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

Infrared (IR) photodissociation spectra of the aniline(+) -Ar-n cations, An(+) -Ar-n (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 An(+)-Ar 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 pi-bound local minimum, in which the Ar atom is attached to the pi-electron system of the aromatic ring. This result is the first unambiguous detection of the most stable H-bound An+-Ar dimer. All previous spectroscopic studies of An+-Ar employed resonance enhanced multiphoton ionization (REMPI) of neutral An-Ar and identified only the less stable pi-bound cation due to restrictions arising from the Franck-Condon principle. The EI-IR spectrum of An(+)-Ar-2 shows that the most stable structure of this trimer features two equivalent H-bonds (C-2nu 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 H-bound global minima (D-e = 513 cm(-1)) and pi-bound local minima (D-e = 454 cm(-1)), with a barrier of V-b approximate to 140 cm(-1) for isomerization from the pi-bound toward the H-bound minimum.