Quantum chemical analysis of intermolecular hydrogen bonds in associates of 4-[(4'-butoxyphenyl)diazenyl]benzenesulfonic acid and its H-complexes with pyridine derivatives

Abstract
A study of conformational properties of 4-[(4-butoxyphenyl)diazenyl]-benzenesulfonic acid have been carried out. During the study 8 conformers were identified. The main geometric parameters were determined for all the conformers and ions, and it was found that they are slightly different. The exception were the values of torsion angles that determine the conformers shape. The vibration frequencies were also calculated. The energy parameters of the process of gas-phase deprotonation of the studied acid were calculated. It was determined that the gas-phase acidity of the studied sulfonic acid is higher than for unsubstituted benzene sulfonic acid and most benzene sulfonic acids with electron-donating substituents, but lower than for benzene disulfonic acids and benzene sulfonic acids with electron-withdrawing substituents. Modeling of homomolecular complexes in the form of dimers and trimers was performed. The energies of complex formation were calculated and the main characteristics of intermolecular hydrogen bonds in H-complexes were determined. The geometric and electronic structure of hydrogen-bonded heteromolecular complexes of the studied acid with pyridine derivatives Py-X-Py of composition 1:1 and 2:1 was also determined. The energies of complex formation were calculated and the main characteristics of intermolecular hydrogen bonds in H-complexes were determined. The strongest H-bonds were formed when the bridging groups –CH₂–CH₂– and –Ph– were introduced into the systems under study. The influence of solvent polarity on the appearance and characteristics of H-complexes is shown. Increasing the solvent polarity leads to a decrease in the HN bond length, while in more polar solvents there is a complete proton transfer from the sulfo group to the pyridine fragment.

Keywords: hydrogen bond, proton transfer, quantum chemical calculations, DFT, sulfonic acids, gas-phase acidity, supramolecular complexes