45 For some ambiguous TSs, intrinsic reaction coordinate (IRC) calculations have been performed at the same level. Moreover, to increase the accuracy of the energy of some essential transition states on the PES of 1,3-dioxane, additional optimizations at the M06-2X/aug-cc-pVTZ level 46 starting from the B3LYP/6- 311+G(d, p) results have been performed. While uncertainties in barrier heights evaluated for the DBH24 database amount to more than 4 kcal mol -1 for B3LYP, the M06-2X/augcc-pVTZ method yields uncertainties below 1 kcal mol -1 . 47 This is the second-best method scaling at fourth order in the benchmark by Zheng et al. and therefore well-suited to eciently provide reliable barrier heights in this study.