Hierarchy of relative bond dissociation enthalpies and their use to efficiently compute accurate absolute bond dissociation enthalpies for C-H, C-C, and C-F bonds.

We have used the high-level W1X-2 and G4(MP2)-6X procedures to examine the performance of a variety of computationally less demanding quantum chemistry methods for the calculation of absolute bond dissociation enthalpies (BDEs) and a hierarchy of relative bond dissociation enthalpies. These include relative bond dissociation enthalpies (RBDEs), deviations from additivity of RBDEs (DARBDEs), and deviations from pairwise additivity of RBDEs (DPARBDEs). The absolute magnitudes of these quantities decrease in the order BDE > RBDE > DARBDE > DPARBDE, and overall, theoretical procedures are better able to describe these quantities in the same order. In general, the performance of the various types of procedures improves in the order pure DFT → hybrid DFT → double-hybrid DFT → composite procedures, as expected. Overall, we find M06-L to be the best-performing pure DFT procedure and M06-2X to be the best among the hybrid DFT methods. A promising observation is that even many pure and hybrid DFT procedures give DARBDE and DPARBDE values that are reasonably accurate. This can be exploited by using reference BDEs calculated at a higher-level of theory, in combination with DARBDE or DPARBDE values obtained at a lower level, to produce BDEs and RBDEs with an accuracy that is close to the directly calculated higher-level values. Strongly π-electron-withdrawing or π-electron-donating groups, however, sometimes represent challenges to these approximation methods when the substrate contains several of these substituents.