Hi all,
One small correction to Rainer’s email. The vicinal couplings can never be,
in principle, averaged to the same value (look at the Newmann projections in
the JCE paper and consider the effects of the X and Y substituents and the
H's on the couplings with respect to their positions). With a sizeable conformational
bias, the difference in the vicinal couplings can be large, hence possibly resulting
in strong effects due to magnetic inequivalence depending on the geminal couplings.
With an equitable conformational distribution, averaging reduces the difference and
the couplings tend to close in value (since for each H there is a trans and two
gauche vicinal couplings). What is small/close ? Close may mean that the effects
due to magnetic inequivalence are easily overlooked as they are so slight, or they
are effectively obscured under the typical conditions and by the usually applied and
acquisition and processing parameters such that the multiplets appear first order,
or the magnetic inequivalence is almost impossible to observe except with extreme
measures.
There have been lots of suitable experiments touted for undergraduates to perform to
examine the coalescence of chemical shift (signals) due to either chemical exchange
or spin exchange, but anything for magnetic inequivalence ? I think not and some of
the systems highlighted by Rainer could be good educational experiments to perform.
In one project for which we hope to submit a manuscript soon we observed that the
strong effects due to magnetic inequivalence could be reduced by going up in temperature
to provide a "first-order" spin system (in that it resembles first-order but in
principle is not and never will be). We were actually looking at a different process
and this was just a sidelight. Such an experiment would be a good way to explain
magnetic inequivalence and to help undergraduates understand when and how it occurs
(and how it can be easily overlooked/reduced).
Regards,
Karel
________________________________________
> From: Rainer Haessner <rainer.haessner_at_tum.de>
> Sent: 04 January 2021 19:55:49
> To: ammrl_at_ammrl.org
> Subject: AMMRL: H-1 spectrum of 1,2-Dipenylethane - Intermediate summary
Hello again,
I really have to thank all people, who made suggestions to understand this issue.
After presenting the first summary i got some more helpful input from Charles DeBrosse,
Robin Stein and Abil Aliev.
The solution is rather simple, but I really wasn't aware of the stereochemical background all my life.
I discussed with a lot of people and fortunately (as excuse for my lack of base knowledge) nearly nobody
was aware of the true solution. Even in some textbooks there are wrong explanations, assuming restricted
rotations, which is not necessary at all.
A good explanation is available from Robert M. Silverstein and Robert T. LaLonde
in J. Chem. Educ. 57(1980), 343 - 344
(Chemical Shift equivalence and Magnetic Equivalence in Conformationally Mobile Molecules).
Because I assume, some of the participants of this list are not familiar with this particular question of
magnetic non equivalence (like me), please let me try to provide an explanation.
The spinsystem of all compounds of the general molecular formula X-CH2-CH2-Y is not of the type A2X2
(or A2B2) but AA'XX' (or AA'BB'). This is a general rule without exception. Sometimes the nonequivalence
is not visible, sometimes the effect if rather impressive. One nice example is 2-chloroethanol in D2O
(Biological Magnetic Resonance Data Bank)
https://bmrb.io/metabolomics/mol_summary/show_data.php?id=bmse000360
another one is 1-bromo-2-chloro-ethane
https://spectrabase.com/spectrum/zRoqNITnsB
In my example, the only role of C-13 was to change a compound of the type X-CH2-CH2-X into X-CH2-CH2-Y, nothing else.
And now let me try an explanation using my words. I am not too happy with my own graphics.
To provide a better one, I have to learn Blender. That's an impressive piece of Software, but
the learning curve is really steep. Anyway, let's try with the low quality models.
The pink and green balls represent X and Y. The black/grey and the blue/red pairs of hydrogen are
chemically equivalent. The starting point could be the two lower rotamers. Both behave like image
and mirror image. The statistical probability of both rotamers is the same. The red hydrogen mirrors
to the blue one and vice versa. They are chemically equivalent as already said above.
Bot now let us inspect the vicinal coupling constants between the hydrogen pairs black/blue and black/red
It is sufficient to look at the dihedral angles.
In the left side rotamer we have
black/blue - 60 degree
black/red - 60 degree
In the right side rotamer we have
black/blue - 60 degree
black - red - 180 degree
If we average over both rotamers the vicinal coupling constants black/blue and black/red are *not identical*
If we take the upper rotamer into account we observe
black/blue - 180 degree
black/red - 60 degree
If we average over all three rotamers both coupling constants become the same
*assuming the statistical probabilities of all three rotamers are identical.*
As soon, there is a difference in this statistic probability there is no longer a perfect averaging of
the black/blue and black/red coupling constant, which means they are magnetically non equivalent.
Why "intermediate summary"?
Novruzh Akhmedov made a very beautiful simulation of the proton spin system in 1,3-Dichloro-propane.
Many, many thanks. You might see this as X-CH2-CH2-Y type compound as well with X - -Cl and Y- CH2Cl.
Actually I am providing a summary of this nice example, but that's really Challenging (and time consuming).
You know: Blender ... :-)
I hope, my description in combination with the lack of english knowledge was not too bad.
Greetings
Rainer
Received on Mon Jan 04 2021 - 23:13:45 MST