Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine trifluoroacetates and their deuteriated analogues in dichloromethane

Abstract

I.r. and ¹H n.m.r. spectra are reported for the trifluoroacetates of 12 substituted pyridines in dry dichloromethane. The carbonyl–carboxylate region shows that the weakest and strongest pyridines form, respectively, hydrogen-bonded complexes (B ⋯ HA ) and hydrogen-bonded ion pairs (B⁺H ⋯ A^–). The intermediate-strength pyridines form a mixture of both species. Dichloromethane, as a more polar solvent than benzene, slightly shifts the equilibrium between these species to the ion-pair form. If the centres of gravity (_H or _D) and the chemical shifts of hydrogen-bonded protons (δ) are plotted versusΔpK_a, the points fall on two intersecting straight lines. The correlations reflect changes in the hydrogen bond strength and the equilibrium between the molecular complex and the ion pair with ΔpK_a. The centres of gravity in dichloromethane are compared with those in benzene and discussed with respect to variations of hydrogen bond strength. A plot of δversus_H gives again two intersecting straight lines, which reflects the effect of a positive charge caused by the proton transfer on the chemical shift. The effect of the solvent on the correlation of δversus_H is examined and discussed. The isotope frequency ratio (_H/_D) varies from 1.07 to 1.21.