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Mechanisms
seen in Table 1. This mechanism has two elementary steps.
Figure 5 shows all computed structures, including the only
transition state. A qualitative profi le is presented in Figure 8.
Figure 4:
Third mechanism
proposed.
Mechanism 2:
As Figure 3 shows, the proposed mechanism has four intermediates
and four transition states linked by fi ve elementary
steps. Unlike mechanism 1, mechanism 2 has two more elementary
steps than mechanism 1. Furthermore, as Table 2 shows,
mechanism 2 has intrinsic activation barriers, net activation
barriers, and well depths, that are lower than those belonging
to the fi rst mechanism. This suggests that mechanism 2 is more
sensible than mechanism 1. It is interesting to note that the
forward reaction energy is the same for mechanism 1: 1.4 kcal
mol -1 Fig.4
. Furthermore, it is interesting to note that loss of HCl
from I is concerted. This change employs six electrons and so
3
the transition state is aromatic. Figure 6 shows all structures
together with the four transition states. A qualitative profi le is
given in Figure 9.
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