Microsolvation of Charged PAH Clusters: Modeling Interaction of Water Aggregates with Cationic Pyrene Dimer

The interaction of polycyclic aromatic hydrocarbons (PAHs) aggregates with water plays a key role in both atmospheric and interstellar chemistry. However, its characterization at the molecular level represents experimental and theoretical challenges. In this work, we propose a detailed characterization of the potential energy surfaces (PES) of cationic pyrene dimer interacting with water molecules and clusters, Py 2 + (H 2 O) n ( n = 1–10). A dedicated PES global exploration algorithm, parallel tempering Monte Carlo, combined with the density functional tight-binding configuration interaction potential, was employed with further refinement at the density functional theory level. Structural patterns were identified, and their relative stabilities were determined. For small clusters, side isomers are found to be the most stable, while face isomers become competitive for larger sizes. Analysis of cohesive energies, relative stabilities, and evaporation energies highlights the subtle effects of noncovalent interactions and reveals size-dependent patterns. Infrared spectral signatures are predicted in the spectral region corresponding to interstellar aromatic infrared bands (AIBs), including modifications of the C–H stretch band with a new red-shifted component, and the appearance of new features in the 10–20 μm region, while the 6–10 μm range remains quite unaffected. These results provide molecular-level insight into microhydrated PAH cationic clusters that are experimentally out of reach. The obtained data are relevant for astrochemical models and the interpretation of astronomical observations, particularly in the context of James Webb Space Telescope observations.