Understanding the Rotational Mechanism of a Single Molecule: STM and DFT Investigations of Dimethyl Sulfide Molecular Rotors on Au(111)

Thioether molecules are an ideal surface-mounted rotor system that allows many fundamental properties of molecular rotors to be studied and quantified. Recent work on thioether molecular rotors has revealed that their rotation can be controlled thermally, electrically, and mechanically. This study combines scanning tunneling microscopy experiments and density functional theory calculations to investigate the rotational properties of a simple thioether molecule, dimethyl sulfide. Experimental studies showed a very low barrier to rotation for dimethyl sulfide (it rotated >10³ Hz at 5 K) and this barrier was calculated theoretically. Also, using theoretical methods the minimum energy adsorption site was determined and the mechanism of rotation was elucidated. Our results indicate that the rotation of a small, simple molecule is actually rather complex; as the CH₃ groups of dimethyl sulfide rotate around the Au−S bond, the central S atom precesses around a surface Au atom.