Cooperative effects in H-bond formation: influence on the acidity of proton donors

Abstract

Linear correlations between the relative frequency shifts of several proton donors XH in polar aprotic solvents B with the relative frequency shift of HOD in the same solvents are obtained. The slopes of these correlations increase with decreasing heterolytic bond dissociation energy of the proton donors as well as with its dissociation constant in water. Thus an order of acidities is established. The cooperative influence exerted by cations on DOH⋯B (B is an H-bond acceptor) in cation⋯OH⋯B complexes increases the acidity of the water molecules involved. The relative OH frequency shift of cation⋯OH⋯B complexes shows the same correlation with the dissociation constant of hydrate water of the cations as found for other proton donors. In H-bonded complexes of proton donor dimers XH⋯XH⋯B a mutual cooperative effect is present. This may lead to such a large increase in the acidity of XH that dissociation occurs.