Rep:Mod:Physicalrg1709
Module 3: Computational (physical) Laboratory
The Cope Rearrangement
1,5-hexadiene can undergo a Cope rearrangement, a form of isomerisation, which is classified as a [3,3]-sigmatropic shift. The concerted pericyclic mechanism for which the Cope rearrangement occurs can either proceed via a chair or boat conformation shape transition state. To determine via which transition state the Cope rearrangement occurs computational modeling will be used, as outlined below.
Optimisation
1,5-hexadiene was constructed in GaussView with the central four carbons having an anti-periplanar geometry (dihedral angle set to 180o). This structure was then optimised using the Hartree-Fock method, in Gaussian, using a basis set of 3-21g where the memory has been set to 250MB.
This produced an optimised structure where the total energy calculated was -231.69254 a.u.(Hartree). The optimisied structure had a point group of Ci (i.e. just one centre of inversion). This corresponds to anti2 in Appendix 1 as it has the same energy and point group.
Another molecule of 1,5-hexadiene was then constructed in GaussView with the central four carbons having a gauche geometry between the four central carbon atoms of the molecule(dihedral angle set to 180o). This gauche structure is expected to have a higher energy than the anti-periplanar structure as C-C bonds are closer to each other (with some overlap) in the gauche conformations and so exert larger repulsive forces (due to repulsion of overlapping electron density and also steric repulsion of the substituent groups present). This structure was then optimised using the Hartree-Fock method, in Gaussian, using a basis set of 3-21g where the memory has been set to 250MB.
This produced an optimised structure where the total energy calculated was also -231.68772 a.u.. The optimisied structure had a point group of C2 (i.e. a 180o rotational axis). This Corresponds to gauche1 in Appendix 1 as it has the same energy and point group.
- Further Optimisation of anti2
Anti2 was further optimised using the DFT/B3LYP method with a basis set of 6-31G*. This further optimisation created an anti2 molecule where the energy was -234.55970 a.u. which is lower (more stable) than that calculated using the previous method. The overall geometry of this further optimised structure is the same as before, Ci. However the C=C bond length has increased from 1.316Â to 1.338Â and also the C-H bonds have also increased by about 0.01Â.