Influence of the pH of the medium on the chemical shifts of the PMR of 3- hydroxypyridine derivatives
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The dependence of the chemical shift of the ring protons on the pH of the medium was studied by the PMR method for a number of derivatives of 3-hydroxypyridine.
A consideration of the family of dependences indicated the presence of three horizontal portions, corresponding to the ranges of acid, neutral, and alkaline media.
A comparison of the spectra of 2-methyl-3-hydroxypyridine and its N-methyl derivative confirms the presence of a bipolar structure in aqueous solution.
Good qualitative agreement was obtained between the estimated chemical shifts of the ring protons of derivatives of 3-hydroxypyridine in the bipolar form and theπ-electron densities calculated according to Hiickel method.
The transition from acid to alkaline medium produces a change in the sequence of arrangement of the proton signals at the 4- and 5-carbon atoms.
The presence of an intramolecular hydrogen bond in 2-dimethylaminomethyl-3-hydroxypyridine substantially influences the nature of the dependence of the chemical shift on the pH of the medium.
An analysis of the dependences of the chemical shift on the pH for an acid medium in the case of 2-dimethylaminomethyl-3-hydroxypyridine permits it to be concluded that separate protonation exists at the nitrogen of the ring and the nitrogen of the side chain.
KeywordsHydrogen Nitrogen Hydrogen Bond Chemical Shift Acid Medium
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