Competition between electron transfer and reactive capture in ion–molecule reactions at low collision energies: isotopic and stereodynamic effects in the reactions of CH₃F with H₂⁺, HD⁺ and D₂⁺

The bimolecular reactions between CH₃F and H₂⁺, HD⁺ and D₂⁺ have been studied in the range of collision energies between ∼0 and k_B × 30 K using a merged-beam approach. The ion–molecule reactions were investigated following photoexciting of H₂ (HD, D₂) to high Rydberg states in a supersonic beam, merging the Rydberg-molecule beam with a cold supersonic beam of CH₃F using a surface-electrode Rydberg–Stark deflector and monitoring the CH₃⁺, CH₂F⁺ and CH₃F⁺ ions generated by the reactions of H₂⁺ (HD+, D₂⁺) with CH₃F within the distant orbit of the Rydberg electron. In all three reaction systems, a strong increase of the rate coefficients was observed at collision energies below k_B × 4 K. Branching ratios for the formation of CH₃⁺, CH₂F⁺ and CH₃F⁺ were measured for all three reactions as a function of the collision energy. The branching ratio for the formation of CH₃⁺ was found to decrease with increasing deuteration of the hydrogen molecular ion and to increase at collision energies below k_B × 4 K. The experimental results were interpreted using model calculations based on a rotationally adiabatic capture model as well as using classical trajectory simulations. The reaction products are shown to be generated in two distinct mechanisms: electron transfer leading to a dominant CH₂F⁺ and a weaker CH₃F⁺ product channel, and short-range complex formation leading predominantly to CH₃⁺ by F⁻ transfer, with a weaker contribution of CH₂F⁺ by H⁻ transfer. The model calculations highlight the role played by quantum-statistical and stereodynamical effects associated with the J = 1, |K| = 1 ground state of para-CH₃F and by the reduced mass of the colliding partners: the orientation of CH₃F molecules induced by the electric field of the ion favours the production of CH₃⁺ by F⁻ transfer at low collision energies and the slower approach of the reaction partners with increasing reduced mass favours electron transfer at intermediate distances.