Studies by electron spin resonance of the reactions of alkylperoxy radicals. Part 2.—Equilibrium between alkylperoxy radicals and tetroxide molecules

Abstract

Photolytically generated alkylperoxy radicals have been detected and their concentration measured by electron spin resonance spectroscopy. The rate of the irreversible decay of tertiary alkylperoxy radicals is negligible below –115°C. However, within the temperature range –160 to –115°C, a reduction in temperature causes the concentration of radicals to decrease to a new value which, once achieved does not vary with time. The process is reversible. This behaviour is attributed to the existence of an equilibrium between two alkylperoxy radicals and a tetroxide molecule. Average values for the associated changes in entropy ΔS° and enthalpy ΔH° are –38 ± 4 cal deg^–1 mol^–1 and –8.9 ± 0.5 kcal mol^–1 respectively. These thermodynamic constants have been combined with the rates of the irreversible decay of alkylperoxy radicals, measured at higher temperatures (–60 to +20°C) to provide a more complete description of the termination process.