Intramolecular isotope effects in thermal unimolecular reaction of $CH_2DCH_2Br$

Abstract

The competitive thermal decomposition reaction of bromoethane-2-$d_1$ was studied in the fall-off region at two temperatures, 697.2 and 741.2 K. Pressure dependence of intramolecular isotope effect in the reaction was observed, and at both temperatures the isotopic rate ratios, $k_H/k_D$, increased with lowering the pressures. The pressure effects on the over-all rate constant were also obtained in the region. Measurements of the isotopic rate ratios were made over the range of temperatures from 641.2 to 781.2 K at 3.60 mmHg of the reactant. The observed activation energy difference, $\triangle{E}_0$, between DBr and HBr elimination reactions for $CH_2DCH_2Br$ was 829$\pm$20 cal/mole, and the ratio of Arrhenius preexponential factors was 0.20. Pressure dependence of the isotope effect in the competitive channel reactions between HBr and DBr eliminations for $CH_2DCH_2Br$ was explained on the basis of the quantum statistical RRKM formulation. This approach leads to conclusion that the normal isotope effect on the micro-scopic rate constant and non-equilibrium isotope effect.