Microsolvation of the formanilide cation (FA+) in a nonpolar solvent: infrared spectra of FA(+)-Ln clusters (L = Ar, N2; n ≤ 8).

Infrared photodissociation (IRPD) spectra of cationic formanilide (N-phenylformamide) clusters, FA(+)-Ln, with L = Ar (n = 1-8) and N2 (n = 1-6), are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges to probe the preferred interaction motifs and the cluster growth. Cold FA(+)-Ln clusters are generated by electron ionization in a supersonic expansion, which generates predominantly the most stable cluster isomers. Size- and isomer-specific νNH frequencies unravel the microsolvation process of FA(+) in a nonpolar (L = Ar) and a quadrupolar (L = N2) solvent. The H-bound FA(+)-L dimer with L binding to the NH proton of the amide group is the most stable isomer, and further ligands are attached to the aromatic ring (π-stacking). Ionization changes the preferred binding motif from π-stacking to H-bonding in FA((+))-L. Quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level confirm the experimentally derived sequential cluster growth and the vibrational and isomer assignments. The calculated FA(+)-L binding energies of D0(H) = 594/1054 cm(-1) for H-bound and D0(π) = 459/604 cm(-1) for π-bound Ar/N2 ligands are consistent with the observed photofragmentation branching ratios. Ionization of FA results from removal of a bonding π-electron delocalized over the phenyl and amide moieties and thus weakens the N-H bond and strengthens the C-O bond.