Carboxylic Sulfuric Anhydrides

Carboxylic acids react with sulfur trioxide to form carboxylic sulfuric anhydrides, RCOOSO₂OH. In this article, new supersonic jet microwave spectra are presented for the anhydride derived from propiolic acid (HCCCOOH), and recent work on a series of carboxylic sulfuric anhydrides is reviewed. For the propiolic acid derivative, computed minimum-energy structures are reported for both the anhydride (HCCCOOSO₂OH) and its precursor complex (HCCCOOH-SO₃), and additional CCSD(T)/CBS(D-T)//M06-2X/6-311++G(3df,3pd) calculations indicate that, after zero-point energy corrections, the barrier to anhydride formation is effectively zero. These results are similar to those for other carboxylic sulfuric anhydrides studied and are consistent with their rapid production under supersonic jet conditions. Carboxylic sulfuric anhydrides, as a class, have not been widely characterized in the chemical literature and thus their study represents a new feature of the chemistry of sulfur oxides and oxyacids. As such, structural and energetic features of the carboxylic sulfuric anhydrides derived from formic, acetic, acrylic, trifluoroacetic, propiolic, pinic, and benzoic acids are compared. Computed vibrational frequencies are provided as Supporting Information and should be useful for possible future observation by infrared and/or Raman spectroscopy. Statistical thermodynamics is used to estimate the equilibrium constants for the formation reactions at a series of temperatures, and the results indicate values ranging from ∼10⁴ atm^-1 for formic acid at 288 K to over 10¹¹ atm^-1 for benzoic acid at 217 K. We speculate that carboxylic sulfuric anhydrides could be active species in the Earth's atmosphere and atmospheric concentrations have, therefore, been estimated assuming an equilibrium state. These estimates are subject to significant uncertainties in the atmospheric SO₃ and carboxylic acid concentrations but may be as high as 10⁷ molecules/cm³ in some locations. Related calculations suggest that equilibrium anhydride concentrations may exceed those of the sulfuric acid precursors SO₃-H₂O and SO₃-(H₂O)₂ by several orders of magnitude. Kinetic modeling will ultimately be necessary to fully assess the role, if any, of carboxylic sulfuric anhydrides in atmospheric processes.