Understanding the Influence of a Water Molecule in the Structure of a Dimer

Understanding how water influences molecular aggregation is essential for interpreting noncovalent interactions in biological and chemical systems. In this study, we investigate the solvation of organic dimers formed by 2-phenylethanol (PEAL), 2-phenethylamine (PEA), and 2-phenethylthiol (PET) through a combination of two-color REMPI and IR-UV double resonance spectroscopy, supported by quantum chemical calculations. Despite the structural and electronic differences among the three molecules, all systemsexcept PET₂form similar monohydrated structures, where water adds to the preexisting hydrogen bond network rather than inserts into it. The observed behavior seems to follow qualitative energetic trends, with the most stable structures favored under the experimental conditions; structural accessibility could also influence the formation of certain isomers. Spectroscopic shifts in OH, NH, and SH stretching regions reveal correlations between OH···S and OH···N interactions, despite their differing electronic characteristics, while the OH···O interaction needs to be stronger to induce comparable shifts. Computational analysis confirms that the experimentally observed structures correspond to the most stable isomers, although alternative cyclic structures were also identified with slightly higher energies. By systematically comparing dimers formed by OH-, NH-, and SH-containing molecules, this work extends previous studies on water insertion versus addition and provides new insights into how donor type influences hydration patterns and spectroscopic signatures.