Microhydrated aromatic cluster cations: Binding motifs of 4-aminobenzonitrile-(H₂O)_n cluster cations with n ≤ 4

Infrared photodissociation (IRPD) spectra of mass-selected 4-aminobenzonitrile-(water)_n cluster cations, ABN⁺-(H₂O)_n with n ≤ 4, recorded in the N–H and O–H stretch ranges are analyzed by quantum chemical calculations at the M06-2X/aug-cc-pVTZ level to determine the evolution of the initial microhydration process of this bifunctional aromatic cation in its ground electronic state. IRPD spectra of cold clusters tagged with Ar and N₂ display higher resolution and allow for a clear-cut structural assignment. The clusters are generated in an electron impact source, which generates predominantly the most stable isomers. The IRPD spectra are assigned to single isomers for n = 1–3. The preferred cluster growth begins with sequential hydration of the two acidic NH protons of the amino group (n = 1–2), which is followed by attachment of secondary H₂O ligands hydrogen-bonded to the first-shell ligands (n = 3–4). These symmetric and branched structures are more stable than those with a cyclic H-bonded solvent network. Moreover, in the size range n ≤ 4 the formation of a solvent network stabilized by strong cooperative effects is favored over interior ion hydration which is destabilized by noncooperative effects. The potential of the ABN⁺-H₂O dimer is characterized in detail and supports the cluster growth derived from the IRPD spectra. Although the N–H bonds are destabilized by stepwise microhydration, which is accompanied by increasing charge transfer from ABN⁺ to the solvent cluster, no proton transfer to the solvent is observed for n ≤ 4.