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The accurate quantum mechanical description of homogeneous catalysis involving transition-metal complexes is a complicated and computationally demanding task. Hence, in this study the performance of different quantum chemical approaches with respect to the ruthenium catalyzed olefin metathesis of ethylene and RuCl₂ (PH₃)₂ CH₂ as a model system is investigated. All intermediates and transition states that are relevant for the rate determining steps of competing reaction mechanisms (associative and two dissociative pathways) are considered. Results from density functional theory calculations employing B-P86, B97-D, B3-LYP, TPSSh, and B2-PLYP functionals, as well as from MP2 and SCS-MP2 perturbation theory are compared to reference values (relative and reaction energies) obtained at the QCISD(T) level of theory. In particular, the applicability of AO basis sets of increasing size ranging from double-ζ to quadruple-ζ quality is evaluated for representative methods. For some reaction steps, large basis set effects on the order of 10 kcal mol^(-1) TPSSh > B-86 ≈ B97-D > SCS-MP2. The often used standard approaches B3-LYP and MP2 provide overall the largest errors. The accurate QCISD(T) computations predict in conclusion (and in agreement with a recent other study) that for the model system considered, the dissociative trans pathway is favored over the dissociative cis pathway and also over the associative reaction mechanism.