Acid to Acid

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Science  11 Jan 2013:
Vol. 339, Issue 6116, pp. 120
DOI: 10.1126/science.339.6116.120-a

Hydration of sulfur trioxide (SO3) to sulfuric acid (H2SO4) in Earth's atmosphere gives rise to acid rain as well as particle formation. A sound mechanistic understanding of this reaction is thus central to the chemistry of climate and pollution mitigation. Although the necessary bonding rearrangements appear straightforward—S-O bond formation accompanied by proton transfer from water to any of the other oxygen atoms—the direct bimolecular reaction is in fact extremely slow. Theoretical and experimental studies have demonstrated that the assistance of a second, catalytic, water molecule is crucial to facilitate the proton transfer. Torrent-Sucarrat et al. now report, on the basis of extensive theoretical calculations, that a second sulfuric acid molecule ought to be an even more potent catalyst; in other words, the reaction is autocatalytic. The authors combine electronic structure calculations with a transition-state theory framework to predict rate-constant ratios, and then estimate the prevalence of this pathway depending on the relative background concentrations of water and sulfuric acid. The sulfuric acid–to–water ratio is too low for significant autocatalysis in the troposphere, but the pathway could play a pivotal role in the stratosphere as well as in aerosols, and perhaps in the atmosphere of Venus.

J. Am. Chem. Soc. 134, 20632 (2012).

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