I have seen a paper (attached) and also observed myself that phenazine/dihydrophenazine-based compounds either didn't show or have very-2 broader peaks in the proton NMR when recorded in the solvents like CDCl3, CD2Cl2, DMSO-d6 or acetone-d6. Interestingly, in the benzene-d6 they gave good results.
Can anybody tell me if such observations were found with 1H-NMR, the C-13 NMR will also exhibit similar results? or will C-13 be ok in all solvents.
It may be dependent on the temperature, or even the trace metals (or acids) in the different solvents. The large linewidths could indicate formation of large complexes or interchange between two or more different structures.
Assuming that the instrument is in good conditions and samples are prepared properly, the major reasons for line broadening in NMR are either slow inter/intramolecular movement (in NMR timescale) or presence of paramagnetic species. For phenazine and dihydrophenazines most likely slow internal rotation, further complicated by intermolecular aggregations (mentioned above) is the cause of line broadening.
Aromatic solvents may interact with the dissolved (in this case also aromatic) molecules, and due to their magnetically anisotropic character would change the appearance of the NMR spectrum. This aromatic solvent induced shift is usually noticeable in both 1H as well as in 13C spectra.
It might be interested to see how the electron-deficient aromatic solvents (like hexafluorobenzene) change the picture – since diaryl-dihydrophenazines are electron-rich, theirs interaction with electron-poor aromatic system could be stronger.
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