Sheng,
I agree that the two small couplings are 4-bond. The larger one (1.9 Hz) is to the 5-exo proton (W coupling) and the small one (0.9 Hz) is to the 5-endo proton (some people have referred to this geometry as the "question mark" coupling: Simpson, J. Organic Structure Determination using 2-D NMR spectroscopy - a problem based approach, 2nd edition, Elsevier, pp. 142-143). These long-range couplings show up because of the rigid and well-defined structure of the molecule.
Bromine has two isotopes, each close to 50% abundance and both spin 3/2. If relaxation were not an issue, there would be two superimposed peaks for the 3-exo proton, one with 5 equally-spaced and equal height lines, and another also with 5 equally-spaced equal height lines, but with different spacing and possibly a different center. This is in addition to the other 1H-1H couplings. But relaxation is probably very rapid for Br due to the quadrupolar moment, so no coupling to 1H is observed.
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: Friday, December 06, 2013 12:59 PM
To: ammrl_at_ammrl.org
Subject: 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
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 11 2013 - 06:53:08 MST