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.