Microsolvation of the acetanilide cation (AA+) in a nonpolar solvent: IR spectra of AA+–Ln clusters (L = He, Ar, N2; n ≤ 10)

Abstract

Infrared photodissociation (IRPD) spectra of mass-selected cluster ions of acetanilide (N-phenylacetamide), AA⁺–L_n, with the ligands L = He (n = 1–2), Ar (n = 1–7), and N₂ (n = 1–10) are recorded in the hydride stretch (amide A, ν_NH, ν_CH) and fingerprint (amide I–III) ranges of AA⁺ in its ²A′′ ground electronic state. Cold AA⁺–L_n clusters are generated in an electron impact ion source, which predominantly produces the most stable isomer of a given cluster ion. Systematic vibrational frequency shifts of the N–H stretch fundamentals (ν_NH) provide detailed information about the sequential microsolvation process of AA⁺ in a nonpolar (L = He and Ar) and quadrupolar (L = N₂) solvent. In the most stable AA⁺–L_n clusters, the first ligand forms a hydrogen bond (H-bond) with the N–H proton of trans-AA⁺ (t-AA⁺), whereas further ligands bind weakly to the aromatic ring (π-stacking). There is no experimental evidence for complexes with the less stable cis-AA⁺ isomer. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra. The calculated binding energies of D_e(H) = 720 and 1227 cm⁻¹ for H-bonded and D_e(π) = 585 and 715 cm⁻¹ for π-bonded Ar and N₂ ligands in t-AA⁺–L are consistent with the observed photofragmentation branching ratios of AA⁺–L_n. Comparison between charged and neutral AA⁽⁺⁾–L dimers indicates that ionization switches the preferred ion–ligand binding motif from π-stacking to H-bonding. Electron removal from the HOMO of AA⁺ delocalized over both the aromatic ring and the amide group significantly strengthens the CO bond and weakens the N–H bond of the amide group.