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 kB × 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 kB × 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 kB × 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.