Force field and tunneling effects in the H–H–Cl reaction system. Determination from kinetic-isotope-effect measurements

An iterative method is used to obtain, within the framework of the transition state theory, sets of force constants from experimental kinetic‐isotope‐effect data for the reaction system ${\mathrm{H}}_2+\mathrm{Cl}\mathrm{\to \; [}\mathrm{H–H–Cl}{]}^{‡}\to \mathrm{H}+\mathrm{HCl}$ . Two sets of force constants, describing quite different transition‐state structures, are found to be equally compatible with all available rate data for the reactions of chlorine atoms with H₂, HD, HT, DT, D₂, and T₂. These force fields are compared with force fields derived by semiempirical methods. Measurements of the intramolecular kinetic isotope effect, i.e., the relative rate of production of HCl/DCl in the reaction $\mathrm{HD}+\mathrm{Cl}$ , are shown to be especially suited for selecting compatible transition‐state models.