Matrix effect on hydrogen atom tunneling: Comparison between hydrogen addition and abstraction

Hydrogen-atom tunneling from and to linear alkenes in organic matrices at 7 7 K has been studied for elucidating the effect of matrices on the rate of the tunneling. The rate of hydrogen-atom addition to the vinyl carbon of al kenes does not depend on the length of allyl chains bonded to the vinyl car bon, whereas the rate of hydrogen; atom abstraction by methyl radicals from the allylic carbon decreases with increasing length of alkyl chains bonded to the allylic carbon. These effects are explained as due to the resistanc e of matrices to the change of the C-C-C bond angle of the reacting carbon during the tunneling. For hydrogen-atom abstraction, the resistance causes the increase of the height of the potential barrier between the entrance an d the exit of the tunneling channel, whereas the resistance does not cause the retardation of the tunneling to the vinyl carbon since the bond angle s carcely changes during the tunneling.