Stepwise microhydration of aromatic amide cations: formation of water solvation network revealed by infrared spectra of formanilide(+)-(H2O)(n) clusters (n ≤ 5).

Hydration of peptides and proteins has a strong impact on their structure and function. Infrared photodissociation spectra (IRPD) of size-selected clusters of the formanilide cation, FA(+)-(H2O)n (n = 1-5), are analyzed by density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the sequential microhydration of this prototypical aromatic amide cation. IRPD spectra are recorded in the hydride stretch and fingerprint ranges to probe the preferred interaction motifs and the cluster growth. IRPD spectra of cold Ar-tagged clusters, FA(+)-(H2O)n-Ar, reveal the important effects of temperature and entropy on the observed hydration motifs. At low temperature, the energetically most stable isomers are prominent, while at higher temperature less stable but more flexible isomers become increasingly populated because of entropy. In the most stable structures, the H2O ligands form a hydrogen-bonded solvent network attached to the acidic NH proton of the amide, which is stabilized by large cooperative effects arising from the excess positive charge. In larger clusters, hydration bridges the gap between the NH and CO groups (n ≥ 4) solvating the amide group rather than the more positively charged phenyl ring. Comparison with neutral FA-(H2O)n clusters reveals the strong impact of ionization on the acidity of the NH proton, the strength and topology of the interaction potential, and the structure of the hydration shell.