Kinetic Isotope Effects for Hydrogen Abstraction from a Series of Cycloalkanes and Branched Alkanes by Hydrogen Atoms in the Gaseous Phase

Article abstract of DOI:10.1021/ja00292a024

Hydrogen atoms produced in the radiolysis of water vapor were used to determine the kinetic isotope effects for the reactions H(.) + RH(RD) -> H2(RD) + R(.) H(K_D)>, where RH is a perprotiated alkane and RD is the corresponding perdeuterated alkane.The alkanes studied include a homologous series of cycloalkanes, cyclopentane through cyclododecane, and isobutane, 2,3-dimethylbutane, 2,3,4-trimethylpentane, and neopentane.The results were expressed in terms of the Arrhenius-type equation k_H/k_D = A_H/A_D exp<(E_D-E_H)(kJ mol^-1)/RT>, over the temperature range of 363-463 K.The values for the ratio A_H/A_D range from 0.32 to 0.75, and the activation energy differences E_D-E_H vary from 6.8 to 11.0 kJ/mol, depending on the molecular structures of the reactants.The variation in the values of E_D-E_H was correlated with the bond dissociation energies of the C-H bond being broken.Theoretical calculations based on transition-state theory combined with the London-Eyring-Polanyi-Sato potetial energy surfaces could reproduce the major features of the experimental results when tunnel effects were taken into consideration.