Nitrogen-15 NMR studies of hydrogen bonding and proton transfer in complexes of pyridine N-oxides with dichloroacetic acid in CDCl3
✍ Scribed by Zofia Dega-Szafran; Mirosław Szafran; Jerzy Sitkowski; Lech Stefaniak
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 387 KB
- Volume
- 9
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
The "N NMR chemical shifts of eight substituted pyridine N-oxides (B) and their complexes (AHB) with methanol and dichloroacetic and trifluoromethanesulfonic acids were measured in chloroform-d at the natural abundance level. The measured chemical shifts are strongly affected by substituents, hydrogen bond strength and protonation. The plot of the relative chemical shifts, A6(l5N) = G(AHB) -6(B), against ApK, gives a titration curve that reflects the variation of the hydrogen bond strength and the proton moves from the acid to the N-oxide. According to previous IR data, in the complexes from the inversion region (ApK, = 1.26), the proton is either delocalized (B...H...A) or more likely the lifetime is so short (<lo-" s) that the method does not recognize the B...H-A and B'H...A-species. Protonation of pyridine N-oxides shifts the nitrogen signal ca 50 ppm to lower frequencies compared with the cu 123 ppm for pyridines.