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.