Comparison of ab initio and DFT methods for studying chain propagation and chain termination processes with group 4 polymerization catalysts. 1. The ansa-bis(cyclopentadienyl)zirconium catalyst

In this paper we present a systematic comparison of the performance of different computational approaches to study the propagation and termination reactions of olefins with a prototype homogeneous group 4 ansa-metallocene catalyst. Chain propagation, β-H transfer to the monomer, and β-H elimination to the metal have been investigated for the H₂Si-(Cp)₂ZrR^- (R = ethyl, n-butyl) + C₂H₄ system using ab initio and density functional theory (DFT) techniques. For all the species investigated, all the computational approaches we considered result in substantially similar geometries. A comparison of the DFT and Møller-Plesset theory (MP2) propagation and termination barriers with extrapolated coupled-cluster calculations with inclusion of single, double, and perturbatively connected triple excitation (CCSD(T)) values indicates that all the pure functionals considered underestimate the difference between termination and propagation by roughly 3-4 kcal/mol. In contrast, hybrid functionals are within 1 kcal/mol from extrapolated CCSD(T) values. For a comparison with experimental results inclusion of zero-point energy contributions and the use of an alkyl group longer than ethyl to simulate the growing chain in both termination reactions are mandatory.