Re: proton NMR
jmk...@columbia.edu
> I am having trouble understanding the H NMR for 5-norbornene-2,3-
> dicarboxylic anhydride (to see it, please
see:http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng).
>
> To begin with, I don't understand why H's D and E are labelled as non-
> identical hydrogens, when you only see one peak for D/E on the
> spectrum. I am also unclear as to why you can't see the difference
> between B and C on the spectrum (they are merged into one peak, when
> it seems that B is closer to the electronegative =O than C, and thus
> more deshielded).
>
> Any help would be appreciated.
> Thanks
Hey. I'm having a lot of the same questions as you. Have you found
the answers to them. I don't think I got the cyclopentene ring hint.
Also, the overlapping triplets - since D and E are in different
electronic states, does that mean they split eachother. In which case
D would be split by E and C? Have you made any progress with it?
Micha? Sobkowski
>> 5-norbornene-2,3-dicarboxylic anhydride
>> http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng).
>
>
> Hey. I'm having a lot of the same questions as you. Have you found
> the answers to them. I don't think I got the cyclopentene ring hint.
It was a superficial explanation, indeed.
A double bond shifts signals of beta protons downfield, roughly to the
same extent, irrespective of the atom in gamma position (systems
>CH-CR=X, X = C, O, N, S). While one may expect stronger shift in the
systems with more electronegative atoms, this is not always true. Thus,
similar shift found for our compound should not be a surprise (however,
the question "why" is always very good! :-) ). I don't know if there is
one and trivial explanation of this phenomenon.
The other factor is geometry of the compound. The SDBS spectrum refers
apparently to the endo isomer (with anhydride residue below norbornene
skeleton). For exo configuration, the difference between B and C is much
larger, ca. 0.5 ppm. Maybe building models of these structures could be
helpful.
Protons involved in 5-member ring are shifted downfield additionally
(this applies to both B and C protons).
> Also, the overlapping triplets - since D and E are in different
> electronic states, does that mean they split eachother. In which case
> D would be split by E and C? Have you made any progress with it?
Sorry, no triplets but doublets. My mistake. Only ^2J coupling (D - E)
is visible at this low resolution. ^3J coupling (D - C, E - C) depends
strongly on conformation od H-C-C-H dihedral angle and may vary from 0.5
ppm (angle 90o) to over 10 ppm (angle 0 or 180o). In this particular
case it is ca. 1.5 Hz. On SDBS's spectrum you may notice only signal
broadening due to this weak coupling.
Michal