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

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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 versusΔpKa, 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 ΔpKa. 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.

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