Redox processes in rigid matrices with specific reference to irradiated nitromethane

Abstract

E.s.r. studies of solids exposed to ionizing radiation at low temperatures suggest that the primary mechanism is one of electron ejection. This is frequently followed by specific electron capture, so that two primary radical species are trapped in the rigid matrix, the electron-loss centre and the electron-gain centre. For non-ionic matrices these are radical cations and radical anions. Because of the initial mobility of the electron these centres are usually too far apart for the detection of spin–spin interactions. By using specific matrices which only trap electron-loss centres efficiently, specific electron-gain centres (S˙^–) can be prepared from a range of solutes (S). Similarly, if the matrix reacts rapidly with electrons, but the hole-centre remains mobile, specific electron-loss centres (S˙⁺) can be prepared. However, if either the electrons or the primary hole-centres are not trapped, electron return may dominate. This may lead to the formation of an excited state of a matrix molecule which may then undergo bond homolysis. The resulting radicals may then be detected by e.s.r. spectroscopy. Sometimes they are trapped in close proximity to give triplet states. All these processes are illustrated by reference to nitromethane (CH₃NO₂, ¹³CH₃NO₂ and CD₃NO₂). Dilute solutions in CD₃OD gave MeNO^–₂ radical anions, and dilute solutions in CFCl₃ gave the cations, MeNO^˙+₂. The latter were found to exist initially as the σ-radicals H₃C˙NO⁺₂, the SOMO comprising σ-orbitals on carbon and nitrogen. These readily rearranged to give radicals identified as ONO—CH₃. In marked contrast, exposure of pure nitromethane to γ-rays at 77 K gave mainly ˙CH₃(˙CD₃) radicals and ˙NO₂ radicals. An important extra species comprised a triplet-state radical pair shown by analysis of the g= 2 and g= 4 features to be MeN(O)(OMe)˙NO₂ pairs with a spin-spin separation of ca. 5.6 Å. Mechanisms for the formation of these species are outlined.