Competition between O-H and S-H Intermolecular Interactions in Conformationally Complex Systems: The 2-Phenylethanethiol and 2-Phenylethanol Dimers

Noncovalent interactions involving sulfur centers play a relevant role in biological and chemical environments. Yet, detailed molecular descriptions are scarce and limited to very simple model systems. Here we explore the formation of the elusive S-H<middle dot><middle dot><middle dot>S hydrogen bond and the competition between S-H<middle dot><middle dot><middle dot>O and O-H<middle dot><middle dot><middle dot>S interactions in pure and mixed dimers of the conformationally flexible molecules 2-phenylethanethiol (PET) and 2-phenylethanol (PEAL), using the isolated and size-controlled environment of a jet expansion. The structure of both PET-PET and PET-PEAL dimers was unraveled through a comprehensive methodology that combined rotationally resolved microwave spectroscopy, mass-resolved isomer-specific infrared laser spectroscopy, and quantum chemical calculations. This synergic experimental-computational approach offered unique insights into the potential energy surface, conformational equilibria, molecular structure, and intermolecular interactions of the dimers. The results show a preferential order for establishing hydrogen bonds following the sequence S-H<middle dot><middle dot><middle dot>S < S-H<middle dot><middle dot><middle dot>O less than or similar to O-H<middle dot><middle dot><middle dot>S < O-H<middle dot><middle dot><middle dot>O, despite the hydrogen bond only accounting for a fraction of the total interaction energy.