Third Year Chemistry Computational Module 3: Transition States and Reactivity

Ferdinand Fai Yang LOO (00641123)

Part I: The Cope Rearrangement

In this exercise the Cope rearrangement of 1,5-hexadiene is investigated using computational chemistry. The outline reaction scheme is as follows:

As shown there are two possible transition states - the chair and boat conformations that are possible for this reaction, and in this exercise the molecules are analysed and rationalised for the actual transition state adopted for this reaction.

Optimising anti 1,5-hexadiene

Setup

A molecule of 1,5-hexadiene with an anti-linkage for the central four C-atoms are setup in GaussView and cleaned.

Job type: OPT
Method: Hartree-Fock
Basis set: 3-21G
Memory limit: 250MB

Results

The .log file can be found here.

The summary table of the optimised molecule is shown below:

The energy of the optimised molecule was reported to be -234.06807191 a.u..

The Items table is included for reference:

Item               Value     Threshold  Converged?
 Maximum Force            0.000020     0.000450     YES
 RMS     Force            0.000006     0.000300     YES
 Maximum Displacement     0.001743     0.001800     YES
 RMS     Displacement     0.000518     0.001200     YES
 Predicted change in Energy=-3.395201D-08
 Optimization completed.
    -- Stationary point found.

All values are converged and having a very small gradient of 0.00000902 a.u. the optimisation was successfully carried out.

Does your final structure have symmetry? Select Symmetrize under the Edit menu (note that sometimes it is necessary to relax the search criteria under the Point Group menu). Make a note of the point group.

Optimising gauche 1,5-hexadiene

Setup

Another molecule of 1,5-hexadiene with a Gauche-linkage for the central four C-atoms are setup in GaussView and cleaned. Optimisation using exactly the same method was used:

Job type: OPT
Method: Hartree-Fock
Basis set: 3-21G
Memory limit: 250MB

Results

The .log file can be found here.

The summary of the optimisation is also included for reference:

The Items table is shown:

Item               Value     Threshold  Converged?
 Maximum Force            0.000033     0.000450     YES
 RMS     Force            0.000005     0.000300     YES
 Maximum Displacement     0.000718     0.001800     YES
 RMS     Displacement     0.000195     0.001200     YES
 Predicted change in Energy=-5.552298D-09
 Optimization completed.
    -- Stationary point found.

The energy of the molecule was computed to be -234.06351571 a.u..

With all converging values and a very small gradient of 0.00001041 a.u. the optimisation was carried out properly.

Energy Comparison of the Two Conformations

Conformation	Energy (a.u.)
anti	-234.06807191
gauche	-234.06351571

The gauche-conformation is 0.0045562 a.u. higher in energy than the anti-conformation.