Energy Distribution among Reaction Products. VII. H + F2

Abstract

The reaction ${\mathrm{H+F}}_2\to \mathrm{HF+F}$ has been studied by means of the ``arrested relaxation'' variant of the infrared chemiluminescence technique. Detailed rate constants k (V′, R′, T′), where V′, R′, and T′ symbolize the vibrational, rotational, and translational energies in the reaction products, are reported in the form of a contour plot. The total detailed rate constants into specified vibrational quantum states for H+F<sub>2</sub> (summed over the rotational levels of each v′ level) are k (v′ = 1) = 0.12, k (v′ = 2) = 0.13, k (v′ = 3) = 0.25, k (v′ = 4) = 0.35, k (v′ = 5) = 0.78, [k (v′ = 6) = 1.00], k (v′ = 7) = 0.40, k (v′ = 8) = 0.26, k (v′ = 9) < 0.16 (relative to the highest rate constant $\text{k(v̂′)=1.00).}$ In common with other members of the H+X<sub>2</sub> family of reactions (X is a halogen) the H+F<sub>2</sub> reaction exhibits a comparatively low fractional conversion of the total available energy into internal excitation of the new molecule. The mean fractions entering vibration and rotation are ${\mathrm{f̄}}_{\mathrm{V\prime }}\text{=0.53}$ and ${\mathrm{f̄}}_{\mathrm{R}\prime }\text{=0.03.}$ This behavior appears to be characteristic of reactions involving substantial ``repulsive'' energy release and a light attacking atom. Evidence is presented to show that H+F₂ occupies a special place in the H+X₂ family in that the repulsive energy release (energy released as the products separate) is restricted to the period when the F atoms are at close range. An approximate relation $\log {\mathrm{\hspace{0.17em}E}}_a{\mathrm{\propto \; \rho }}_2^f[{\mathcal{a}}_{\perp }\text{/(1−}{\mathcal{a}}_{\perp }\mathrm{)],}$ relating the activation energy E_a to the $\mathrm{X–X}$ bond extension at the termination of energy release, ${\rho }_2^f,$ and the attractive energy release ${\mathcal{a}}_{\perp },$ is derived and is tested for the family of reactions H+X₂. There is only a very small increase in product rotational excitation with decreasing vibrational excitation. It follows that the translational energy of the products is markedly greater for successively lower v′ states.