Temperature and Pressure Dependences of the Reactions of Fe⁺ with Methyl Halides CH₃X (X = Cl, Br, I): Experiments and Kinetic Modeling Results

The pressure and temperature dependences of the reactions of Fe⁺ with methyl halides CH₃X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300–600 K. FeX⁺ was observed for all three halides and FeCH₃⁺ was observed for the CH₃I reaction. FeCH₃X⁺ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX⁺ products come via an insertion mechanism, while the FeCH₃⁺ can be produced from either insertion or S_N2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH₃I reaction, indicating that some more direct process must be important.