Solvent-dimer-mediated clusters as a microscopic model of solution: IR spectroscopy and density functional theory of jet-cooled pyrrole–tetrahydrofuran clusters

A solvated cluster in a supersonic jet serves as a microscopic model of a bulk solution. Generally, a 1-1 cluster of an organic solute with a simple solvent molecule is considered the smallest solvated cluster. However, a bulk solution consists of a solute surrounded by multiple solvent molecules. Therefore, the smallest solvated cluster is practically a 1-2 cluster comprised of one solute and two solvents, allowing us to investigate intermolecular interactions not only between the solute and the solvent but also among the solvents themselves. In this study, we focus on solvent-dimer-mediated clusters of pyrrole (Py) and tetrahydrofuran (THF) dimers to elucidate the mechanism of micro-solvation. The solvation structures of Py-THF clusters are determined through a combination of IR cavity ringdown spectroscopy and density functional theory computations. The hydrogen-bonded structures of Py₁-THF₁, Py₂-THF₁, and Py₁-THF₂ are analyzed in terms of the geometric deformation of the THF backbone. Notably, we found that the geometric deformation of the solvent THF dimer in Py₁-THF₂ is significantly larger than that of the other solvent dimers. To discuss the relationship between the deformation of the solvent dimer and the solubility of the bulk solution, we introduced Hansen Solubility Parameters. Consequently, we concluded that the flexible solvation by a solvent dimer, due to its large deformation, correlates with higher solubility in a bulk solution. The concept of solvent-dimer-mediated clusters preliminarily provides new insights into connecting micro-solvation with macro-solutions.