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