Transient radical species paramagnetic complexes

Coupling of rapid chemical quench or freeze-quench methods with ERR, MS, or NMR has been a very useful strategy for detecting reaction intermediates that are paramagnetic such as transition metal complexes in metalloenzymes and radical species " " or to determine the chemical structure of transient reaction intermediates or reaction products." ... 

Because of the sensitivity and selectivity of the ESR approach, it is a powerful method for studying free radicals in low concentrations in complex systems. ESR can be helpful in two ways to characterize and structurally identify radical intermediates and to obtain information on the kinetics and mechanisms of their reactions. These reactions can be very fast one of the main differences between ESR of biological free radicals and ESR of spin labels and metal ions is that in the former case one generally is dealing with transient paramagnetic species. 

---