Transfer hydrogenation between ethane and ethene: a critical assessment of theoretical procedures§

Abstract

The prototypical transfer-hydrogenation reaction between ethane and ethene has been examined with quantum chemistry procedures. Methods used include high-level single-reference procedures such as CCSD(T), BD(T) and CCSDT, high-level multi-reference procedures such as CASPT2, CAS-ACPF and CAS-AQCC, and less computationally demanding density functional theory procedures such as B3-LYP, MPWB1K and BMK. The concerted pathway for this reaction is clearly favoured over the stepwise process. The best prediction of the concerted barrier is 210 kJ mol–1. An anti transition structure for the stepwise pathway lies 70 kJ mol–1 higher in energy. It is found that the concerted transition structure has relatively little biradical character but that incorporation of dynamic correlation is very important for its accurate theoretical description. The stepwise transition structure has considerable biradical character and, among traditional methods, either high-level multi-reference procedures (e.g. CAS-AQCC) or broken-spin-symmetry single-reference procedures (e.g. UBD(T) or UCCSD(T)) are required for reliable results. Broken-spin-symmetry density functional theory methods such as UMPWB1K and UBMK provide a cost-effective alternative for examining both pathways. § In celebration of the 60th birthday of Professor Michael Robb and in recognition of Mike's contributions to science.