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.