Hello again,
many, many thanks for all the sugggestions. I would especially like to thank Alan Kenwright, Joseph Barchi, Lukas Hintermann, Michael Engelhardt, Marc-Oliver Ebert, Daniel Rentsch,
Stefan Berger (!), Svetlana Simowa, Sebastian Kemper, Akhmedov Novruz, Kirk Marat, Clemens Anklin, Paul White, Serge Lavoie, Ronald Crouch, Zdenek Tosner and Glenn Facey.
Yes, the exclamation marks labels a special contribution seen with my eyes.
Let me try to sort the main ideas.
The key explanation should be described in two ancient papers:
Internal Rotation in Substituted Ethanes and Cyclic Ethers
Author(s): N. Sheppard and J. J. Turner
Source: Proceedings of the Royal Society of London. Series A, Mathematical and Physical
Sciences, Vol. 252, No. 1271 (Oct. 27, 1959), pp. 506-519
and
The Magnitudes and Relative Signs of Hydrogen Spin-Spin Coupling Constants
in Hydrocarbon Groupings
By C. N. BANWELL, A. D. COHEN, N. SEEPPARD, and J. J. TURNER
I haven't read them yet and I believe to understand the content will require at least one week.
Furthermore I got a lot of comments to see the whole spin system as AA'BB'X. To be honest,
actually I am not able to see that kind of magnetic nonequivalence. But on the other side the well
known adipic acid with its AA'A''A'''XX'X''X''' spin system might be seen as a symmetrically substituted
ethane. To think about this, will require some walks at the fresh air. Fortunately, after retirement
there is enough time for such walks ...
A very interesting question was related to the magnitude of the coupling constants in different solvents.
Unfortunately the spectrum of 1,2-Diphenylethane is very old and after retirement it is not that easy
to acquire new spectra. But ... from three different sources (thanks to Lukas Hintermann, Gerd Gemmecker
and Ralf Stehle in Garching and John Markley in Madison/Wisconsin) I had related proton spectra form
glycerol in two different solvents. And yes! There are significant differences with respect to the vicinal
coupling constants. While they differ by about 3 Hz in D2O, they are almost the same in DMSO-d6.
The solvents really change the probability of the rotamers significantly.
If you want to see the spectra by yourself, all of the glycerol spectra are stored as a zip file at
https://www.nmr.tips/AMMRL/Glycerol.zip
Another set of data is available under
https://www.nmr.tips/AMMRL/broken_symmetry.zip
which contains 1,2-Diphenylethane (Garching), 1,2-Dichloroethane (Madison/Wisconsin) und
1,3-Dichloropropane (Garching)
The last compound is for reasons of curiosity. The proton spectrum is of higher order as well, visible
as soon as you change the default processing with line broadening of 0.3 Hz to a better resolution.
This seems to be a related problem, because about 50% of this molecule are asymmetric as well.
And finally many thanks to Daniel Rentsch for a generous gift of two proton spectra (maleic and fumaric
acid). I wanted to measure these compounds for educational purposes in the future,
but now I already have them.
I hope, this was not too confusing. A long explanation and my bad english knowledge might result in
a toxic mixture ...
Best wishes
Rainer
Received on Mon Dec 14 2020 - 07:53:17 MST