A DYNAMICAL DENSITY-FUNCTIONAL STUDY ON THE REACTION OF ETHYLENE WITHCP(2)ZR(C2H5)(+)

Possible pathways for the reaction of ethylene with Cp(2)Zr(C2H5)(+) a re studied with the Car-Parrinello Projector Augmented Wave (CP-PAW) t echnique, which is based on density functional theory (DFT). ''Slow gr owth'' constrained first-principles molecular dynamics at 300 K were u sed to sample the phase space in the vicinity of the saddle points of the reactions. The simulations considered the growing chain in a resti ng state between insertions where a hydrogen on the beta-carbon is att ached to the metal. We have investigated three mechanisms, namely fron tside (FS) insertion with ethylene approaching toward the beta-agostic Zr-H bond, backside (BS) insertion with ethylene approaching toward t he Zr-C-alpha bond, and hydrogen transfer (HT) chain termination with transfer of the beta-agostic hydride to the incoming ethylene. Which p athway is followed is determined by the out-of-plane rotation of the e thyl group, which in turn is determined by the agostic interactions of the ethyl group. Agostic interactions are found to shift rapidly, wit h typical lifetimes of 0.5 ps for the FS insertion and 2 ps for the FS insertion. The activation barriers increase in the order front-side i nsertion < back-side insertion < hydrogen transfer. The shape of the p otential surface obtained from previous static calculations was confir med. The ethylene molecule is mobile enough to partake in fast exchang e equilibria with surrounding substrate in solution.