Ionization-induced switch in aromatic molecule-nonpolar ligand recognition:: Acidity of 1-naphthol+ (1-Np+) rotamers probed by IR spectra of 1-Np+-Ln complexes (L = Ar/N2, n≤5)

The interaction of the trans (t) and cis (c) rotamers of the 1-naphthol cation (1-C10H8O+ = 1-Np+ = 1-hydroxynaphthalene(+)) with nonpolar ligands in the ground electronic state is characterized by IR photodissociation spectra of isolated 1-Np+-L-n, complexes (L = Ar/N-2) and density functional calculations at the UB3LYP/6-311(2df,2pd) level. Size-dependent frequency shifts of the O-H stretch vibration (Deltav(1)) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1Np(+) rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and pi-bonding, the estimation of ligand binding energies, and the acidity of t/c-1Np(+). t-1Np+is predicted to be more stable than c-1Np(+) by 9 kJ mol(-1), with an isomerization barrier of 38 U mol-1. The OH group in t-1-Np+ is slightly more acidic than in c-1-Np+ leading to stronger intermolecular H-bonds. Both 1-Np+ rotamers are considerably less acidic than the phenol cation because of enhanced charge delocalization. The 1-Np+-Ar spectrum displays v, bands of the more stable H-bound and the less stable pi-bound t-1-Np+-Ar isomers. Only the more stable H-bound dimers are identified for t/c-1-Np+-L-2. Analysis of the Deltav(1) shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (similar to908 +/- 30 kJ mol(-1)). The Deltav(1) shifts of 1-Np+-L-n (n less than or equal to 2 for Ar, n less than or equal to 5 for N-2) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np+-L, which is further solvated by (n-1) pi-bound ligands. Ionization of 1-Np-L-n drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate-nonpolar ligand recognition pattern.