Isotope effect on hydrogen atom abstraction reaction by recoil tritium atoms in xenon-ethane-perdeuteroethane mixtures at 77 K

Abstract

Recoil T atom reactions have been studied in Xe-C/sub 2/H/sub 6/-C/sub 2/D/sub 6/ mixtures as 77 K. The ratio of the HT yield to the DT yield at 0.2 mol % ethane (C/sub 2/H/sub 6/+C/sub 2/D/sub 6/) is much higher than the C/sub 2/H/sub 6//C/sub 2/D/sub 6/ ratio. The H/D abstraction isotope effect for the T atom reaction with C/sub 2/H/sub 6/ and C/sub 2/D/sub 6/ is 3.5-6 at .02 mol % ethane, while the effect decreases with an increase of ethane concentration down to 1.4 at a concentration higher than 10 mol %. The remarkable isotope effect at the low concentration of ethane is much higher than the isotope effect (1.3) for the hot T atom reaction in the gas phase and is ascribed to a quantum-mechanical tunneling reaction by thermalized T atoms with ethane. The rate constant for the tunneling abstraction is calculated by using the unsymmetrical Eckart potential function and the effective mass in the linear triatomic system. It is concluded that T atoms can react with ethane by quantum-mechanical tunneling as easily as H atoms do.