Semiempirical Study of the H2Cl Transition Complex through the Use of Hydrogen Isotope Effects

Abstract

Linear and triangular structures for the H₂Cl transition complex have been analyzed in terms of normal vibration theory and by an evaluation of the experimental data on the effect of hydrogen isotope substitution on the rate of reaction of hydrogen molecules and chlorine atoms. The frequencies of the bending vibrations are probably sufficiently small that their contribution to the relative rates can be expressed in terms of a small quantum correction of the order of (hc/kT)²(ω_H²—ω²_D,T/24). The near classical behavior of the bending frequencies for both linear and triangular structures of the H₂Cl complex serves to reduce the ratio of the frequency factors, A_H/A_D,T, in the Arrhenius equation from that expected from structural considerations alone. Quantitative agreement is found between the calculated and experimentally determined frequency factor ratios for the triangular complex. The ``symmetrical'' stretching frequencies, for both linear and triangular complexes respectively, have been evaluated from the relative rates of reaction of H₂ and HT with chlorine atoms at 0°C and the known properties of the isotopic hydrogen molecules. This empirically evaluated parameter, together with thermodynamic data, and an estimate of the tunnel correction, suffice to calculate the relative rates of reaction of the isotopic hydrogen molecules with chlorine atoms as a function of temperature. Good agreement is found between the calculated rates for both linear and triangular structures and the available experimental data on HD, D₂, and HT.