Infrared investigation of hydrogen bonding and proton transfer of some pyridine trifluoroacetates in benzene

Abstract

I.r. spectra are reported for the trifluoroacetates of ten substituted pyridines in dry benzene. 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. A plat of the gravity centre (v_H) of v_AH complex absorption against ΔpK_a consists of two intersecting straight lines. This correlation is considered in the light of the previously reported similar relationship between the chemical shift of hydrogen-bonded protons (δ) and ΔpK_a. If δ is plotted versus v_H, again two intersecting straight lines are found. The correlation reflects the effect of a positive charge caused by the proton transfer on the chemical shift. All these results confirm the long-held assumption that no general correlation exists for hydrogen-bonded complexes and hydrogen-bonded ion-pairs.