Proton-exchange mechanisms in phenol–tertiary amine–aprotic solvent systems

Abstract

¹ H NMR experiments at different temperatures exhibit perturbations of the phenol–amine 1:1 complexes by the phenol or amine molecules, in solutions with a molar excess of either the acid or base. In the former, (phenol)₂–amine 2:1 complexes with OHO and OHN hydrogen bonds are formed. The exchange between these bridging protons require the rupture and reformation of the OHN bonds. In the latter, the phenol–amine 1:1 complexes weakly interact with the base molecules. The exchange between the complexing and surrounding base molecules is achieved by the rupture and reformation of the OHN bonds when the phenols are less acidic. For more acidic systems, phenol–(amine)₂ transition complexes with bifurcated hydrogen bonds are formed. This phenomenon is accompanied by the reversible proton transfer inside the hydrogen bridges, O—H⋯N ⇄ O^–⋯H—N⁺, of the complexes dissolved in halogenated solvents at low temperatures.