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Phenol-Amine Hydrogen Bonds with Large Proton Polarizabilities. Position of the OH…N<*>O(1-)…H(1+)N Equilibrium as a Function of the Donor and Acceptor
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 Ka, and the equilibrium constants, KPT, of proton transfer in the equilibrium OH…N<*>O(1-)…H(+)N need be considered.Linear relations exist between log Ka and the DeltapKa values for both families of systems.Log KPT increases with both systems in proportion to DeltapKa, i.e. the Huyskens equation is valid.This increase is caused by a decrease in the enthalphy term DeltaH<*>0 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 DeltaH<*>I values due to the increasing interaction of these hydrogen bonds with their environment is responsible for this shift in the equilibrium. DeltapK50percenta 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(1-)…H(1+)N have almost the same probability.
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Reference£º
Chiral nitrogen ligands in late transition metal-catalysed asymmetric synthesis¡ªI. Addressing the problem of ligand lability in rhodium-catalysed hydrosilations,
Nitrogen-Containing Ligands for Asymmetric Homogeneous and Heterogeneous Catalysis