Deuterium Kinetic Isotope Effect in the Hydrogen—Iodine Reaction

Abstract

Rates of reaction of deuterium with iodine were measured in the range 633°—800°K. The elementary reactions are: $${\mathrm{I}}_2\text{=2}\mathrm{I}$$ $${\mathrm{D}}_2+{\mathrm{I}}_2\text{→2}\mathrm{DI}$$ $$2\mathrm{DI}\to {\mathrm{D}}_2+{\mathrm{I}}_2$$ $$\mathrm{I}+{\mathrm{D}}_2\to \mathrm{DI}+\mathrm{D}$$ $$\mathrm{D}+\mathrm{DI}\to {\mathrm{D}}_2+\mathrm{I}$$ $$\mathrm{D}+{\mathrm{I}}_2\to \mathrm{DI}+\mathrm{I}$$ $$\mathrm{I}+\mathrm{DI}\to {\mathrm{I}}_2+\mathrm{D}.$$ The three independent constants, k₁, k₃, k₄/k₅, in (mole/cc)^—1sec^—1, are given by log(k₁/T^½) = 12.50— 40 790/4.575T, log(k₃/T^½) = 12.41–33 770/4.575T, and k₄/k₅ = 0.073. From a comparison of these with previously obtained rates in the H₂ system, the kinetic isotope effects for Reactions [1] and [3] are: $$\log {(}_{\mathrm{H}}{k}_1{/}_{\mathrm{D}}{k}_1\text{)=0.225+150/4.575T,\hspace{0.17em}\hspace{0.17em}}\log {(}_{\mathrm{H}}{k}_3{/}_{\mathrm{D}}{k}_3\text{)=0.124+1020/4.575T.}$$ The results for [3] were fitted to absolute rate theory including tunneling (unsymmetrical Eckart barriers) to obtain permissible ranges of tunneling factors and vibrational frequencies of the linear IH₂ complex. Internuclear distances in triangular IH₂ complexes were found to be only slightly greater than r_e(HI) = 1.60 Å and r_e(H₂) = 0.74 Å. Sato energy surfaces were calculated for a number of Sato parameters; tunneling for these surfaces was found to be negligible when computed from Eckart potentials fitted to the Sato surfaces. Agreement of theory with experiment could not be obtained for both the pre‐exponential factor and the activation energy of I+H₂ for any value of the Sato parameter.