Microhydration of PAH⁺ cations: evolution of hydration network in naphthalene⁺-(H₂O) clusters (n ≤ 5)

The interaction of polycyclic aromatic hydrocarbon molecules with water (H₂O = W) is of fundamental importance in chemistry and biology. Herein, size-selected microhydrated naphthalene cation nanoclusters, Np⁺-W (n ≤ 5), are characterized by infrared photodissociation (IRPD) spectroscopy in the C-H and O-H stretch range to follow the stepwise evolution of the hydration network around this prototypical PAH⁺ cation. The IRPD spectra are highly sensitive to the hydration structure and are analyzed by dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to determine the predominant structural isomers. For n = 1, W forms a bifurcated CH···O ionic hydrogen bond (H-bond) to two acidic CH protons of the bicyclic ring. For n ≥ 2, the formation of H-bonded solvent networks dominates over interior ion solvation, because of strong cooperativity in the former case. For n ≥ 3, cyclic W solvent structures are attached to the CH protons of Np⁺. However, while for n = 3 the W₃ ring binds in the CH···O plane to Np⁺, for n ≥ 4 the cyclic W clusters are additionally stabilized by stacking interactions, leading to sandwich-type configurations. No intracluster proton transfer from Np⁺ to the W solvent is observed in the studied size range (n ≤ 5), because of the high proton affinity of the naphthyl radical compared to W . This is different from microhydrated benzene⁺ clusters, (Bz-W )⁺, for which proton transfer is energetically favorable for n ≥ 4 due to the much lower proton affinity of the phenyl radical. Hence, because of the presence of polycyclic rings, the interaction of PAH⁺ cations with W is qualitatively different from that of monocyclic Bz⁺ with respect to interaction strength, structure of the hydration shell, and chemical reactivity. These differences are rationalized and quantified by quantum chemical analysis using the natural bond orbital (NBO) and noncovalent interaction (NCI) approaches.