User talk:Np1612

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Atropisomerism of Taxol (0.25) 92%

"A hypothesis to explain the greater stability of molecule 10 could be the antiperiplanar orientation of the C=O bond with the neighbouring C-H bond."

Good discussion!

"The Total Energy results of the optimised molecule 18 conformation was compared with colleagues and it was found that the lowest energy optimised molecule 18 conformation had a Total Energy of 402 kJmol-1."

Nice approach!

"Table 5. 1H NMR Data of a Taxol Intermediate 18 "

This would be much easier to read if your sorted it by shift.

"The reason for this large discrepancy between values is that the chemical shifts reported in literature were obtained experimentally. Hence, they reflect a time-weighted average of all the possible conformations of Taxol intermediate 18 yet, only one conformer was used to generate the 1H NMR quantum mechanically."

Well, yes and no - you make it sound like it's the experiment's fault that the calculation doesn't match - maybe phrase that more carefully next time.

It's not very likely that the different conformers interconvert into ech other. You can only say that if you've got evidence that for the molecule there's no kinetic barriers between the conformations.

It's actually more likely that the lowest energy conformation in solution is a different one from your (calculated) vacuum-model.

"However, the chemical shift values reported in the literature[4] are not assigned to each hydrogen in each multiplet, therefore literature values were also averaged to obtain a mean shift for each multiplet"

I'm not quite sure I understand what you did there. But there's no need to average the literature. HNMR is a highly reliable technique and in this case you should treat the literature as 'hard fact' (as opposed e.g. to alpha values in optical rotation).

You explained what you did and why you did it, and that's okay. However, I'd like to give you a general warning. Any changes to the experimental data 'to make it match better' could end up being viewed as trying to change reality, i.e. cheating.

What is actually happening is that you have a model that's not in agreement with experimental data. I understand your intentions here, but you should really know that if you do something like this in a thesis or a paper that will at the least be viewed as a bad scientific work style. So, once again, I'm not implying anything, just warning not to mess with 'hard data'.

In any case, adding the original literature values to the table would have been helpful. Make it easy on your readers to verify your thought-process without having to look up the data themselves. ;)

" The reason for this large discrepancy between values is that the chemical shifts reported in literature were obtained experimentally. Hence, they reflect a time-weighted average of all the possible conformations of Taxol intermediate 18 yet, only one conformer was used to generate the 1H NMR quantum mechanically. "

Same thing again, you mentioned in your intro already that it takes time to convert from one isomer to another. That indicates a kinetic barrier of sufficient height between the conformers. Again, it will be the difference between computation 'in vacuo' and experiment in solution.

Crystal structures(0.05) 100%

good!

Epoxide NMR (0.05) 40%

Discussion missing.

Optical Rotation and TS (0.35) 35%

Discussion missing and the thermal energy values missing..

.

Sorry, wish I could give you a better grade.