Table 1 shows the computed activation energies DE and
reaction enthalpies DH at various levels for all three
substitution reaction channels. The results indicate that the
substitution reactions from CH3FCH to CH3BrCH tend to
lower the energy barrier and favor the exothermicity, leading to
more reactant-like transition structures, in agreement with
Hammond’s postulate [26]. The trend can be explained by the
C–X bond dissociation enthalpies.