PNNL

Abstract

The highly exoergic nucleophilic substitution reaction F- + CH3I shows strikingly different reaction dynamics than substitution reactions of larger halogen ions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show dominant large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH3F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl- + CH3I [Science 2008, 319, 183] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F-–HCH2I hydrogen-bonded complex, a direct rebound and a direct stripping reaction. The indirect mechanism is found to contribute ~ 60% of the overall substitution reaction at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, since the barrier for the F-–HCH2I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.