EPR AND ENDOR STUDY ON MIXED-CRYSTALS OF AN INDOLINONE NITROXIDE RADICAL AND THE ISOELECTRONIC KETONE .2. EPR AND ENDOR INVESTIGATION ON RADICAL PAIRS IN MIXED-CRYSTALS

The EPR spectrum due to radical pairs was studied in mixed crystals containing 5% of a nitroxide indolinone radical and 95% of the isoelectronic ketone. Each radical pair gives rise to a pair of lines in the wings of the main spectrum. The first order analysis of the angular dependence of the fine splittings gave the electron-electron dipolar tensors due to three different pairs. The comparison of the experimental tensors with the calculated ones allowed us to identify the radical pairs in the crystal structure. The temperature dependencies of the EPR intensities of two types of radical pairs were analysed and the singlet-triplet energy separations were obtained. The magnetic susceptibility of the pure radical crystal has been determined and compared with the results of the EPR of the mixed crystals. An ENDOR investigation was performed on the radical pairs at T = 5 K. The spectra can be explained by taking into account the inter radical electron-nuclear dipolar interaction.