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

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 (S N 2) reaction F - + CH 3 Cl → CH 3 F + 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 + CH 2 Cl - and chlorine abstraction to form CH 3 + FCl - . The threshold energy for the proton-transfer reaction is E 0 = 97 ± 9 kJ/mol, which yields Δ acid H 298 (CH 3 Cl) ≤ 1653 ± 9 kJ/mol and EA 0 (CH 2 Cl) ≥ 0.77 ± 0.14 eV. The threshold energy for the chlorine abstraction reaction to form FCl - is E 0 = 170 ± 40 kJ/mol, which yields EA 0 (FCl) ≥ 2.6 ± 0.4 eV. Potential energy surfaces for the three reaction paths are calculated using the coupled cluster and density functional theory methods at the CCSD(T)/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels. Geometry optimizations of stationary points along the surfaces show that a hydrogen-bonded F - ···H−CH 2 Cl complex is 5 kJ/mol lower in energy than the C 3 v F - ···CH 3 Cl complex. An additional feature observed in the Cl - reaction cross section at collision energies above 2 eV is attributed to further dissociation of CH 2 Cl - and FCl - products.