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 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 a reaction CHF + HF → CF2 + H2. The experimental value of the rate constant of the latter indicates that this reaction does not proceed by an addition-elimination process as assumed before, but by a more direct abstraction pathway involving elements of roaming dynamics