Dear John,
Usually cyclic (bi) compounds such bicycle [2.1.1] hexane reveal in their 1H NMR spectrum long-range (4JHH) couplings. The magnitude of 4JHH are in this range: 0.5 to 8 Hz . Even the magnitude of 4JHH in bicycle[1.1.1]pentane is so high (10 and 18 Hz).
Nowadays using the NMR techniques it is very easy to determine all nJHH (n=2,3,4,5,6 etc) .
I certainly believe that coupling of 0.9 Hz observed in 3-Bromocamphor is 4JHH.
Thanks,
Novruz
Novruz Akhmedov, Ph.D.
Director of NMR Facility
Web page: http://nmr.wvu.edu/
West Virginia University
Department of Chemistry,
217 Prospect St., Clark Hall, Morgantown WV, 26506
Phone: 304 293 0918
304 293 5366
303 293 0995
From: John Fowble [mailto:fowble.1.osu_at_gmail.com]
Sent: Monday, December 09, 2013 8:01 PM
To: Cai, Sheng
Cc: ammrl_at_ammrl.org
Subject: AMMRL: Re: AMMRL: 1H NMR of bromide compounds
I would not be surprised to see two long-range couplings: both the W and the inverted "zig-zag" protons geminal to each other. If they're shift dispersed, you might have the chance to see these fine splittings amid the expected large geminal doublet.
-Jack
Sent from my iPad
On Dec 6, 2013, at 11:59 AM, "Cai, Sheng" <sheng.cai_at_marquette.edu<mailto:sheng.cai_at_marquette.edu>> wrote:
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 Tue Dec 10 2013 - 09:01:37 MST