Formation of Distinct Isomers via Chemical Reaction in a Supersonic Jet: The Reaction of Sulfur Trioxide and Thioacetic Acid Studied by Rotational Spectroscopy

The reaction between sulfur trioxide (SO₃) and thioacetic acid (thiol form, CH₃COSH) is studied by microwave spectroscopy in a supersonic jet. With multiple isotopic substitutions, and supported by MP2 and density functional theory calculations, the spectra unambiguously establish the formation of two distinct isomers. The first, denoted the O-S isomer, is CH₃C(=S)OSO₂OH and involves a new bond between the oxygen of the thioacetic acid and the sulfur of the SO₃. The second, denoted the S-S isomer, CH₃C(=O)SSO₂OH, involves the new bond between the sulfur of the acid and the sulfur of the SO₃. The O-S isomer is entirely analogous to the product identified in previous studies of the RCOOH + SO₃ and C₆H₅COSH + SO₃ reactions, but the formation of the S-S isomer is new. Calculations place the S-S isomer ∼8 kcal/mol lower in energy than the O-S form, suggesting that the latter is a kinetically controlled product. The mechanism by which the S-S isomer forms is unclear but may involve either a set of independent pathways or initial formation of the O-S isomer, followed by rapid rearrangement. Participation by a third body is likely. Without input from an external energy source, the formation of two isomers in a supersonic expansion is unusual.