An EPR characterisation of stable and transient reactive oxygen species formed under radiative and non-radiative conditions

Electron paramagnetic resonance (EPR) spectroscopy is the ideal method of choice when detecting and studying the wide variety of paramagnetic oxygen-centred radicals. For simple diatomic radicals, such as the superoxide (O-2(-)) or peroxy (ROO center dot) species, the CW EPR profile (in particular the g-values) of these species can appear similar and indeed indistinguishable in some cases. Experiments using O-17-enriched oxygen, revealing a rich O-17 hyperfine pattern, are therefore essential to distinguish between the two species. However, in many cases, particularly involving TiO2 photocatalysis, the peroxy-type (ROO center dot) radicals or other intermediate species such as the [O-2(-) ... organic]-type adducts can be transient in nature and once again can produce similar g-values. In general terms, these reactive oxygen species (ROS) are formed and detected at low-temperature conditions. Hence, the application of EPR spectroscopy to studies of surface-stabilised oxygen-centred radicals must be performed under carefully selected conditions in order to confidently distinguish between the differing types of diatomic radicals, such as O-2(-), ROO center dot or [O-2(-) ... organic].