Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine N-oxide trifluoroacetates and trichloroacetates in chloroform. Correlation of OH stretching frequencies and proton chemical shifts with pyridine N-oxide pKa values

Abstract

The i.r. and ¹H n.m.r. spectra are reported for the trifluoro-and trichloro-acetates of 12 substituted pyridine N-oxides in dry chloroform. If the centre of gravity of the OH stretching absorption (_H)and the chemical shift of hydrogen-bonded protons (δ) are plotted against the pK_a value of the N-oxides, the points fall on two intersecting straight lines. The correlations reflect changes in the hydrogen-bond strength. Plots of _H and δ of trifluoroacetates against the corresponding values for the trichloroacetates are used to show that the proton-donor properties of the acids are comparable. A direct plot of δversus_H gives a straight line consistent with data for the monomeric acids. This correlation is discussed with respect to the difference between hydrogen bonds in molecular and ion-pair complexes.