1H NMR Studies of Solvent and Substituent Effects on Strong Intramolecular Hydrogen Bonds

Article abstract of DOI:10.1002/mrc.1270150116

Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds.In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used.The chemical shift value of picolinic acid N-oxide varies with the solvent.These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide.The chemical shifts of substituted picolinic acid N-oxides were correlated with ?_p, ?_m and ΔpK_a.The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.