Interaction between Aromatic Amine Cations and Quadrupolar Ligands: Infrared Spectra of Aniline+−(N2)n (n = 1−5) Complexes

Abstract

Infrared (IR) photodissociation spectra of mass-selected aniline+−(N2)n complexes with n ≤ 5, produced in an electron impact (EI) cluster ion source, are presented in the range of the N−H stretch vibrations of the aniline cation (An+). Analysis of the An+−N2 spectrum is consistent with a H-bound structure in which the N2 ligand forms a nearly linear hydrogen bond to one proton of the NH2 group of An+. No other isomers are observed in the n = 1 spectrum, implying that H-bound An+−N2 corresponds to the global minimum of the dimer potential. This conclusion is supported by ab initio calculations at the UMP2/6-311G(2df,2pd) level, which predict H-bound An+−N2 (De = 1431 cm-1) to be more stable than other local minima. Systematic vibrational frequency shifts in the spectra of the larger An+−(N2)n clusters provide information about the microsolvation process of An+ in a quadrupolar solvent. The first two N2 ligands fill a first subshell by solvating the protons of the amino group (H bonds), whereas further ligands bind to the aromatic ring of An+ (π bonds). Analysis of the photofragmentation branching ratios yields approximate dissociation energies of the H and π bonds of D0(H) ∼ 1100 ± 300 cm-1 and D0(π) ∼ 700 ± 200 cm-1, respectively.