Solvent effect on the competition between weak and strong interactions in phenol solutions studied by near-infrared spectroscopy and DFT calculations

Near-infrared (NIR) spectra of phenol in a series of non-aromatic and aromatic solvents were recorded to study the competition between various types of solute-solute and solute-solvent interactions. Depending on the phenol concentration, the free OH and OH involved in the OHMIDLINE HORIZONTAL ELLIPSISOH interactions in the dimers and higher associates are present in cyclohexane solutions. On the other hand, free OH does not appear in Cl-containing solvents since at a low phenol content the OH groups participate in the OHMIDLINE HORIZONTAL ELLIPSISCl interactions. In CCl4 and tetrachloroethylene this interaction is weak, while in chlorobenzene the strength of this interaction is higher. In the aromatic solvents the solute-solute OHMIDLINE HORIZONTAL ELLIPSISOH interactions compete with the solute-solvent OHMIDLINE HORIZONTAL ELLIPSIS pi and aromatic CHMIDLINE HORIZONTAL ELLIPSISOH ones. Consequently, the degree of self-association of phenol in aromatic solvents is smaller than that in non-aromatic ones. The strength of the OHMIDLINE HORIZONTAL ELLIPSIS pi interactions increases with growing electron-donating ability of the substituents in the benzene derivatives. This observation obtained from the NIR spectra is in line with the results of the theoretical calculations (DFT). A clear correlation appears between the number of methyl groups in aromatic solvents and the population of the free OH groups. The methyl groups are steric hindrances and impede the formation of the OHMIDLINE HORIZONTAL ELLIPSISOH and OHMIDLINE HORIZONTAL ELLIPSIS pi interactions. Our results suggest the presence of aromatic CHMIDLINE HORIZONTAL ELLIPSISOH solute-solvent interactions, not observed in previous studies. NIR spectroscopy appears to be a powerful tool for exploration of free and weakly-bonded OH groups.

Automated summary

The study examined the competition between solute-solute and solute-solvent interactions in phenol dissolved in various non-aromatic and aromatic solvents by recording NIR spectra. It was found that the degree of self-association of phenol in aromatic solvents is lower compared to non-aromatic solvents, with the strength of OH pi interactions increasing with the electron-donating ability of benzene derivatives. Additionally, the presence of aromatic CH solute-solvent interactions was suggested, which was not observed in previous studies.