Deuterium Isotope Effect in the Reaction of Hydrogen Molecules with Chlorine Atoms and the Potential Energy of the H2Cl Transition Complex

Abstract

The relative rates of reaction of H₂ and HD with chlorine atoms have been measured over the temperature range of 243–350°K. In this temperature interval, the ratio of the second‐order rate constants, $${\mathrm{R=k}}_{{\mathrm{H}}_2}{\mathrm{/\Sigma k}}_{\mathrm{HD}},$$ is equal to (1.24∓0.03) exp (490±6/RT). A search was made for HD after an unequilibrated mixture of H₂ and D₂ was half converted to hydrogen chloride by photochemical reaction with Cl₂. From the failure to detect 0.02% HD in the unreacted hydrogen, a lower limit is set for the ratio of the rate constants of the reactions $$\mathrm{H}+{\mathrm{Cl}}_2\to {\displaystyle {lim}_{\text{k16}}}\mathrm{HCl}+\mathrm{Cl\; and\; H}+\mathrm{HCl}\to {\displaystyle {lim}_{\text{k17}}}{\mathrm{H}}_2+\mathrm{Cl}.$$ It is shown that the pre‐exponential factors in R for the HD experiments and in the analogous experiments on HT are in quantitative agreement with theoretical calculations for either linear or triangular transition states, subject to the sole restriction that k(HD+Cl→HCl+D) is approximately equal to k(HD+Cl→DCl+H). An intercomparison is made between the experimental difference in activation energies between H₂, HD, HT, and D₂ for reaction with chlorine atoms and theoretical calculations for linear and triangular transition states. With the most favorable potential function for H₂Cl, the maximum deviation between ΔE_exp‡ and ΔE_theor‡ is fifteen percent. Inasmuch as there are more experimental data than parameters necessary to construct the potential function of H₂Cl, the agreement between theory and experiment constitutes a positive test of the transition state formulation of the effect of isotopic substitution on the rates of chemical reactions.