Talk:Mod:YYTmod1

FEEDBACK

Q1: Your calculated energies are good, but it is not necessary to give so many significant figures; MM2 is an approximate method and there is some degree of error to consider. You should have included at least a jpg image of your structures and ideally a jmol or link to a jmol structure. The differences between the isomers are discussed, but it would have been better to relate this analysis to the structures of the compounds in question. The discussion of kinetic vs thermodynamic control is good for the dimerisation reaction. For the hydrogenation, it is not really possible to comment without further information about the conditions. Normally this type of reduction is metal-catalysed, irreversible, and therefore under kinetic control. To predict the kinetic product, DFT methods would need to be used to calculate the relevant transition state energies.

Q2. The lowest energy isomer is correctly identified, but the structures are not correct. The lowest energy forms of both isomers have the 6-ring in a chair conformation not a twist-boat conformation. There is no discussion here of how you optimised the compounds – since there is not much more to discuss in this question this would have added a bit more detail. The reason the oxy-cope rearrangement you have shown is often irreversible is that the product (an enol) rapidly isomerises to a more stable form (the appropriate carbonyl compound) which is unreactive in the backwards sense. In this case, the double bond is unreactive in a more general sense (to any reagents); this is because it is a “hyperstable alkene” – an alkene for which the parent alkane is more strained due to the bridgehead position on a medium –sized ring.

Q3. Your calculations, MOs and IR stretches are all good. The explanation for the regioselectivity in the reaction with dichlorocarbene is correct. You have identified the pi-sigma* interaction that lowers the C-Cl bond energy, but did not correlate this with the IR data. The IR stretch of the C-Cl bond is more energetic (stronger bond) when the double bond is removed because the donation into its antibonding orbital is removed.

Q4. The choice of R=methyl is correct and the PM6 calculations are indeed more appropriate than MM2. As you have shown the A/B/C/D set is lower in energy than the A*/B*/C*/D* set. Your values are overall higher than expected; I suspect that the carbonyl oxygen of the acetyl group is not sufficiently close to the oxonium carbon. The structures C* and D* were the trans fused bicycles – any fused bicycle can potentially be cis or trans-fused for 6.5 structures, the trans-fused scenario is usually much higher in energy. The table of bond distances is not helpful – it would be better to list the relevant properties you want to discuss or at least highlight them in the text. Comparison of data to ideal bond lengths and angles becomes less relevant when using semi-empirical methods as you are no longer relying solely on a set of empirical reference points.

MP. This is a good simple reaction for a mini-project as it produces a mixture of isomers which are conformationally restricted. However, I’m not sure how well simple computational methods can account for an ion pair. You are right in saying that the diastereomers could be distinguished by different coupling constants. Looking at your NMR spectra, I think it is possible that you didn’t apply the TMS reference which will dramatically change the ppm values. When comparing lit and calculated NMR data it is better to give the differences in ppm rather than just both sets of data; the clearest way to do this is to present the differences graphically, so that the error and the overall deviation can be easily assessed.