A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and Computational Studies of CF₃COOSO₂OH

Trifluoroacetic sulfuric anhydride (CF₃COOSO₂OH, TFASA) and its deuterated isotopologue have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. TFASA was generated in situ in a supersonic expansion from the reaction of CF₃COOH or CF₃COOD with SO₃. The spectrum, which was notably weaker than those of previously studied carboxylic sulfuric anhydrides, is that of a simple asymmetric rotor with no evidence of internal rotation of the CF₃ group. Calculations at the M06-2X/6-311++G(3df,3pd) level indicate that the title compound is produced via a mechanism involving a concerted cycloaddition, analogous to that found for other carboxylic sulfuric anhydrides. The calculations further show that the equilibrium orientation of CF₃ relative to the C═O bond changes upon formation of the anhydride, indicating that any path connecting the equilibrium structures of CF₃COOH and CF₃COOSO₂OH necessarily includes both cycloaddition and internal rotation. CCSD(T)/complete basis set with double and triple extrapolation [CBS(D-T)] single-point energy calculations at key points on the potential surface indicate that the barrier to form TFASA from a putative CF₃COOH···SO₃ complex is about 1.2 kcal/mol after zero-point energy corrections. This value is significantly larger than the near-zero or slightly negative barriers previously reported for the reactions of SO₃ with nonfluorinated carboxylic acids and likely accounts, at least in part, for the reduced spectral intensity. Thus, TFASA is a somewhat unique addition to the series of carboxylic sulfuric anhydrides studied to date. Theoretical values of certain structural parameters, atomic charges, and vibrational frequencies also support this point of view. Despite the differences, however, this work clearly demonstrates that the reaction RCOOH + SO₃ → RCOOSO₂OH readily occurs in the gas phase and is not restricted to acids with hydrocarbon R groups.