We report microwave spectra and theoretical calculations for the carboxylic sulfuric anhydrides RCOOSO2OH (R = CH3, CF3). Both species are produced by the reaction of SO3 and the corresponding carboxylic acid in a supersonic jet. For the CH3COOH derivative, A and E internal rotor states are observed and a barrier to internal rotation of 241.093(30) cm-1 is determined. For the CF3 analog, no evidence of internal rotation was observed in the spectrum. M06-2X/6-311++G(3df,3pd) calculations indicate that both species form via a cycloaddition reaction within a putative RCOOH×SO3 complex and that in both cases, the equilibrium orientation of the CH3 or CF3 rotor differs between the free acid and the anhydride. Thus, internal rotation must accompany cycloaddition. For the acetic acid derivative, CCSD(T)/CBS//M06-2X/6-311++G(3df,3pd) electronic energies corrected for zero point energy using M06-2X frequencies reveal that the reaction is essentially barrierless regardless of whether the internal rotation and the cycloaddition are sequential or simultaneous. In the CF3 species (whose spectra were noticeably weaker than those of the CH3 analog) there is a barrier of ~0.65 kcal/mol. Microwave spectra and calculations on the water complex of the acetic acid derivative, CH3COOSO2OH×H2O, are also presented and the internal rotation barrier is experimentally determined to be 219.60(2) cm-1. This system represents the first stage of hydration of an acid anhydride. A possible role for the acetic acid related species in atmospheric aerosol formation is described.

Internal rotation and monohydration of carboxylic sulfuric anhydrides

255th National Meeting (2018)

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