F^-(H_\textbf2O)+CH_\textbf3I Ligand Exchange Reaction Dynamics

Single hydration of the gas phase F^-+CH_3I\rightarrow I^-+CH_3F reaction allows to probe solvent effects on a fundamental nucleophilic substitution reaction. At the same time, the addition of a solvent molecule opens alternative product channels. Here, we present crossed beam imaging results on the dynamics of the F^-(H_2O)+CH_3I\rightarrowFCH_3I^-+H_2O ligand exchange pathway at collision energies between 0.3 and 2.6 eV. Product kinetic energies are constrained by the stability requirement of the weakly bound product complexes. This implies substantial internal excitation of the water molecule and disfavors efficient energy redistribution in an intermediate complex, which is reflected by the suppression of low kinetic energies as collision energy increases. At 0.3 eV, internal nucleophilic displacement is important and is discussed in light of the competing nucleophilic substitution pathways that form I^- and I^-(H_2O).