Phenol–amine hydrogen bonds with large proton polarizabilities. Position of the OH···N ⇌ O–···H+N equilibrium as a function of the donor and acceptor

Abstract

1:1 mixtures of octylamine + chlorophenols and pentachlorophenol + aromatic amines have been studied in CCl₄ solution using infrared spectroscopy. It has been shown by conductivity measurements that no charged species are present in these systems. Therefore only the association constants K_a, and the equilibrium constants, K_PT, of proton transfer in the equilibrium OH ··· N [graphic omitted] O^–··· H⁺N need be considered. Linear relations exist between log K_a and the ΔpK_a values for both families of systems. Log K_PT increases with both systems in proportion to ΔpK_a, i.e. the Huyskens equation is valid. This increase is caused by a decrease in the enthalpy term ΔH^o₀ due to increasing acidities of the donors or (in the other family of systems) increasing basicities of the acceptors. In addition to this direct effect, an increase in the size of the negative ΔH^o_I values due to the increasing interaction of these hydrogen bonds with their environment is responsible for this shift in the equilibrium. ΔpK^50%_a amounts to 3.6 in the case of the family containing the aliphatic amine and 1.6 in the case of the family containing the aromatic amines, since with the latter the charge is spread over a more extended region if the proton approaches the N atom. The intensity of the continuum (and thus the proton polarizability) is largest if both proton-limiting structures OH ··· N and O^–··· H⁺N have almost the same probability.