Characterization of a solvent-separated ion-radical pair in cationized water networks: Infrared photodissociation and Ar-attachment experiments for water cluster radical cations (H₂O)_n⁺ (n = 3-8)

We present infrared spectra of nominal water cluster radical cations (H₂O)_n⁺ (n = 3-8), or to be precise, ion-radical complexes H⁺(H₂O)_n-1(OH), with and without an Ar tag. These clusters are closely related to the ionizing radiation-induced processes in water and are a good model to characterize solvation structures of the ion-radical pair. The spectra of Ar-tagged species show narrower bandwidths relative to those of the bare clusters due to the reduced internal energy via an Ar-attachment. The observed spectra are analyzed by comparing with those of the similar system, H⁺(H ₂O)_n, and calculated ones. We find that the observed spectra are attributable to ion-radical-separated motifs in n ≥ 5, as reported in the previous study (Mizuse et al. Chem. Sci.2011, 2, 868-876). Beyond the structural trends found in the previous study, we characterize isomeric structures and determine the number of water molecules between the charged site and the OH radical in each cluster size. In all of the characterized cluster structures (n = 5-8), the most favorable position of OH radical is the next neighbor of the charged site (H₃O⁺ or H₅O₂⁺). The positions and cluster structures are governed by the balance among the hydrogen-bonding abilities of the charged site, H₂O, and OH radical. These findings on the ionized water networks lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions.