Kinetic Isotope Effects for Hydrogen Abstraction from Various Saturated Hydrocarbons by Deuterium Atoms in the Gas Phase

Abstract

AbstractUsing deuterium atoms produced in the radiolysis of heavy water vapor, the kinetic isotope effects kH/kD were determined by the isotope competitive method for the abstraction reaction D* + RH(R′D)→HD(D2) + R′(R′), where RH is an alkane and R′D is the corresponding perdeuterated alkane. The alkanes RH investigated include isobutane, 2,3,4‐trimethylpentane, neopentane, cyclopentane, cyclohexane, cycloheptane, and n‐heptane. The results were represented in terms of the Arrhenius‐type expression kH/kD = AH/ADexp((ED‐EH) (kJ mol−1)/RT), over the temperature range 363–473 K. The pre‐exponential factor ratios AH/AD vary from 0.36 for neopentane to 0.70 for 2,3,4‐trimethylpentane, the ED—EH values from 6.4 for 2,3,4‐trimethylpentane to 10.4 kJ mol−1 for neopentane. The variation in the ED—EH values was correlated with the bond strength of the C‐H bond being broken. The deuterium and hydrogen atoms, as the attacking species, are found to show essentially the same magnitude of the kinetic isotope effects in the hydrogen abstraction from a given alkane. The theoretical calculations of the isotope effects have been carried out on the basis of transition‐state theory combined with the London‐Eyring‐Polanyi‐Sato potential energy surface. Good agreement was obtained between experiment and theory, when tunnel effects were taken into consideration.