qw098
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NMR Tree Diagram
Hi Guys,
For the peak seen at 7.85ppm (shown below) which is pretty "clean", it seems to be a ddd. However, the smallest coupling constants don't add up. The
two outside coupling constants are equal, but the two interior coupling constants are different than the outside coupling constants, and by a fairly
large amount. Shouldn't the coupling constants in the same level all be equal for a ddd? I have attached a .jpg file to show what I mean.
In final my question is... what is wrong with this NMR Spectrum? If this is indeed a ddd why aren't the coupling constants adding up?!
Thanks!
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turd
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You seem to miss the fact that in the bottom part of the tree the "branches" can cross. Think about it: what would a "ddd" look like for J=4,5,6 Hz?
Pretty much exactly like your diagram, isn't it?
NMR has always been black magic to me, but this seems to make some sense. BTW: Are you sure you are looking at the correct proton? What does the
signal of the proton at the other side of the double bond look like?
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Satan
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Post whole spectra.
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qw098
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Thanks. Yes, you are right, I mislabeled my hydrogen. The chemical shift should correspond to the adjacent hydrogen.
Quote: Originally posted by turd  | | You seem to miss the fact that in the bottom part of the tree the "branches" can cross. Think about it: what would a "ddd" look like for J=4,5,6 Hz?
Pretty much exactly like your diagram, isn't it? |
So... How would I draw my tree diagram and find the rest of my coupling constants?
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turd
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Come one... I gave you the answer.  If you make the tree for J=4,5,6 Hz it will be
obvious which peaks form pairs.
BTW: This example is obviously simulated. No way a real NMR spectrum would look like this with the shifts being exact to the 5th significant place.
(Answer for the lazy: 6.0, 7.1, 7.9 Hz)
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qw098
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Thanks for the response!
I did draw a diagram! And I got the answer!!! You sir, are AMAZING!  Thank you a
ton!!! To prove to you I drew the diagram please find it attached below
By the way... how was I suppose to know beforehand that there was overlapping between the bottom "branches"? Is this simply a guess I have to do?
Also, may I add... how do I know which hydrogen the coupling constants (6.0, 7.1, 7.9 Hz) couple too. Of course it couples with one of the hydrogens I
am drawing the "tree" for, but how do I know which other hydrogen as well?
Thanks a TON again, I really understand this now Awesome!
[Edited on 25-10-2011 by qw098]
[Edited on 25-10-2011 by qw098]
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qw098
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Nevermind, I finished my whole assignment thanks to you sir!!! I've had this assignment for THREE weeks now... thinking about it EVERYDAY and thanks
to YOU sir.. I have figured it out!!!
Thank you x 10000000!!!!!!!! I can't thank you enough!!! That idea that the smallest branches can overlap was ingenious!! Thank you x infinity!
You are the best!!
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