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© c+b 3/04
Mechanisms
4. Results and Discussions
In connection with earlier experiments, students should have
analyzed experimental evidence to decide which is the plausible
mechanistic alternative (see Figures 2, 3 and 4). Although RHF
computations do not give exactly the same structures as those
given by traditional physical evidence, the criterion for selecting
the more superior mechanism takes into account the number
of elementary steps and the activation barrier heights for each
proposed mechanism.
In order to ensure, that the computational structures are intermediates,
reactants, products or transition states, the student
should carry out a frequency calculation on each structure. If
all frequencies are positive then the structure is a reactant, product
or intermediate, depending on the input data. But if one
and only one frequency is negative and the remaining ones are
positive, the structure corresponds to a transition state.
When looking for a transition state by the algorithm mentioned
in section 3 (or another algorithm), a frequency calculation
is mandatory.
Finally, once all structures have been found
and their energy values have been converted
to kcal mol -1 , the use of an energy v/s reaction
coordinate qualitative profi le will allow
one to visualize the energetic aspects of each
mechanism. That profile can be obtained
by putting the energy points in an arbitrary,
but sensible way into the qualitative profi les
depicted later.
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