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+–Ln 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+–Ln
(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−Ln drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate–nonpolar ligand recognition pattern.
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