Interaction of ionic biomolecular building blocks with nonpolar solvents:: Acidity of the imidazole cation (Im+) probed by IR spectra of Im+-Ln complexes (L = Ar, N2; n ≤ 3)

The intermolecular interaction between the imidazole cation (Im(+) = C3N2H4+) and nonpolar ligands is characterized in the ground electronic state by infrared photodissociation (IRPD) spectroscopy of size-selected Im(+)-L-n complexes (L = Ar, N-2) and quantum chemical calculations performed at the UMP2/6-31 1G(2df,2pd) and UB3LYP/6-311G(2df,2pd) levels of theory. The complexes are created in an electron impact cluster ion source, which predominantly produces the most stable isomers of a given cluster ion. The analysis of the size-dependent frequency shifts of both the N-H and the C-H stretch vibrations and the photofragmentation branching ratios provides valuable information about the stepwise microsolvation of Im(+) in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between various intermolecular binding motifs (H-bonding and pi-bonding) and their interaction energies, and the acidity of both the CH and NH protons. In line with the calculations, the IRPD spectra show that the most stable Im(+)-L dimers feature planar H-bound equilibrium structures with nearly linear H-bonds of L to the acidic NH group of Im(+). Further solvation occurs at the aromatic ring of Im(+) via the formation of intermolecular pi-bonds. Comparison with neutral Im-Ar demonstrates the drastic effect of ionization on the topology of the intermolecular potential, in particular in the preferred aromatic substrate-nonpolar recognition motif, which changes from pi-bonding to H-bonding.