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 H2 + Cl → [H–H–Cl]‡ → H+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 HD+Cl, are shown to be especially suited for selecting compatible transition-state models.