Observation of Two Conformers of Acrylic Sulfuric Anhydride by Microwave Spectroscopy

The rotational spectrum of acrylic sulfuric anhydride (CH₂═CHCOOSO₂OH, AcrSA) has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. The species was produced from the reaction between acrylic acid and sulfur trioxide in a supersonic jet. Spectroscopic constants are reported for both the s-cis- and s-trans-AcrSA conformers of the parent and monodeuterated (OD) isotopologues. Geometries were optimized for both conformers using M06-2X/6-311++G(3df,3pd) methods. Single-point energy calculations at the M06-2X geometries were calculated using the CCSD(T)/complete basis set method with double and triple extrapolation [CBS(D-T)]. Further calculations indicate that the anhydride results from a π₂ + π₂ + σ₂ cycloaddition reaction within the acrylic acid-SO₃ complex. Because the C═O double bond of the acrylic acid migrates from one of the COOH oxygens to the other during the reaction, the s-cis form of acrylic acid leads to the s-trans form of the anhydride and vice versa. With zero-point energy corrections applied to the CCSD(T) energies, the s-cis and s-trans forms of CH₂═CHCOOSO₂OH are 19.0 and 18.8 kcal/mol lower in energy than that of SO₃ + their corresponding CH₂═CHCOOH precursor conformation. The zero-point-corrected transition state energies for formation of the s-trans and s-cis anhydrides are 0.22 and 0.33 kcal/mol lower than those of the complexes of SO₃ with s-cis and s-trans acrylic acid, respectively, indicating that the reaction is essentially barrierless. This system adds to a growing body of examples demonstrating that carboxylic acids readily add to SO₃ in the gas phase to produce the corresponding carboxylic sulfuric anhydride.