Proton transfer and correlations between the C-O, O-H, N-H and O⋯N bond lengths in amine phenolates

A relationship between the geometrical parameters of the O-H⋯ N bridge based on the results of X-ray diffraction studies of a number of amine-phenol hydrogen-bonded complexes covering a broad Δp Ka range from −3.91 to 5.93 are proposed. The analysis shows that the shortest bridges are realized when the proton transfer degree, reflected in the C-O bond length, reaches about 50%. The position of the hydrogen atom is then close to the centre of the bridge. Such quasi-symmetric bridges behave critically, being sensitive to temperature, pressure and deuteration.