Hydration of an Acid Anhydride: The Water Complex of Acetic Sulfuric Anhydride

The complex formed from acetic sulfuric anhydride (CH₃COOSO₂OH, ASA) and water has been observed by pulsed nozzle Fourier transform microwave spectroscopy. ASA was formed in situ in the supersonic jet via the reaction of SO₃ and CH₃COOH, and subsequently complexed with water using a concentric, dual injection needle that allows reagents to be introduced at different points along the expansion axis. Spectroscopic constants for the parent, fully deuterated, and CH₃¹³COOSO₂OH species are reported. Both A and E internal rotor states have been observed and analyzed. The fitted internal rotation barrier of the methyl group is 219.598(21) cm^-1 for the parent species, indicating that complexation with water lowers the internal rotation barrier of the methyl group by 9% relative to that of the free ASA. M06-2X/6-311++G(3df,3pd) calculations predict at least two distinct isomeric forms of ASA···H₂0. Spectroscopic constants for the observed species agree with those for the lower energy isomer in which the water inserts into the intramolecular hydrogen bond of the ASA monomer. CCSD(T)/CBS(D-T) calculations place the binding energy of this isomer at 13.3 kcal/mol below that of the isolated ASA and H₂O monomers. The calculations further indicate that the doubly hydrogen bonded complex CH₃COOH···H₂SO₄, which contains the hydrolysis products of ASA, lies even lower in energy, but this species was not observed in this study. This system represents the first stage of microsolvation of an acid anhydride, and the results indicate that a single water molecule does not induce spontaneous hydrolysis in a cold molecular cluster.