Spectroscopic identification of carbenium and ammonium isomers of protonated aniline (AnH+):: IR spectra of weakly bound AnH+-Ln clusters ( L = Ar, N2)

Infrared photodissociation (IRPD) spectra of mass-selected clusters composed of protonated aniline (C6H8N+) = AnH(+)) and a variable number of neutral ligands (L = Ar, N-2) are obtained in the N-H stretch range. The AnH(+)-L-n complexes (n <= 3) are produced by chemical ionization in a supersonic expansion of An, H-2, and L. The IRPD spectra of AnH(+)-L-n 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(+)-L-n 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 N-2 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.