Dynamics of the Gas-Phase Reactions of Fluoride Ions with Chloromethane

Abstract

Guided ion beam tandem mass spectrometry techniques are used to examine the competing chemical dynamics of reactions of fluoride ions with chloromethane in the center-of-mass collision energy range 0.05−30 eV. The exothermic bimolecular nucleophilic substitution (SN2) reaction F- + CH3Cl → CH3F + Cl- predominates at the lowest collision energies (0.05−0.1 eV) but decreases by a factor of ∼100 over the range 0.1−2 eV. Two endothermic product channels are detected at collision energies ∼1−20 eV, corresponding to proton transfer to form HF + CH2Cl- and chlorine abstraction to form CH3 + FCl-. The threshold energy for the proton-transfer reaction is E0 = 97 ± 9 kJ/mol, which yields ΔacidH298(CH3Cl) ≤ 1653 ± 9 kJ/mol and EA0(CH2Cl) ≥ 0.77 ± 0.14 eV. The threshold energy for the chlorine abstraction reaction to form FCl- is E0 = 170 ± 40 kJ/mol, which yields EA0(FCl) ≥ 2.6 ± 0.4 eV. Potential energy surfaces for the three reaction paths are calculated using the coupled cluster and density functional theory meth...