Single Crystal EPR Studies of Radical Pairs in Dibenzoyl Peroxide

Abstract

Four different phenyl-benzoyloxy (PB) radical pairs and two different benzoyloxy-benzoyloxy (BB) pairs, each existing in four symmetry-related orientations, have been observed by epr in single crystals of dibenzoyl peroxide after brief uv irradiation at low temperature. At 5 K two PB pairs are formed; PBT disappears in less than 2 minutes at 5 K. but PB5K persists to 15 K, where it converts to PB15K in a process that involves 39° in-plane rotation of the benzoyloxy radical. Further warming to 24 K generates PB24K from an unidentified precursor which gives no epr spectrum and is destroyed by visible light. PB15K and PB24K persist to 65 K. Irradiation at 10 K forms a BB pair which converts cleanly to a second BB at 25 K. Stability of the pairs and distinguishability of PB pairs are discussed in light of a crystal packing analysis.