[Question from NMRWiki Q&A forum] About paragnetic impurities, inorganic salts, bad workups, etc. and their impact on small-molecule NMR spectra

[nmrlearner]

About paragnetic impurities, inorganic salts, bad workups, etc. and their impact on small-molecule NMR spectra

I sometimes face the challenge of explaining to my organic chemists that, even if no impurity signals are visible in their 1H spectrum, other signs (like FID shape, very poor s/n compared to what could be expected with the sample qty, broad peaks, bad resolution...) indicate that clearly, something is wrong with their sample.

Everybody would agree that it's way much easier to convince an invidual based on concrete facts ("look, here we can see solvent/starting material/side product/isomer signals integrating for x% in your proton-nmr") rather than just doubts ("well, your structure is correct however something seems weird with the spectrum...Yet I can't see anything so I can't tell you exactly what"). Ultimately in these cases, I can't think of an easy way to get indisputable evidence of a purity issue, other than using quantitative NMR techniques maybe (which still won't tell me the nature of impurities).

Purpose of my post is firstly for us to share personal experiences and discuss that topic. I'm also very interested if someone could provide me with a simple definition + common examples of paramagnetic impurities in order to explain to lab technicians the potential effects in small-molecule NMR measurements. Finally, I'm all ears if someone has good arguments, a straightforward method or eventually an idea of a good test sample that could be performed as an example to prove the point that having a nice LCMS chromatogram and no obvious imps in your 1H-NMR doesn't necessarily mean your compound is pure.

Thanks in advance



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