I am sometimes asked to acquire 13C NMR spectra of highly paramagnetic compounds. Those of you attempting to acquire such spectra for the first time should be aware of the following:
1. You should use a large spectra width as the 13C chemical shifts can be very extreme. In order to get a wide uniform excitation, use very short high power pulses.
2. Both the T1's and the T2's are short, so you can use short acquisition times and short recycle delays.
3. The 1H chemical shift range for paramagnetic compounds can span hundreds of ppm. The 1H decoupling schemes used to collect 13C data do not permit such a broad decoupling bandwidth and you may have to collect several 13C spectra with different 1H offsets. It is always best to collect a proton spectrum first to evaluate the 1H decoupling needs in the 13C spectrum.
4. The lines are often very broad and therefore many scans are needed to build up the signal-to-noise ratio.
5. The 2H lock signal may be shifted by the paramagnetic compound, so don't be surprised if your automatic locking routines do not work.
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4 comments:
I have a stable radical containing compound. I basically saw nothing either in proton or carbon spectra. Can you suggest a strategy?
Thanks.
Anonymous,
Thank you for the question. The problem you are having is due to the dipolar coupling between the unpaired electron and the nuclear spins. Your spectra must be extremely broad and since all of the intensity is spread over very broad signals, the signal-to-noise ratio is very low. The best advice I can give is use a large spectra width, short recycle delay and short acquisition time. Process the data with lots of exponential line broadening.
Glenn
Dear Glenn,
Regarding the effect of an unpaired electron on the detection of 13C nuclei nearby: is there some kind of distance dependence? For NOE the distance dependence is 1/r6. I am wondering "how far" the effect of a localized unpaired electron or radical can reach. Is it just one carbon atom, or two, or ...? Thank you for your help!
Dear Kris,
The fast dipolar relaxation due to coupling between unpaired electrons and nuclear spins which causes broadening in the NMR lines also has a 1/(r6) dependence however the situation is quite a bit more complicated than that. Although I would predict that the closest nuclei to the localized unpaired electron would be affected most, I am not aware of a simple way to predict by how much more distant spins are affected. I have seen the NMR resonances of the solvent affected by paramagnetic solutes.
Glenn
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