Rotational Spectroscopy and Conformational Flexibility of 2-Phenylethanethiol: The Dominant S-H⋅⋅⋅π Intramolecular Hydrogen Bond.

We present a rotational-computational investigation of the aromatic mercaptan 2-phenylethanethiol, addressing its potential energy surface, conformational equilibrium, internal dynamics and intramolecular interactions. The experiment used broadband chirped-pulse Fourier transform microwave spectroscopy in a supersonic jet expansion, recording the rotational spectrum in the 2-8 GHz frequency region. Two different conformers were detected in the spectrum. The most intense transitions correspond to a skew (gauche-gauche) conformation, identified as the global minimum. The spectra of ten different isotopologues were assigned for this species, leading to accurate effective and substitution structures. The weaker spectrum presents small tunnelling doublings caused by the torsional motion of the thiol group, which are only compatible with an antiperiplanar skeleton and a gauche thiol. The larger stability of the global minimum is attributed to an intramolecular S-H⋅⋅⋅π weak hydrogen bond. A comparison of the intramolecular interactions in the title molecule and 2-phenylethanol, similarly stabilized by a O-H⋅⋅⋅π hydrogen bond, shows the different strength of these interactions. Density functional (B3LYP-D3, B2PLYP-D3) and ab initio (MP2) calculations were conducted for the molecule.