Mechanist challenge

A number of mechanistic challenges remain. Unlike neutral free radicals, radical ions also possess charge and thus their reactivity is sensitive to environmental effects (i.e. counterion, solvent). Thus, there remains much that needs to be learned both about this important class of intermediates, as well as about the role of these environmental factors, before this chemistry can be completely understood and exploited. 

In addition to thermodynamic considerations, the mechanistic challenge of the formation of the (0-0) group from two water molecules is a vital factor in the design of a realistic model. Systems that include di-fi-oxo bridging between two Mn(III) or —(IV) ions, such as the... 

The oxidation of water by Photosystem II in oxygenic photosynthesis is an energetically and mechanistically challenging process, in which... 

Mechanistic challenges faced by the enzyme include the mismatch in redox potentials for the free enzyme (—145 mV) and substrate (—40 mV), and the basicity of the substrates a-protons ( 20). Much informa-... 

Although the mechanistic insights for the model complex of [FeFe] hydrogenase became clearer little by little, those for the [NiFe] hydrogenase are still challenging topics. 

Strategies to determine the nature of reacting RIs in solids are often based on product analyses, stereochemical correlations, kinetics measurements, and so forth, just as they are generally applied in solution media3 While there are challenges associated with the implementation of spectroscopic methods in the solid state, the direct observation of short-lived RIs can be used to support or reject mechanistic models. 

---