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)

Abstract

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/N₂) and density functional calculations at the UB3LYP/6-311G(2df,2pd) level. Size-dependent frequency shifts of the O–H stretch vibration (Δν₁) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1-Np⁺ rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and π-bonding, the estimation of ligand binding energies, and the acidity of t/c-1-Np⁺. t-1-Np⁺ is predicted to be more stable than c-1-Np⁺ by 9 kJ mol⁻¹, with an isomerization barrier of 38 kJ 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 ν₁ bands of the more stable H-bound and the less stable π-bound t-1-Np⁺–Ar isomers. Only the more stable H-bound dimers are identified for t/c-1-Np⁺–L₂. Analysis of the Δν₁ shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (∼908 ± 30 kJ mol⁻¹). The Δν₁ shifts of 1-Np⁺–L_n (n ≤ 2 for Ar, n ≤ 5 for N₂) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np⁺–L, which is further solvated by (n−1) π-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.