Tuesday, January 24, 2017
Improved 1H Resolution with 14N Decoupling
The J coupling between 13C and quadrupolar nuclides can be resolved, for example, in the cases of the 13C NMR spectra of deuterated compounds, some cobalt complexes and some tetraalkyl ammonium salts. The ability to resolve the coupling depends on the relaxation rates among the Zeeman levels of the quadrupolar nuclide with respect to the reciprocal coupling constant. When the relaxation is slow, the J coupling can be resolved and when it is very fast, the 13C is a sharp singlet and said to be "self decoupled". When the relaxation rates among the Zeeman levels of the quadrupolar nuclide are on the same order of the coupling constant, the NMR resonance of the 13C will be broadened. This is a very common observation for the 13C resonances of nitrogen bearing carbons. It is also possible to see broadened 1H or 19F resonances due to coupling to 14N. Such is the case for the resonances of the proton on C6 and the fluorine on C2 in 2,3-difluoropyridine as can be seen from the figure below which clearly shows these resonances broadened compared to the resonances of 1H or 19F further removed from the nitrogen.
The broadening of the resonance of the 1H on C6 can be reduced by applying 14N decoupling during the acquisition time, thus providing much improved resolution. This is demonstrated in the figure below.
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10 comments:
I can't seem to get any resolution improvement with the nicotinic acid/DMSO-d6 sample I had lying around, but maybe this one doesn't benefit from the 14N-decoupling. What sort of decoupling parameters are you using?
Hello Glenn - another simple but good example of this can be obtained from 1H-{14N} of the "15N sensitivity test" sample (90% formamide, 10% DMSO-d6).
Hello Glenn,
Two counterintuitive examples of where the 14N relaxation rates are slow due to small EFGs at the nitrogen are isocyanides (the nitrogen is linear but the EFGs from the two carbons nearly cancel) and tetranitromethane (I have no idea in terms of molecular structure why the 14N relaxation should be slow). In both cases J coupling to 14N have been observed. I wish we would have had 14N decoupling methods when I was a grad. student!
Glenn Penner.
ErrHuman,
I collected an 14N spectrum of the sample to obtain the correct 14N frequency. I used GARP decoupling with the power level set to 15 dB (Bruker AVANCE II console).
Glenn
Glenn,
What sort of probe did you use?
Phil,
I used a standard 300 MHz BBOF probe.
Glenn
Finally got it working with the formamide peak (thanks Craig!) - the TopSpin default referencing meant that the 14N and 15N chemical shifts were not the same. I was attempting to use my 15N-HMBC-derived 15N chemical shifts to set the decoupling parameters, but unfortunately my solution of nicotinic acid was too dilute and/or the 14N peak was too broad to sensibly observe it.
Incidentally I got better decoupling performance with waltz65 than garp (~0.2 W) on a BBO probe at 400.
Glenn,
The reason I asked about the probe was that we have had no reason to use frequencies as low as 14N. I thus checked on the Bruker web site to see how low the probes we have might go. So far as I can see, most broad band probes are listed as tuneable between 31P and 15N with 19F thrown in if you have the F version. I have a memory of there being a low frequency option but I saw no mention of it. So do you have such a low frequency probe or is the range wider than advertised?
Thanks again
Phil.
Hi Phil,
Our broadband probes tune from 109Ag to 31P. Even if your probe is specified to go only as low as 15N, it may tune down to 14N. It is worth a quick check.
Glenn
Perhaps better late than never for those seeing this useful post. Used to be (long time ago) all Bruker probes BBO-type probes tuned to 109Ag. But with the newer ATM units, they now will tune only 31P-15N as default. You can however request the lower frequency tuning range that will include 14N when you order a new probe/spectrometer. I don't think it costs much to have that included.
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