies have succeeded in isolating and characterizing pure carbonic acid; it was even shown that solid carbonic acid could be sublimed and recondensed. Loerting et al. now show that water vapor plays a decisive role in the kinetic stability of gas-phase carbonic acid. They have used variational transition state theory, corrected for multidimensional tunneling effects, to study the decomposition of carbonic acid as the isolated molecule or in the presence of one or two water molecules. The reaction barrier is lowered dramatically by additional water molecules, which accelerates the rate of decomposition. Quantum effects also play an important role—the reaction path deviates from the classical minimum energy path as a result of tunneling. The authors conclude that in the absence of water, carbonic acid should be stable over a wide range of temperatures, which increases the likelihood of the detection of carbonic acid in interstellar space.—JU

Angew. Chem. Int. Ed.39, 892 (2000).

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