Solvent assisted proton transfer processes in phenol–amine complexes revealed in effective dipole moments

Abstract

The dipole moment of the 4-nitrophenol–tri-n-butylamine (PNP·TBA) adduct of the 1∶1 stoichiometry was studied in tetrachloroethylene, chloroform and 1,2-dibromoethane as a function of temperature. Based on the results obtained, the thermodynamic parameters for the O–H⋯N ⇆ O⁻⋯H–N⁺ proton transfer (PT) reaction were evaluated. The solvent effect on ΔG_PT^○, ΔH_PT^○ and ΔS_PT^○ is discussed in terms of two contributions. The short range solvation results from a specific direct interaction between the adduct and solvent molecules. The second solvation contribution can be ascribed to a non-specific long range bulk dielectric permittivity effect (reaction field effect). By using heterogeneous models of dielectric medium as well as empirical solvent parameters an attempt was undertaken to analyse quantitatively the solvent effect on thermodynamics of proton transfer.