PHENOL RETINAL SCHIFF-BASE HYDROGEN-BONDS - INFLUENCE OF STERIC HINDRANCE AND PHENOL ACIDITY ON THE THERMODYNAMIC DATA OF FORMATION AND PROTON-TRANSFER

CD2Cl2 solutions of four trans-retinal Schiff base (containing methylamine, n-butylamine, t-butylamine and 5-butyl-nonylamine), their perchlorates and their 1:1 complexes with 3,4-disubstituted and 4-monosubstituted phenols were studied as a function of temperature using Fourier transform infrared (FTIR) spectroscopy. The thermodynamic quantities ΔH0F and ΔS0F of hydrogen-bond formation and ΔH0PT and ΔS0PT of the proton transfer process across the hydrogen bonds were determined. All (I) OH⋯N⇌O-⋯H+N (II) bonds show large proton polarizability as indicated by continua in the IR spectra. In all the systems studied, the ΔH0PT and ΔS0PT values of the (I) OH⋯N⇌O-⋯H+N (II) equilibrium were negative. This is a result of the highly ordered arrangement of the solvent molecules around these complexes. ΔH0F and ΔS0F of formation of the OH⋯N⇌O-⋯H+N bond and ΔH0PT and ΔS0PT decrease significantly with decreasing acidity of the phenol. Furthermore, ΔH0PT and ΔS0PT decrease significantly with increasing screening of the hydrogen bonds from the solvent environment by bulky groups. This indicates that the ΔH0PT and ΔS0PT values are determined by the interaction of the hydrogen bonds with their environment. © 1990.