Determination of unimolecular rate data from reaction mid-point temperatures derived from EPR Spectra

The activation energies of unimolecular homolytic reactions (E-r) are predicted to correlate linearly with the mid-point temperatures (T-mild) defined as the temperature at which the rearranged and unrearranged species are of equal concentration, for photochemically initiated processes under EPR conditions. A linear relationship (E-r/kcal mol(-1) = 0.044T(mid) + 0.22) has been demonstrated from experimental data and shown to hold for a range of solvents and instrumental configurations. The relationship will facilitate the determination of kinetic parameters, particularly for processes where the spectra of the intermediates are of low intensity.