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Microsolvation of the acetanilide cation (AA(+)) in a nonpolar solvent: IR spectra of AA(+)-L-n clusters (L = He, Ar, N-2; n <= 10)

Schmies, Matthias; Patzer, Alexander; Schütz, Markus; Miyazaki, Mitsuhiko; Fujii, Masaaki; Dopfer, Otto

Inst. Optik und Atomare Physik

Infrared photodissociation (IRPD) spectra of mass-selected cluster ions of acetanilide (N-phenylacetamide), AA+–Ln, 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, νNH, νCH) and fingerprint (amide I–III) ranges of AA+ in its 2A′′ ground electronic state. Cold AA+–Ln 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 (νNH) 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+–Ln 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 De(H) = 720 and 1227 cm−1 for H-bonded and De(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+–Ln. 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 C[double bond, length as m-dash]O bond and weakens the N–H bond of the amide group.
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