Key properties of monohalogen substituted phenols: interpretation in terms of the electron localization function

This paper is an attempt to bridge the key properties of monohalogen substituted phenols with the electronic localization function, which has a vivid 3D topological pattern and the vector gradient field of which is determined by the electron transition current density. A primary goal is to interpret the ‘anomalous’ strength of the intramolecular hydrogen bond O—H···X formed in cis ortho-X substituted phenols, depending upon the halogen atom (X = F, Cl, and Br) in terms of the populations of the electronic localization function basins and a so-called core valence bifurcation index. A theoretical model is considered aiming to explain convincingly the cis-trans conversion in ortho-X phenols occurring in some solvents and resulting in the experimentally observed splitting of the ν_OH stretch and based on the Pauling model. Characteristic harmonic vibrational modes of all monohalogen substituted phenols are discussed thoroughly. The order of stability of monohalogen substituted phenols is established at a high level of computational performance, showing the ‘anomalous’ order of stability of fluorophenols, the result being that, in contrast to Cl and Br, the F atom favours the trans meta position over the cis ortho with formation of the intramolecular hydrogen bond.