Temperature effect on proton-transfer equilibrium and IR spectra of chlorophenol–tributylamine systems

Abstract

Binary solutions of chlorophenols in tributylamine (TBA) have been studied by UV and IR spectroscopy at temperatures down to the glassy transition state of TBA (123 K). The pentachlorophenol–TBA system has also been studied at 13–123 K in TBA matrices prepared by co-deposition of the gaseous components on a cooled window. 2,4-Dichlorophenol, 2,4,5-trichlorophenol, 2,6-dichlorophenol and 2,4,6-trichlorophenol in solution show proton-transfer (PT) equilibria which shift sharply toward the PT species on cooling. No PT is detected in the case of 2-chlorophenol. Pentachlorophenol appears entirely as a PT ion pair across the whole temperature range both in solution and in a matrix. Significant influence of ion-pair self-association on the IR spectra of solutions has been found by comparison with spectra obtained in rigid matrices. All systems studied reveal broad, intense, temperature-dependent bands corresponding to protonic vibrations. An extremely intense and broad absorption, with a maximum shifted to 700–500 cm^–1, is observed when the PT equilibrium constant is close to unity. The contribution of different processes, e.g. hydrogen-bond strengthening, proton transfer and association of ion pairs, to the temperature transformations of IR spectra is discussed.