The transition states for the substitution reactions exhibit
simple structures of C3v symmetry. H atom attacks the C atom of
CH3X, kicking off theXatomand forming theCH4molecule. For
the HCCH3F reaction, the forming C–H bond is 1.243 A ° , which
is 0.153 A ° longer than that in the CH4 product. The breaking C–F
bond is stretched to 1.691 A ° . The energy barrier for the reaction is
rather high, 129.5 kJ/mol.Moreover, the formation of F and CH4
is endothermic by 27.6 kJ/mol. Therefore, it could be anticipated
that this substitution channel should play a negligible role in the
reaction.TheHabstraction channel is expected to dominate under
all conditions because the barrier to the H abstraction channel is
significantly lower than the F abstraction and substitution
channels. For the HCCH3Cl reaction, the breaking C–Cl bond
is elongated by 0.235 A ° . The forming C–H bond is 1.469 A ° ,
which is 0.379 A ° longer than that of the CH4. In view of these
structural characteristics, TSb3 is more reactant-like and thus an
early barrier, as can be anticipated from the reaction exothermicity
of 88.4 kJ/mol. The corresponding barrier height is
79.4 kJ/mol, which is 35.5 and 30.1 kJ/mol higher than the Cl
abstraction and H abstraction channels, respectively. It is noted
that the product CH4CCl is most exothermic channel in the
reaction. The results show that in the CH3ClCH reaction, the
abstraction channels still dominate but the substitution channel
should contribute at high temperature. Similarly, the breaking C–
Br bond is elongated by 0.207 A ° in the CH3BrCH reaction. The
forming C–H bond is relatively long, 1.536 A ° , which is 0.446 A °
longer than that in the CH4. Evidently, TSc3 is a rather early
barrier, in accordance with the very high exothermicity of
134.0 kJ/mol. The barrier height is calculated to be 59.3 kJ/mol.
Note that the energy of TSc3 is 8.2 and 34.6 kJ/mol higher than