Talk:Mod:ht309mod1

Q1

• Correct energies for 1, 2, 3 and 4.

• kcal.mol-1 and kJ.mol-1

• Diels-Alder cycloadditions are not all under kinetic control. If you see the conditions being >200 C, it’s probably thermodynamic controlled (reversible).

• You could have explained the origin of the difference in energy between 1 and 2: Torsion, and the part of the structure responsible for this.

• Correct ananlysis of bending energy and bon angles.

• If you want to compare 4 and 2, you MUST balance the equation with energy of H2. If the equation isn’t balanced, they can’t be compared.

Q2

• Correct optimised structures and energies for both MM2 and MMFF94.

• For comparison between different techniques, use the relative difference in energy between different isomers/conformers.

• I think you understand what an ‘hyperstable alkene’ is, although more elaboration would be appreciated.

• Convincing display of energy and strain in the hydrogenated product.

Q3

• Correct MOs. Although you should have specified how you arrived at the optimised structure, before the MO calculation.

• Decent analysis of MOs and what they mean. You could have backed your analaysis up with the two different C=C bond lengths, which are indicative of the strength of these bonds (also related to the IR calculation later).

• Correct vibrational frequencies. Good way of displaying them.

• However, you miss the change in C-Cl frequency, which was the results of not having the pi bond to donate into its sigma* orbital.

Q4

• You need to explain why you chose Me group. Which is the better method.

• Which method were the Jmol structures from?

• Your optimised energies are close, but not quite yet the lowest energies. Consider rotating the CH2OH group.

• From which method were the angles and distance you showed come from?

• You don’t seem to spot the huge difference between MM2 and PM6 results.

Mini project

• Did you correct your 13C NMR shift calculated by Gaussian as instructed?

• I think a common mistake for all students here is to try to compare all chemical shifts. For NMR calculation, there’s always an error margin. What you should have focus on is one or two carbon, which literature values are very different between A and B, and see if calculation can reproduce it. C15 is perfect here and is unlikely to be affected by changes in conformation. Gratifyingly the calculation holds up against scrutiny.

• H-H coupling would be rather uninformative in this case, as most of the structure is flat. For IR, only the N-N stretch should really be different between the two structure, but I doubt calculation would be accurate enough.