Discrimination of σ‐Bond Metathesis Pathways in H/D Exchange Reactions on [(SiO) 3 ZrH]: A Density Functional Theory Study

The mechanism of the H/D exchange reaction in alkane/hydrogen mixtures on silica‐supported zirconium hydride was investigated by a modelling study using density functional theory (DFT) calculations. The electronic activation enthalpy (Δ H ${^{\ne }_{\rm elec}}$ ) for the CH bond activation step (TS 3 ) was calculated to be around 92 kJ mol −1 , whereas it would be 258 kJ mol −1 for a direct exchange process (TS 1 , also called the kite TS). These data clearly speak in favour of the former as a mechanism for CH bond scrambling. Moreover, the calculated enthalpy of activation (Δ H ${^{\ne }_{{\rm elec}}}$ ) for H/D exchange in H 2 /D 2 mixtures (TS 2 ) is 33.5 kJ mol −1 , which shows that this reaction is much faster than the H/D scrambling in alkane/H 2 mixtures, as shown experimentally. Additionally, the calculated activation entropies (For TS 1–4, Δ S ${^{\ne }_{{\rm tot}}}$ ranges between −129 and −174 J mol −1  K −1 ) are very negative. Although the calculated activation entropies are also in full agreement with experimental data (Δ S ${^{\ne }_{{\rm exp}}}$ =−113 J mol −1  K −1 ), overall, the calculated activation enthalpies are much higher than the experimental ones. This suggests that the actual catalyst is probably more electrophilic than the model chosen for the calculations .