Kinetic Isotope Effects in the Reaction between Atomic Chlorine and Molecular Hydrogen. Tunnel Coefficients of the Hydrogen Atom through an Asymmetric Potential Barrier

Abstract

Kinetic isotope effects for the reactions between atomic chlorine and molecular hydrogen have been measured in the range of −30° to +70°C. The following expressions were obtained: $$\begin{array}{c}{R}_{{\mathrm{H}}_2/\mathrm{HT}}\text{=(1.27±0.03)\hspace{0.17em}}\exp \text{[(797±14)/RT],}\\ R_{{\mathrm{H}}_2/{\mathrm{D}}_2}\text{=(1.44±0.06)}\exp \text{[(1128±17)/RT],}\\ R_{{\mathrm{H}}_2/\mathrm{DT}}{\text{=1.53}}_4\exp \text{(1422/RT),}\\ R_{{\mathrm{H}}_2/{\mathrm{T}}_2}{\text{=1.54}}_5\exp \text{(1693/RT).}\end{array}$$ The isotope effect of HD, $$R_{{\mathrm{H}}_2/\mathrm{HD}}\text{=(1.24±0.03)}\exp \text{[(490±6)/RT],}$$ has been redetermined and found to agree with previous measurements. Theoretical calculations of these isotope effects, using (1) a Sato model, (2) a generalized Sato model, and (3) the Johnston—Parr method, were made to compare the calculated effects with experimental results. Tunnel corrections were applied using (1) an asymmetric Eckart barrier, or (2) the Johnston—Rapp method with an asymmetric barrier. Best agreement (within 15%) of calculated values with experiment was obtained for a generalized Sato model including Johnston—Rapp tunnel corrections. Empirical sets of four force constants describing the transition state H–H–Cl are also given. These are used to calculate isotope effects which are in excellent agreement with experimental values.