Dr. Cai was kind enough to send me the 1H data on endo-3-bromo camphor and
I think I found the source of the 1.0 Hz long-range coupling he was asking
about.
Surprisingly, the 3-exo proton (ddd, J = 4.8, 2.1, 1.0) is coupled to the
6-exo proton with a 1.0 Hz coupling. This is a 5-bond relationship, and I
think the reason it shows up is the alignment of the C6-H6exo bond with the
p orbital of the C=O pi bond on C2. This allows it to couple with H6exo
via the anti H6exo-C6-C1-C2 relationship.
The coupling shows up in the 6-exo peak (1.684 ppm, dddt, J = 13.7, 11.6,
4.0, 0.85 Hz) as one of the triplet couplings (0.85 Hz). This is not resolved
but with resolution enhancement (unshifted sine-bell) you can see the triplet.
The triplet is probably actually a dd (J = 1.0, 0.7) that appears as a triplet,
matching the 1.0 Hz coupling on H3. Where does the 0.7 Hz coupling come from?
I'm guessing the bridgehead H-4 (2.298 ppm, t, J = 4.5 Hz). The lines in this
triplet are more than twice the linewidth (1.78 Hz) of the sharpest lines in the
spectrum (0.82 Hz), so it's probably hiding this and other unresolved long-range
couplings. The H6exo to H4 coupling is a distorted W coupling (4 bonds).
The following table shows my complete analysis. In the SDBS database the H5exo and
H5endo assignments are backwards, so their J coupling values are also wrong.
H3: delta 4.619 (ddd, J = 4.8, 2.1, 1.0 Hz, 1H) (linewidth 0.96 Hz)
H4: delta 2.298 (t, J = 4.5 Hz, 1H) (linewidth 1.78 Hz)
H5exo: delta 1.876 (ddddd, J = 13.3, 11.4, 5.1, 4.4, 2.0 Hz, 1H) (linewidth 0.82 Hz)
H5endo: delta 2.081 (ddd, J = 13.4, 9.4, 4.0 Hz, 1H) (linewidth 1.44 Hz)
H6exo: delta 1.684 (dddt, J = 13.7, 11.6, 4.0, 0.85 Hz, 1H)
H6endo: delta 1.426 (ddd, J = 14.0, 9.3, 5.0 Hz, 1H) (linewidth 1.30 Hz)
Here is a list of J couplings:
H3-H4: 4.8 Hz vicinal
H3-H5exo: 2.0 Hz W coupling
H3-H6exo: 1.0 Hz 5-bond
H4-H5: 4.4 Hz vicinal
H4-H6exo: 0.7 Hz distorted W
H5exo-H6exo: 11.5 Hz cis vicinal
H5exo-H6endo: 5.0 Hz trans vicinal
H5endo-H4: zero (90 degree dihedral)
H5endo-H6endo: 9.3 Hz cis vicinal
H5endo-H6exo: 4.0 Hz trans vicinal
H5endo-H5exo: 13.3 Hz geminal
H6endo-H6exo: 13.9 Hz geminal
These bicyclo[2.2.1] systems are very rigid and show lots of long-range couplings.
I would like to confirm these finding with COSY or selective decoupling, but I don't
have a sample here.
Neil
________________________________________
> From: Cai, Sheng [sheng.cai_at_marquette.edu]
> Sent: Monday, December 16, 2013 4:04 PM
> To: Jacobsen, Neil E - (neil)
> Subject: RE: RE: AMMRL: 1H NMR of bromide compounds
Hi Neil,
Attached please find the raw data of the compound, zipped by winrar.
Sheng
________________________________________
> From: Jacobsen, Neil E - (neil) <neil_at_email.arizona.edu>
> Sent: Wednesday, December 11, 2013 2:12 PM
> To: Cai, Sheng
> Subject: RE: RE: AMMRL: 1H NMR of bromide compounds
Sheng,
Can you send me the FID? I think there must be a peak that is broadened by
a combination of long-range couplings, and this hides the 0.9 Hz coupling.
I would love to have a look at the data, send my a zipped version of your
1H spectrum.
Neil
Neil E. Jacobsen, Ph.D.
NMR Facility Manager
Department of Chemistry and Biochemistry
University of Arizona
1306 E University Blvd.
Tucson, AZ 85721
520-621-8146
________________________________________
> From: Cai, Sheng [sheng.cai_at_marquette.edu]
> Sent: Tuesday, December 10, 2013 3:11 PM
> To: ammrl_at_ammrl.org
> Subject: RE: RE: AMMRL: 1H NMR of bromide compounds
Hi all,
Thank you for your time. Most people suggest long range coupling. But my problem
is I don't see this 0.9 Hz from any other signal. I cannot find its coupling parter.
All other peaks show perfect coupling patterns (multiplicity), without this 0.9 Hz.
I am on a trip now. Once I get back to my office, I will upload the spectrum, so
you can see it more clear.
Sheng
RE: AMMRL: 1H NMR of bromide compounds
Hi all,
Thank you all for your suggestion. We did some further investigation and found
two of our Br compound have isomers and caused the spliting.
But I still cannot explain the spectrum of one compound, ((1R)-endo)-(+)-3-Bromocamphor
(CAS # 10293-06-8). This compound was purchased from SigmaAldrich. The proton attached
to the bromocarbon (position 3) shows a doublet (4.6 Hz) of doublet (1.9 Hz) of
doublet (0.9 Hz) at 4.6 ppm (400 Hz instrument in CDCl3). The coupling of 4.8 Hz and 1.9 Hz
can be easily assigned to 3-bond H-H coupling and long range coupling, respectively. But
I have no idea where the 0.9 Hz splitting come from. None of rest protons show this 0.9 Hz
coupling. I can perfectly fit the entire spectrum using the measured J coupling and chemical
shifts, except for the 4.6 ppm one. Thus, I am sure this 0.9 Hz is not a coupling. It comes
from different isomers.
The SigmaAldrich catalog says this compound is a sum of enantiomers. I thought all
enantiomers should give same NMR spectra, if the compound they sold me is pure
(no 3-bromocamphor, CAS # 76-29-9). So, I was left with only one choise, Br isotope
effect. I looked through literature and cannot find any information on Br effect on
1H chemical shifts.
Did I miss something? Thank you very much.
Sheng
________________________________
> From: Cai, Sheng
> Sent: Tuesday, November 26, 2013 2:40 PM
> To: ammrl_at_ammrl.org<mailto:ammrl_at_ammrl.org>
> Subject: 1H NMR of bromide compounds
Hi all,
I found some bromine-containing organic compounds showing strange splitting
patterns in proton NMR. I cannot explain these splittings (usually 1-2 Hz)
using any long-range spin-spin coupling. I suspect they may come from Br79/81
isotope effect. Has anybody observe this before ? A link to references will
be highly appreciated. Thank you.
Sheng Cai
Marquette University
Received on Wed Dec 18 2013 - 07:12:17 MST