Chemical exchange dynamics and structure of intramolecular OHO bridges: an ESR and indo study of 2-hydroxy- and 2,6-dihydroxy-phenoxyl

Abstract

ESR spectra of phenoxyl-type radicals of 1,2-dihydroxy benzene (pyrocatechol), 1,2,3-trihydroxy benzene (pyrogallol) and their deuteroxy modifications have been studied in CCl 4 and CCl 3F solution over a temperature range −100 ÷ +70°C. The spectrum of 2-hydroxy (deuteroxy)-phenoxy radical shows a pronounced temperature dependence, which will be analyzed by a chemical exchange model involving the proton (deuteron) of the internal H-bridge. The exchange process is described by a double minimum potential. The barrier found for this process is 8.2 kcal/mol for the proton exchange and 9.5 kcal/mol for the deuteron exchange. In contrast the ESR spectrum of the radical produced from pyrogallol is found to be temperature independent in the temperature range −100 ÷ +25°C. Analysis of the spectrum of this radical will be shown to result from the 2,6-dihydroxyphenoxy radical. The kinetic isotope effect in the chemical exchange process of the 2-hydroxyphenoxy radical is found to be extremely large, with k H/ k D ≈ 100 at room temperature. An INDO study is presented for the proton hyperfine coupling constants and for the potential function of the proton in the hydrogen bridge. Some discussion is devoted to the line width effects of the deuterated radical.