Hydrogen-Deuterium Kinetic Isotope Effect at Very High Temperature

The kinetic isotope effect for the reaction CF3+CH2D2→CF3H+CHD2 or CF3D+CH2D has been studied in a shock tube between 1000° and 2000°K where CF3 radicals were produced by thermal decomposition of CF3N2CF3. In the reaction zone gas compositions were: noble gases≫CH2D2≫CF3N2CF3; and in this way ratio of final products CF3H/CF3D was equal to the ratio of rate constants kH/kD. The data are represented empirically by lnkH/kD=ln1.400+0.42×106/T2. A London-Polanyi-Eyring-Sato potential energy surface was constructed, where Sato's parameter was fitted to give the correct activation energy, and the frequencies and zero-point-energy of the activated complex were then found from curvatures through the saddle-point of the potential energy surface. At very high temperatures activated complex theory predicts lnkH/kD=ln1.402+0.502×106/T2 —0.069×1012/T4, in fair agreement with experiment.