Stepwise microhydration of aromatic amide cations: water solvation networks revealed by the infrared spectra of acetanilide+–(H2O)n clusters (n ≤ 3)

Abstract

The structure and activity of peptides and proteins strongly rely on their charge state and the interaction with their hydration environment. Here, infrared photodissociation (IRPD) spectra of size-selected microhydrated clusters of cationic acetanilide (AA⁺, N-phenylacetamide), AA⁺–(H₂O)_n with n ≤ 3, are analysed by dispersion-corrected density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the stepwise microhydration process of this aromatic peptide model. The IRPD spectra are recorded in the informative X–H stretch (ν_OH, ν_NH, ν_CH, amide A, 2800–3800 cm⁻¹) and fingerprint (amide I–II, 1000–1900 cm⁻¹) ranges to probe the preferred hydration motifs and the cluster growth. In the most stable AA⁺–(H₂O)_n structures, the H₂O ligands solvate the acidic NH proton of the amide by forming a hydrogen-bonded solvent network, which strongly benefits from cooperative effects arising from the excess positive charge. Comparison with neutral AA–H₂O reveals the strong impact of ionization on the acidity of the NH proton and the topology of the interaction potential. Comparison with related hydrated formanilide clusters demonstrates the influence of methylation of the amide group (H → CH₃) on the shape of the intermolecular potential and the structure of the hydration shell.