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!
turd - 25-10-2011 at 12:32
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?Satan - 25-10-2011 at 12:46
Post whole spectra.qw098 - 25-10-2011 at 12:55
Thanks. Yes, you are right, I mislabeled my hydrogen. The chemical shift should correspond to the adjacent hydrogen.
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?
turd - 25-10-2011 at 13:15
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)qw098 - 25-10-2011 at 13:42
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]qw098 - 25-10-2011 at 14:15
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!
If you make the tree for J=4,5,6 Hz it will be
obvious which peaks form pairs.
Thank you a
ton!!! To prove to you I drew the diagram please find it attached below