A theoretical study of three gas-phase reactions involving the production or loss of methane cations

Abstract

Hydrocarbon ions are important species in flames, spectroscopy and the interstellar medium. Their importance is reflected in the extensive body of literature on the structure and reactivity of carbocations. However, the geometry, electronic structure and reactivity of carbocations are difficult to assess. This study aims to contribute to the current knowledge of this subject by presenting a quantum mechanics description of methane cation dissociation using multiconfigurational methods. The geometric and electronic parameters of the minimum structure were determined for three main reaction paths: the dissociation CH₄⁺ → CH₂⁺ + H₂ and the dissociation–recombination processes CH₄⁺ ↔ CH₃⁺ + H. The electronic and energetic effects of these reactions were analyzed, and it was found that each reaction path has a strong dependence on the methodology used as well as a strong multiconfigurational character during dissociation. The first doublet excited states are inner-shell excited states and may correspond to the ions that are expected to be formed after electron detachment. The rate coefficient for each reaction path was determined using variational transition state theory and RRKM/master equation calculations. The major dissociation paths, with their rate coefficients at the high-pressure limit, are CH₄⁺(²B₁) → CH₃⁺(A²A₁′) + H(²S) (k_∞(T) = 1.42 × 10⁺¹⁴ s⁻¹ exp(−37.12/RT)) and CH₄⁺(²B₁) → CH₂⁺(A²A₁) + H₂(²Σ_g⁺) (k_∞(T) = 9.18 × 10⁺¹⁴ s⁻¹ exp(−55.77/RT)). Our findings help to explain the abundance of ions formed from CH₄ in the interstellar medium and to build models of chemical evolution.