Weak hydrogen bonding as the driver of aromatic hydration.

Abstract

Intermolecular forces are the fundamental architects of supramolecular structure, where subtle interplays of charge distribution, entropy, and sterics determine the outcome. In aromatic molecules, the distribution of delocalised π electrons is modulated by the substituents, affecting their intermolecular interactions and introducing "holes" in the π orbitals. While increasingly well-understood in the solid state, the influence of π-holes on solvation and miscibility in the liquid remains unknown. Here, total neutron scattering and modelling-based refinement reveal the solvation of phenol, aniline, and p-nitrophenol in water. The in-plane solvation is dominated by strong classical hydrogen bonds between water and the substituents. Out of the ring plane, perpendicular OH···π weak hydrogen bonds between water and phenol or aniline are cooperative and modulated by differences in electron density. By contrast, in p-nitrophenol, the presence of the electron-withdrawing nitro group enhances the overall molecular dipole, and introduces a pronounced π-hole that significantly disrupts the overall out-of-plane solvation. The latter is dominated by close water-O···N contacts at ≈ 3.35 Å resulting from localized charge depletion accompanied by O···π* motifs (≈ 3.96 Å). These interactions template the structure of the surrounding water, redefining the solubility of these aromatics in water and underscoring the complex solvation of organic molecules.