Talk:Mod:HAS1501

FEEDBACK

Q1: Your calculated values are all good, although there is no need to include so many significant figures: There is some error in the calculation which is after all an approximation and you have probably seen using ChemBio3D that the latter decimal figures won’t be the same if you do the same calculation twice from scratch. The major difference between compounds 3 and 4 is correctly identified as bending strain – for a more complete answer, the structures could be compared to find the origin of this difference, namely the difference bond angles in the double bonds present. Your comment on the hydrogenation reaction is right – under thermodynamic control you would get 4 and in order to get compound 3 the reaction would have to be under kinetic control.

Q2. Your calculated structures are not the lowest energy forms although isomer B is indeed the lowest energy one. Both of your compounds, in fact, have the 6-ring in the twist-boat form. In the lowest energy form the 6-rings should be in the chair conformation. It would have been worth detailing briefly the measures you took to minimise the structures, an analysis of the different possibilities for the 6-ring would have lead you to the right answer. The discussion of the strain contributions is good. Hyperstable alkenes are well defined, but it is not possible to carry out that comparison of the hydrogenated “parent” compounds using molecular mechanics, because the compounds are not isomeric. It would be possible using the absolute scale of a DFT calculation.

Q3. Your MOs look good, as do the calculated IR spectra. The energies obtained for the main compound with the different calculation methods are not reported – in the future modules make sure you report these different energy values if appropriate, as you may be assessed on the numbers. You have identified the possibility of a pi-sigma* interaction, but did not discuss the impact this has on your various IR stretches. The anti pi bond has a lower energy stretch than the syn pi bond and the C-Cl stretch is stronger when the anti pi bond is removed (because the donation into its antibonding orbital is no longer present and hence the bond is stronger).

Q4. I think you might have slightly misunderstood the object of this question. You were supposed to calculate energies for A and B as well as conformational isomers A’ and B’ which have the nucleophilic acetyl group twisted so that its orientation with respect to the oxonium group is the opposite. It doesn’t make sense to compare the reactivity of A with that of B to determine a reaction outcome because they are different compounds giving two different products (C and D) and the pathways can never interchange because A cannot convert into B (at least in the scope of this question). If there is any similar confusion in the future modules please ask a demonstrator. In general, the choice of methyl for the R group is the best one to go for and the semi-empirical method is the best to use. You can see from your energy values that A=C and B=D for the PM6 calculations – this is because MOPAC treats the structure as a non-classical carbocation, incorporating the neighbouring group effect into a hybrid structure.

MP. This is a very thorough mini-project and shows how NMR prediction can be very accurate for conformational restricted compounds. You have given all of your calculated and literature data and I can see on analysing it that there is a good correlation, however it could have been presented in a more explicit way. Even stating the differences in ppm in the table would be good, but better would be a graphical display (bar chart is often used) of these differences to see at a glance the general strength of the correlation. Further than this there are countless error analysis methods that could be applied. It would also have been interested to see how the data calculated for isomers of the molecules analysed compared to the lit data for different structures. If you could show that the lit data is closer to the calculated data for the real structure than for other possible isomers then it would be possible to use calculated NMR spectra as a diagnostic tool. Did you find out how exactly the different isomers were distinguished experimentally. There are numerous 2D NMR techniques for doing so and an explanation of this would have been complimentary to the investigation. The IR data is a surprisingly good match – as you say it has associated inaccuracies (in part because it is calculated in the gas phase). Also your optical rotations all match the experimentally observed sign which is perhaps the most important aspect of the technique.