Gas phase observation and microwave spectroscopic characterization of formic sulfuric anhydride

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Science  03 Jul 2015:
Vol. 349, Issue 6243, pp. 58-61
DOI: 10.1126/science.aaa9704

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An unexpected gaseous sulfur species

Sulfuric acid plays a central role in both industrial and atmospheric contexts. As such, the behavior of SO3 mixtures in gas phases has been studied for over a century. In gas-phase experiments on wet SO3 and formic acid, Mackenzie et al. discovered a previously unrecognized covalent adduct: formic sulfuric anhydride, or HC(O)OSO3H. The combination of microwave spectroscopy and theoretical calculations reveals its structural properties. The compound may play a role in the nucleation of atmospheric aerosols by serving as an intermediate to H2SO4 formation.

Science, this issue p. 58


We report the observation of a covalently bound species, formic sulfuric anhydride (FSA), that is produced from formic acid and sulfur trioxide under supersonic jet conditions. FSA has been structurally characterized by means of microwave spectroscopy and further investigated by using density functional theory and ab initio calculations. Theory indicates that a π2 + π2 + σ2 cycloaddition reaction between SO3 and HCOOH is a plausible pathway to FSA formation and that such a mechanism would be effectively barrierless. We speculate on the possible role that FSA may play in the Earth’s atmosphere.

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