Benchmark Ab initio calculations of the barrier height and transition-state geometry for hydrogen abstraction from a phenolic antioxidant by a peroxy radical and its use to assess the performance of density functionals

This Letter presents benchmark results for the transition-state geometry and classical barrier height for the hydrogen atom abstraction from phenol by an organic peroxyl radical. We use multireference Møller-Plesset perturbation theory (MRMP2) based on a complete active space self-consistent field (CASSCF) wave function with a previously defined well-balanced nom-CPO+π active space and a triple-ζ one-electron basis set for the benchmark calculations, including full geometry optimization of the saddle point at the MRMP2 level. The classical barrier height for the abstraction reaction by the methylperoxyl radical is found to be 7.4 kcal/mol. A variety of density functionals are tested for their ability to reproduce the benchmark calculations for this reaction to provide guidance for selecting a reliable density functional in future calculations of larger systems involving phenolic antioxidants. The best-performing density functional is M05, and other functionals with above-average performance (for both transition-state geometry and transition-state barrier height) are B1LYP, B3LYP, B98, B97-1, and M06.