title: 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 < or = 3).
abstract: 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+-Ln complexes (L = Ar, N2) and quantum chemical calculations performed at the UMP2/6-311G(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. .
authors.affiliation: Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
authors.forename: Horia-Sorin
authors.initials: HS
authors.lastname: Andrei
authors.forename: Nicola
authors.initials: N
authors.lastname: Solcà
authors.forename: Otto
authors.initials: O
authors.lastname: Dopfer
mesh_terms: D020816:Amino Acid Motifs; D001128:Argon / Q000737:chemistry; D002412:Cations; D019281:Dimerization; D004583:Electrons; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D007477:Ions; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D012997:Solvents; D013055:Spectrophotometry, Infrared / Q000379:methods
chemical_list: D002412:Cations; D007093:Imidazoles; D007477:Ions; D008024:Ligands; D012997:Solvents; D001128:Argon; C029899:imidazole; D009584:Nitrogen