Radical Addition Reactions: Hierarchical Ab Initio Benchmark and DFT Performance Study

Abstract

We performed a hierarchical ab initio benchmark study of the gas-phase radical addition reactions of X⋅+C2H2 and X⋅+C2H4 (X⋅ = CH3⋅, NH2⋅, OH⋅, SH⋅). The hierarchical series of ab initio methods (HF, MP2, CCSD, CCSD(T)) were paired with a hierarchal series of Dunning basis sets with and without diffuse functions ((aug)–cc–pVDZ, (aug)–cc–pVTZ, (aug)–cc–pVQZ). The HF ground-state wavefunctions were transformed into quasi-restricted orbital (QRO) reference wavefunctions to address spin contamination. Following extrapolation to the CBS limit, the energies from our highest- QRO–CCSD(T)/CBS+ level converged within 0.0–3.4 kcal mol−1 and 0.0–1.0 kcal mol−1 concerning the ab initio method and basis set, respectively. Our QRO–CCSD(T)/CBS+ reference data was used to evaluate the performance of 98 density functional theory (DFT) approximations. The MAE of the best functionals for reaction barriers and energies were: OLYP (1.9 kcal mol−1), BMK (1.0 kcal mol−1), M06–2X (0.9 kcal mol−1), MN12–SX (0.8 kcal mol−1) and CAM−B3LYP (0.8 kcal mol−1). These functionals also accurately reproduce key geometrical parameters of the stationary points within an average 2 % deviation from the reference QRO–CCSD(T)/cc–pVTZ level.