Silber spiegel - SwissEduc

40
3/04
Mechanisms
A good approach to a mechanism problem, for undergraduate
students, is running a Gaussian calculation to obtain results that
can be compared with experimental results. When a discrepancy
appears in output data, the student must be able to criticize that
result by analyzing what was ignored when initial conditions
were input.
Salgado et al.[2, 3] established an experiment designed to help
develop scientifi c research abilities in undergraduates students.
This experiment requires the determination of an hypothesis
for explaining the maleic–fumaric isomerization by means of a
plausible mechanism. It included spectroscopic data.
We now propose to extend
an examination of this mechanism-based
experiment
to include Gaussian 98
calculations. Students examine
three mechanisms and
choose one.
Figure 1:
Figure 1:
Maleic–Fumaric
isomerization.
Students should decide which is the plausible mechanism. Moreover they
should assess the calculation process itself. Solvent effects are not considered in
this theoretical study, which leads to high values in intrinsic activation barriers.
Hence, the theoretical study describes a process carried out in gas phase and
not in solution phase.
Finally, students should choose the correct mechanism by
recognizing the mechanism with more elementary steps. The
second mechanism has more elementary steps and its activation
energies are smaller than the alternatives.
© c+b 3/04