Shock Wave and Theoretical Modeling Study of the Dissociation of CH₂F₂. I. Primary Processes

The unimolecular dissociation of CH₂F₂ leading to CF₂ + H₂, CHF + HF, or CHF₂ + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400-1800 K, monitoring the formation of CF₂. It is shown that the energetically most favorable dissociation channel leading to CF₂ + H₂ has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH₂F₂ → CHF + HF is followed by the reaction CHF + HF → CF₂ + H₂. The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH₂F₂* intermediates, as assumed before.