Proton transfer in intramolecular hydrogen bonds with large proton polarizability in 1-piperidinecarboxylic acids. Temperature, solvent and concentration dependence

Abstract

1-Piperidinecarboxylic acids C₅H₁₀N(CH₂)_nCO₂H (n= 1–4)(PCA) solutions have been studied by NMR and IR spectroscopy as well as by osmometric measurements. NMR measurements have shown that with the compounds n= 2–4 relatively strong intramolecular hydrogen bonds are formed. The osmometric measurements demonstrate that compound n= 4 is always dimerized owing to dipole–dipole interactions. At low concentrations in relatively polar solvents the compound with n= 2 is present as a monomer. With increasing concentration it also dimerizes. Because of this dimerization the equilibrium OH⋯N(I)⇌ O^–⋯H⁺N(II) is shifted in favour of the polar structure. In the case of the compound with n= 4, from temperature-dependent measurements the enthalpy ΔH° and the entropy ΔS° have been determined with two solvents. Both quantities are large and negative, in agreement with all other systems with AH⋯B ⇌ A^–⋯H⁺B equilibria studied up to now. For the compound with n= 4 the strong influence of the CH acidity of the solvent is illustrated. This specific interaction effect is responsible for the double minimum in the proton potential.