Theoretical analysis of molecular structure, hydrogen bond strength, and proton transfer energy in O-H••O aromatic compounds

Molecular geometries for a set of 2-hydroxybenzoyl compounds were obtained at B3LYP/6-31G** level and analyzed in view of a parametric model of intrinsic substituent effects by Taft and Topsom. The structural study of the non- and hydrogen-bonded species, together with proton transferred forms, resulted as very useful in understanding the different factors determining the intramolecular hydrogen bond strength and the proton transfer process in this family of molecules. In addition, the previous study was extended to a sequence of other related six-membered hydrogen-bonded structures (alkane, naphthalene, and alkene derivatives) with increasing aromaticity. The results clearly showed the influence of the covalent and electrostatic (acid-base) nature of the hydrogen bond system on its commonly related chemical properties, hydrogen bond strength, and proton-transfer energy. A significant finding in this paper is the approach between the oxygens that yields the internal hydrogen bond, which occurs in the midpoint of the proton transfer, depends on the acid-base characteristics of the proton donor and acceptor groups, and it is not substantially affected by the aromaticity of the system.