Eur. Phys. J. D 20, 469-480 (2002)
https://doi.org/10.1140/epjd/e2002-00147-6

Interaction between aromatic amine cations and nonpolar solvents: Infrared spectra of isomeric aniline⁺–Ar_n (n = 1,2) complexes

Institut für Physikalische Chemie, Universität Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland

Corresponding author: ^a otto.dopfer@unibas.ch

Received: 4 February 2002
Published online: 13 September 2002

Abstract

Infrared (IR) photodissociation spectra of the aniline⁺–Ar_n cations, An (n=1,2), are analyzed in the vicinity of the N–H stretch fundamentals. The complexes are produced in an electron impact (EI) ion source which produces predominantly the most stable cluster isomers. Two isomers of are identified by their characteristic N–H stretch frequencies: the planar proton-bound global minimum, in which the Ar ligand forms a nearly linear H-bond to the amino group, and the less stable local minimum, in which the Ar atom is attached to the π-electron system of the aromatic ring. This result is the first unambiguous detection of the most stable dimer. All previous spectroscopic studies of An⁺–Ar employed resonance enhanced multiphoton ionization (REMPI) of neutral and identified only the less stable cation due to restrictions arising from the Franck-Condon principle. The EI-IR spectrum of shows that the most stable structure of this trimer features two equivalent H-bonds ( symmetry). The interpretation of the experimental data is supported by quantum chemical calculations. The ab initio potential of An⁺–Ar calculated at the UMP2/6-311G(2df, 2pd) level features global minima ( cm^-1) and local minima ( cm^-1), with a barrier of  cm^-1 for isomerization from the toward the minimum.