Kinetics of Submerged Intersystem Crossings in Strongly Coupled Ion-Molecule Reactions: I⁺ + CH₃X (X = F, Cl, Br, I)

The kinetics of the reaction between I⁺ and the methyl halides CH₃X (X = F, Cl, Br, I) are measured at temperatures ranging from 300 to 600 K using a selected-ion flow tube apparatus. Exothermic product channels for X = F, Cl, Br forming CH₂I⁺ + HX or CH₂X⁺ + HI require an intersystem crossing, making the series a model system for the kinetics involving consistently large spin-orbit coupling. The reaction efficiencies relative to capture rates increase steeply along the series (X = F: 0.07, Cl: 0.22, Br: 0.67, I: 0.95); however, this is shown not to be a function of the halide mass or increasing coupling. Nonadiabatic transition state theory was applied to reaction coordinates calculated using density functional theory by treating minimum energy crossing points between singlet and triplet surfaces as proxies for the adiabatic transition states. This quantitatively reproduced both the magnitude and temperature dependence of the X = F, Cl, and Br reactions. The reaction efficiencies are controlled by the energy of the adiabatic transition state as the incident I⁺ approaches a hydrogen atom, leading to abstraction. The energies of those transition states are, in turn, a function of the entrance well depth of the triplet I⁺(H₃CX) complexes, which are electrostatically bound and scale with the polarizabilities of the methyl halides. Charge transfer processes (dominating the X = I reaction and a minor product for X = Br) behave nonstatistically.