Structural elucidation of 2-fluorothiophenol from Fourier transform microwave spectra and ab initio calculations

Abstract

Pure rotational transitions corresponding to the ground vibrational state of 2-fluorothiophenol (2FTPh) were recorded via Fourier transform microwave (FTMW) spectroscopy in the range of 4–26 GHz. The observed transitions were assigned to two planar conformers in which the SH bond is directed toward (cis-2FTPh) or away from (trans-2FTPh) the fluorine substituent with the former predicted to lie 3.86 kJ/mol lower in energy from ab initio calculations (MP2/6-311++G(2d,2p)). The rotational constants determined from the spectral analysis were used to derive effective ground state (r0) structures of the lower energy cis-2FTPh conformer using spectra of the corresponding 13C and 34S isotopologues which were observed in natural abundance. Geometry optimization at the MP2/6-311++G(2d,2p) level provided the equilibrium (re) structure which is in close agreement with the experimentally-derived geometry. Comparison with results from natural bond orbital (NBO) calculations provide evidence of a weak intramolecular interaction between the lone pair on sulfur and the CF moiety in cis-2FTPh that is not found in trans-2FTPh or thiophenol (TPh). This interaction stabilizes the cis-2FTPh conformer by 2.18 kJ/mol.