A plausible reaction that could compete with hydride abstraction is the heterolytic C-H bond activation across a Zr-O sites in this SZO model, analogous to that shown in Figure . This reaction is energetically unreasonable. Heterolytic C-H bond activation of a -CH2-in the tetramer is predicted to have a very high barrier (DE # = 55.2 kcal mol -1 ) to form an unstable Zr-R species (DErxn = 36.8 kcal mol -1 ) on this flat ZrO2 model. These values are significantly higher than those obtained in C-H bond activation reactions with amorphous ZrO2 models where this mechanism has more favorable energetics. Once the carbocation is formed b-scission cleaves the polymer chain to give an olefin and a secondary carbocation. In the presence of H2, hydrogenation of the olefin forms alkanes. ZrO2 materials are known to heterolytically activate H2 to form Zr-H sites, and to catalyze olefin hydrogenation reactions. Reactions performed in the presence of D2 incorporate deuterium in the -CH2and -CH3 positions of the oils (Figure ), suggesting the sequence of reactions shown in Scheme 2. After chain cleavage from the 3 o carbocation, the secondary carbocation transfers a proton to the vinylidene fragment to form a terminal olefin and a new 3 o carbocation. Addition of D2 to the terminal olefin gives the observed selectivity. Isomerization of the terminal olefin in the absence of H2 forms the internal olefins that are observed under those conditions.