Entrance channel complexes of cationic aromatic SN2 reactions: IR spectra of fluorobenzene+–(H2O)n clusters

Microhydrated fluorobenzene cation clusters, C6H5F+–(H2O)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 C6H5F+–H2O potential features several minima with charge–dipole orientation and comparable binding energies (D0 ∼ 9 ± 1 kcal/mol). The C6H5F+–H2O spectrum is consistent with a structure in which the O atom of H2O approaches the C6H5F+ cation from above the aromatic plane. The C6H5F+–(H2O)2 spectrum reveals the presence of two isomers, in which either a (H2O)2 dimer or two single H2O ligands are attached to C6H5F+. The detected C6H5F+–(H2O)1,2 complexes were unreactive with respect to nucleophilic substitution.