A PERICYCLIC REACTION IN THE GAS PHASE IDENTIFIED BY ROTATIONAL SPECTROSCOPY: REACTION OF A THIOCARBOXYLIC ACID WITH SULFUR TRIOXIDE.

The reaction of sulfur trioxide (SO3) and thiobenzoic acid (C6H5COSH) is investigated in the gas phase under supersonic jet conditions. Rotational spectroscopy of the parent and several isotopically substituted derivatives, in conjunction with DFT calculations at the M06-2X/6-311++G(3df,3pd) level of theory, identify the product as thiobenzoic sulfuric anhydride, C6H5C(=S)OSO2OH. Single point CCSD(T)/CBS(D-T)//M06-2X/6-311++G(3df,3pd) calculations place the electronic energy of the product anhydride 114 kJ/mol lower than that of SO3 + C6H5COSH at infinite separation. The calculations further indicate that the reaction proceeds through a cyclic transition state which lies 11.3 kJ/mol higher in energy than a C6H5COSH·SO3 complex, but 83.3 kJ/mol lower in energy than that of the separated reactants. The reaction is rapid under the experimental conditions of this study: based on the duration of the collisional phase of the supersonic expansion, it is clear that the product is formed within tens of microseconds after mixing. While the analogous reaction of carboxylic acids with SO3 has been demonstrated, the ability of a thiocarboxylic acid to undergo similar chemistry has not previously been established. Although rotational spectroscopy is best known for its precise interrogation of molecular and electronic structure, this work demonstrates its ability to study chemical transformations as well.