View source for Talk:Jn307Module1:h1iamachemist

Q1: Your energy values are all good, and the dimerisation is correctly assigned as being under kinetic control. The main difference between the products of the hydrogenation is the relative bending strain; this is due to differences in the deviation from ideal sp2 bond angles of the two compounds. The definitions of the different energy contributions can be found in the Chem3D “help” index. Stretching strain is in fact defined as deviation from ideal bond length.

Q2: Your calculations are good and the table showing the variation of energy with carbonyl dihedral angle was a good idea. The reason for the selectivity of the Grignard addition is correct; the interaction between the magnesium and oxygen is merely electrostatic, leading to coordination rather than bond formation it is likely to happen because both species are relatively hard. For the attack of aniline you are right to say that the reaction is governed by sterics, the repulsion is specifically bad in this case because both the nucleophile and the carbonyl group are electron rich.

Q3: You have correctly identified the “down” isomer as the most stable atropisomer. It is good that you have discussion of the factors that affect the differences in energy, but it would have been nice to see a demonstration of the energies of different conformations that you analysed during energy minimisation. The definition of hyperstable olefins is correct: the double bond has less strain  associated with it than the corresponding alkane that would be formed from hydrogenation. This phenomenon is more of an observation than a calculated result however.

Q4: You should have seen in the HOMO that there is significantly more electron density on the double bond syn to the C-Cl bond than on the other double bond; is it possible that this information became lost in changing the display settings of the MOs (e.g. the isocontour value)? The IR frequencies you have obtained are good, but there should be more discussion. The reason for the weakening of the C-Cl bond in the presence of an anti double bond is a consequence of electron density being donated from the double bond into the sigma* orbital of the C-Cl bond.

MINI PROJECT: The discussion of the differences in properties of the two isomers is good, although perhaps you could have analysed the mechanism and the reason for selectivity at the start. The reaction will go via metalated intermediates and therefore the stability of such parts of the catalytic cycle may be more important than the energies of the end products. The molecules themselves are nice and constrained, aiding calculations. Your calculations have shown that the computational methods used give a good level of accuracy. There are a number of ways to present the errors you see when comparing the experimental and calculated data; one of the best ways is to display this information is in the form of a bar chart allowing an easy visual aide to evaluating the method. Also, it would have been helpful to compare the data that you calculated for one isomer to both sets of experimental data to see how well you can distinguish the isomers. It is good however that you have focussed on the most important data, i.e. the alkene 13C shifts. Picking out key structural differences is often the way in which predictive computational chemistry is performed. The reason that you have more peaks in the calculated spectrum is that the molecule is modelled in a stationary arrangement, whereas the experimental conditions are dynamic. This means that chemically equivalent peaks merge in experiment but not when calculated.