Microhydration Effects on Hemibonds in [H ₂ O–X] ⁺ –(H ₂ O) n (X = O ₂ and CS ₂ ; n = 0–2): Infrared Spectroscopic Characterization toward Understanding Charge-Resonance Interactions in Aqueous Environments

The hemibond, a nonclassical covalent interaction arising from charge-resonance between a radical and a neutral molecule, represents a distinctive bonding motif in open-shell systems. Its role has been widely discussed in radical reactions, radiation chemistry, and related biochemical processes. While hemibonds involving water molecules have garnered considerable interest, it remains unclear whether these interactions can persist under bulk solvation conditions. Here, we investigate hemibond formation in gas-phase [H₂O-X]⁺ clusters and examine the structural evolution upon microhydration. Infrared photodissociation spectroscopy of [H₂O-X]⁺-(H₂O)n (X = O₂ and CS₂; n = 0-2) reveals that the hemibonded structure of [H₂O-X]⁺ persists during microhydration. These results elucidate the interplay between charge-resonance and charge-(induced) dipole interactions that govern hemibond stability and suggest that certain molecules may retain the ability to form stable hemibonds with water even in aqueous environments.