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

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 N-2 (n = 1-10) are recorded in the hydride stretch (amide A, nu(NH), nu(CH)) and fingerprint (amide I-III) ranges of AA(+) in its (2)A '' 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 (nNH) provide detailed information about the sequential microsolvation process of AA(+) in a nonpolar (L = He and Ar) and quadrupolar (L = N-2) 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 (pi-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(pi) = 585 and 715 cm(-1) for p-bonded Ar and N-2 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 p-stacking to H-bonding. Electron removal from the HOMO of AA(+) delocalized over both the aromatic ring and the amide group significantly strengthens the C=O bond and weakens the N-H bond of the amide group.