Barrier permeabilities for a symmetric Eckart potential as studied by the kinetic isotope effects for hydrogen/deuterium abstraction from neopentane by hydrogen atoms in the gas phase

Abstract

Kinetic isotope effects k_H/k_D have been determined in the gas phase over the temperature range 363–473 K for the abstraction reactions [graphic omitted] where RH is an alkane and R′D is the corresponding perdeuterated alkane. The alkanes studied include some n-alkanes, cycloalkanes and branched-chain alkanes. The ratio of the Arrhenius pre-exponential factors A_H/A_D varies from 0.32 to 0.70 and the difference in Arrhenius activation energy E_D–E_H ranges from 7.1 to 11.0 kJ mol,^–1 depending on the strength of the C—H bond being broken. The small A factor ratios, coupled with the large activation energy differences, are taken to indicate the importance of the tunnelling effect in these reactions. A theoretical interpretation of the kinetic isotope effects (KIEs) was made for abstraction of a hydrogen atom from neopentane within the framework of transition-state theory using a London–Eyring–Polanyi–Sato (LEPS) potential-energy surface. Approximating the LEPS profile along the reaction path by a symmetric Eckart barrier, some characteristics of tunnelling phenomena are discussed in great detail, although in an elementary way. Good agreement between experiment and theory was obtained when the barrier permeabilities were taken into consideration.