The effects of substituents and solvents on the conformation of benzophenones

Abstract

Motivation. Natural and synthetic benzophenones are compounds of great scientific and applied interest due to the varied biological and physicochemical properties that they posses. As part of a program aimed at investigating new physicochemical properties of substituted benzophenones, we carry out a study on the influence of the substituents and solvents upon the molecular conformations of these compounds. In the future, we expect to propose relationships between the molecular structural parameters with diverse experimental properties of the compounds that are currently being determined in our laboratory. Method. The HF/6–31G(d) and B3LYP/6–31G(d) methods were used for calculations. Onsager’s and Tomasi’s models were used to analyze the solvent effects on the nine benzophenones studied. Results. A conformational equilibrium was proposed between the two conformers of benzophenones with the greatest thermodynamic stability. The total energies, torsional angles, dipole moments, structural and reactivity molecular parameters, and conformational equilibrium constants were calculated in vacuum, ethanol, methanol and water. The calculated magnitudes were correlated with Hammett’s substituent constants and parameters of solvating solvents. Conclusions. The benzophenones investigated have non–planar structures. Very good linear relationships between the molecular dihedral angles and Hammett’s p–substituent constants were obtained. Electron–donating substituents increase the planarity of the substituted benzoyl group that involves phenyl ring A of the molecules, as well as the folding of phenyl ring B. The conformational equilibrium constants of o–hydroxy–benzophenones ranged approximately between 10–7–10–6. On the other hand, it was concluded that greater hydrogen–bond donor capability of a solvent increases the polarization of the compounds and consequently, increases their dipole moments. It was further concluded that the electron–donating groups increase the strength of the intramolecular hydrogen bond and that in the solvents used the o–hydroxy–benzophenones possess an important intramolecular hydrogen bond of the moderate class