Microwave spectrum and substitution structure of syn thiobenzoic acid

[Display omitted] •The microwave spectrum of the syn conformer of thiobenzoic acid has been observed.•Multiple isotopic substitutions have enabled the determination of a complete structure of the molecular frame.•DFT calculations indicate that the trans conformer lies 2.6 kcal/mol higher in energy than the syn conformer. The rotational spectrum of the syn conformer of thiobenzoic acid (C6H5COSH) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectra of all singly-substituted isotopologues involving 13C, as well as the 18O, 33S, 34S, and −SD derivatives have also been recorded. The isotopic data have been used to derive structural parameters of the molecular frame. The results are compared with density functional theory calculations at theM06-2X/6-311++G(d,p) level and show excellent agreement. The molecule is planar at the equilibrium geometry, and calculations of the energy profile for out-of-plane torsion of the SH group are presented. The anti conformer was not observed, consistent with calculations which indicate that it lies 2.6 kcal/mol higher in energy than the syn form.