Entrance channel complexes of cationic aromatic S N2 reactions: IR spectra of fluorobenzene +–(H 2O) n clusters

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Microhydrated fluorobenzene cation clusters, C 6H 5F +–(H 2O) n with n = 1 and 2, were characterized by IR photodissociation spectra in the O–H stretch range and UB3LYP/6-31G* calculations. The intermolecular C 6H 5F +–H 2O potential features several minima with charge–dipole orientation and comparable binding energies ( D 0 ∼ 9 ± 1 kcal/mol). The C 6H 5F +–H 2O spectrum is consistent with a structure in which the O atom of H 2O approaches the C 6H 5F + cation from above the aromatic plane. The C 6H 5F +–(H 2O) 2 spectrum reveals the presence of two isomers, in which either a (H 2O) 2 dimer or two single H 2O ligands are attached to C 6H 5F +. The detected C 6H 5F +–(H 2O) 1,2 complexes were unreactive with respect to nucleophilic substitution.