Nitrogen-15 NMR studies of hydrogen bonding and proton transfer in complexes of pyridineN-oxides with dichloroacetic acid in CDCl3

Abstract

The 15N 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, Δδ(15N) = δ(AHB) − δ(B), against ΔpKa 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 (ΔpKa = 1·26), the proton is either delocalized (B…H…A) or more likely the lifetime is so short (⩽10−13 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 ca 123 ppm for pyridines.