Probing sulfur-centered intra- and intermolecular interactions in mercaptoethanol from the gas to liquid phase: a...

Sulfur-centered hydrogen bonding plays an important yet insufficiently understood role in molecular structure and intermolecular interactions due to the relatively low electronegativity of sulfur. Mercaptoethanol (HSCH₂CH₂OH, ME), containing both S-H and O-H functional groups, serves as an ideal prototype for examining sulfur-involved hydrogen bonds in different environments. In this work, high-resolution gas-phase Raman spectra of ME were recorded in the S-H stretching region using stimulated photoacoustic Raman spectroscopy and analyzed in conjunction with quantum chemical calculations and temperature-dependent measurements. Comparison between experimental and simulated spectra enables an unambiguous assignment of individual Raman bands to specific conformers and reveals that gauche conformers stabilized by O-H⋯S intramolecular hydrogen bonds dominate in the gas phase, while S-H⋯O intramolecular interactions are present but extremely weak. In contrast, liquid-phase Raman spectra exhibit a red shift of the S-H stretching band in neat mercaptoethanol relative to that in an aqueous solution. Quantum chemical calculations on ME-ME and ME-H₂O dimers demonstrate that this red shift originates from a significantly enhanced S-H⋯O intermolecular hydrogen bond in the liquid state. These results provide direct spectroscopic evidence for the environment-dependent nature of sulfur-centered hydrogen bonding and offer molecular-level insights into the interplay between intramolecular and intermolecular interactions involving sulfur.