Internal rotation and structure of thiophenol and 4-fluorothiophenol studied by microwave spectroscopy and quantum chemistry

Microwave spectra of two isotopologues of thiophenol, (C6H5–SH and C6H5–SD), and one of 4-fluorothiophenol (4-F–C6H5–SD) were investigated. As a consequence of the internal rotation the vibrational ground state consists of two torsional sub-states and the rotational transitions appear as doublets in the spectra. Rotational constants and centrifugal distortion constants for both sub-states and energy difference and a Coriolis type interaction parameter between the sub-states were determined. For thiophenol eleven parameters, including potential constants, structure parameters and parameters describing relaxation of structure during internal rotation, were adjusted to give an accurate global fit to microwave spectroscopic constants for the two isotopologues and far infrared data for the parent species. For 4-fluorothiophenol the microwave spectra were assigned for the SD-species only, good infrared data were not available, and only seven parameters could be determined. The barrier to internal rotation was determined to 277 cm−1 for thiophenol and 62.5 cm−1 for 4-fluorothiophenol. Both molecules were found to be planar in the equilibrium configuration. Ab initio calculations using B3LYP, MP2 and CCSD(T) were used not only to compare with the experimental parameters, but also in the choice of experimental parameters, in particular the relaxation parameters. Problems due to linear dependencies for larger basis sets are discussed.