Sheng...John is absolutely correct...in these restrained ring systems W
couplings are quite common, as well as the zig-zag couplings he
mentioned....0.9 Hz is also a reasonable magnitude for these splittings
Cheers
Joe
On 12/9/13 8:01 PM, John Fowble wrote:
> 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
--
Joseph J Barchi Jr, Ph.D.
Head, Glycoconjugate Chemistry and NMR Section
Chemical Biology Laboratory
Center for Cancer Research
National Cancer Institute, Frederick
376 Boyles Street
Bldg 376 Rm 209
Frederick, MD 21702
301-846-5905
301-846-6033 (Fax)
barchij_at_helix.nih.gov
Received on Tue Dec 10 2013 - 08:22:50 MST