Relaxation processes in some intramolecularly hydrogen-bonded compounds
Dielectric studies on 2,4,6-tri- and penta-substituted phenols have been carried out in a polystyrene matrix. Two sets of absorption curves were observed for 2,4,6-tri- and pentahalogenophenols in widely separated temperature ranges. The high-temperature process was identified as molecular relaxation while the low-temperature process for all the halogenophenols is intramolecular. Both proton tunneling and group relaxation were considered as candidates for the intramolecular process, and the ΔHE values and the Eyring plots suggest that there was no need to invoke tunneling. There is a significant increment in these intramolecular ΔHE values between the 2,4,6-tri- and the pentahalogenophenols. Pentachlorobenzenethiol was also examined, and the ΔHE value for the intramolecular process is 12 kJ mol–1 less than for the corresponding phenolic compound. This is related, at least in part, to the weaker type of intramolecular hydrogen bond formed by the SH relative to the OH group.