title: Spectroscopic identification of carbenium and ammonium isomers of protonated aniline (AnH+): IR spectra of weakly bound AnH+ -Ln clusters (L = Ar, N2).
abstract: Infrared photodissociation (IRPD) spectra of mass-selected clusters composed of protonated aniline (C6H8N+ = AnH+) and a variable number of neutral ligands (L = Ar, N2) are obtained in the N-H stretch range. The AnH+ -Ln complexes (n < or = 3) are produced by chemical ionization in a supersonic expansion of An, H2, and L. The IRPD spectra of AnH+-Ln feature the unambiguous fingerprints of at least two different AnH+ nucleation centers, namely, the ammonium isomer (5) and the carbenium ions (1 and/or 3) corresponding to protonation at the N atom and at the C atoms in the para and/or ortho positions, respectively. Protonation at the meta and ipso positions is not observed. Both classes of observed AnH+-Ln isomers exhibit very different photofragmentation behavior upon vibrational excitation arising from the different interaction strengths of the AnH+ cores with the surrounding neutral ligands. Analysis of the incremental N-H stretch frequency shifts as a function of cluster size shows that microsolvation of both 5 and 1/3 in Ar and N2 starts with the formation of intermolecular H bonds of the ligands to the acidic NH protons and proceeds by intermolecular pi bonding to the aromatic ring. The analysis of both the photofragmentation branching ratios and the N-H stretch frequencies demonstrates that the N-H bonds in 5 are weaker and more acidic than those in 1/3, leading to stronger intermolecular H bonds with L. The interpretation of the spectroscopic data is supported by density functional calculations conducted at the B3LYP level using the 6-31G* and 6-311G(2df,2pd) basis sets. Comparison with clusters of neutral aniline and the aniline radical cation demonstrates the drastic effect of protonation and ionization on the acidity of the N-H bonds and the topology of the intermolecular potential, in particular on the preferred aromatic substrate-nonpolar ligand recognition motif.
authors.affiliation: Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany.
authors.forename: Felix M
authors.initials: FM
authors.lastname: Pasker
authors.forename: Nicola
authors.initials: N
authors.lastname: Solcà
authors.forename: Otto
authors.initials: O
authors.lastname: Dopfer
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D001128:Argon / Q000737:chemistry*; D006838:Hydrocarbons / Q000737:chemistry; D007536:Isomerism; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D011522:Protons; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013055:Spectrophotometry, Infrared / Q000379:methods
chemical_list: D000814:Aniline Compounds; D006838:Hydrocarbons; D008024:Ligands; D011522:Protons; D000644:Quaternary Ammonium Compounds; C030011:carbene; D001128:Argon; D009584:Nitrogen; D008697:Methane