title: Solvent migration in microhydrated aromatic aggregates: ionization-induced site switching in the 4-aminobenzonitrile-water cluster.
abstract: The dependence of the preferred microhydration sites of 4-aminobenzonitrile (4ABN) on electronic excitation and ionization is determined through IR spectroscopy of its clusters with water (W) in a supersonic expansion and through quantum chemical calculations. IR spectra of neutral 4ABN and two isomers of its hydrogen-bonded (H-bonded) 4ABN-W complexes are obtained in the ground and first excited singlet states (S0, S1) through IR depletion spectroscopy associated with resonance-enhanced multiphoton ionization. Spectral analysis reveals that electronic excitation does not change the H-bonding motif of each isomer, that is, H2O binding either to the CN or the NH site of 4ABN, denoted as 4ABN-W(CN) and 4ABN-W(NH), respectively. The IR spectra of 4ABN(+)-W in the doublet cation ground electronic state (D0) are measured by generating them either in an electron ionization source (EI-IR) or through resonant multiphoton ionization (REMPI-IR). The EI-IR spectrum shows only transitions of the most stable isomer of the cation, which is assigned to 4ABN(+)-W(NH). The REMPI-IR spectrum obtained through isomer-selective resonant photoionization of 4ABN-W(NH) is essentially the same as the EI-IR spectrum. The REMPI-IR spectrum obtained by ionizing 4ABN-W(CN) is also similar to that of the 4ABN(+)-W(NH) isomer, but differs from that calculated for 4ABN(+)-W(CN), indicating that the H2O ligand migrates from the CN to the NH site upon ionization with a yield of 100%. The mechanism of this CN→NH site-switching reaction is discussed in the light of the calculated potential energy surface and the role of intracluster vibrational energy redistribution.
authors.affiliation: Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama 226-8503 (Japan).
authors.forename: Takashi
authors.initials: T
authors.lastname: Nakamura
authors.forename: Matthias
authors.initials: M
authors.lastname: Schmies
authors.forename: Alexander
authors.initials: A
authors.lastname: Patzer
authors.forename: Mitsuhiko
authors.initials: M
authors.lastname: Miyazaki
authors.forename: Shun-ichi
authors.initials: S
authors.lastname: Ishiuchi
authors.forename: Martin
authors.initials: M
authors.lastname: Weiler
authors.forename: Otto
authors.initials: O
authors.lastname: Dopfer
authors.forename: Masaaki
authors.initials: M
authors.lastname: Fujii
mesh_terms: D006860:Hydrogen Bonding; D007536:Isomerism; D008958:Models, Molecular; D009570:Nitriles / Q000737:chemistry*; D012997:Solvents; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*
chemical_list: D009570:Nitriles; D012997:Solvents; D014867:Water; C045271:4-aminobenzonitrile
keywords: IR spectroscopy; cluster compounds; hydrogen bonds; isomerization; solvation