13C NMR spectra was obtained for a conjugated planar pyridine - based ligand. Two peaks out of the six expected are very resolved and have a high signal:noise ratio. The other peaks can be brought out of the noise by cooling and freezing out peaks that show there are four conformers. However, when the sample is heated, the rest of these peaks go completely and the two resolved peaks increase in height. The sample is pure and the 1H NMR spectra can be assigned perfectly, but these two very conspicuous peaks Can anyone help explain this?
You should acquire Heteronuclear correlation 2D-NMR, variable-temperature if necessary, such as HSQC and HMBC:
http://chem.ch.huji.ac.il/nmr/techniques/2d/2d.html
After assigning all carbons and protons in your spectra it should be easy to explain the differences in signal-to-noise ratios.
I belive you are dealing with tautomerism and as a result chemical exchange with intermediate rate. We faced with a lot of examples of piridines giving a proper 1H and bad 13C NMR spectra lacking most of signals. Usually it is the case of DMSO or other prototropic solvent solutions. You can send me your spectra and formula if you need the answer and I expect your problem will be solved in short time
Will you see my recent open acsess paper New cases of prototropic tautomerism in substituted pyridines
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