Jump to content

Talk:Mod:Janie192

From ChemWiki

Introduction

Good introduction to the Cope rearrangement exercise.

Cope rearrangement Some conformers successfully optimised ­ not all of them. Is it good to discuss about their relative energy but be careful “Based on these optimsisation calculations, it can be assumed that the most stable structure of 1,5-hexadiene adopts a gauche conformation.” Making a generalisation out of 1 example is risky! Plus, do not forget that you are using low level method HF/3-21G. It is worth checking the relative energies with a higher level method. Transition states successfully optimised with the different methods suggested. Good interpretation of the frequency calculations. Good interpretation of the IRC calculations. Good comment on the theoretical/experimental agreement and good conclusion about the preferred mechanism.

Diels­Alder cycloaddition

It is suprising the HOMO and LUMO are the same symmetry as they should result from the formation of either the anti­symmetric pair of fragment orbitals, giving rise to a fragment MO (or the same for the symmetric pair). In fact the forming C­C bonds at the TS are far too short indicating you have not found the right TS but a symmetry breaking TS at a bifurification point after the TS defining the reaction energy has already been passed. The imaginary frequency shows this is the case. You would expect the bond forming distance to be between the equalibrium C­C bond length (product) and 2x the vdW distance (reactant) where the two electron clouds start to interact. The lowest real frequency is symmetry breaking, suggesting that this motion is uphill in energy (and would not occur). Can you label the secondary orbital overlap interactions in the computed MO diagrams? Extra discussion on this: <http://pubs.acs.org/doi/abs/10.1021/ar0000152>

Overall conclusion?