Solvent Effects g-Tensors of Nitroxide Radicals

The above two examples made it clear that the immediate environment of the radicals has a distinct effect on the g-values and hyperfine couplings of die spincarrying fragments. There are two principal types of effects to be distinguished (a) bulk or unspecific effects that are mainly related to the polarity of die solvent, and (b) specific effects diat depend on selective interactions between solvent and solute. In most cases these effects are related to the formation of hydrogen bonds. 

In a systematic study of these effects, the EPR parameters of two nitroxide radicals (Fig. 5) were studied as a function of the polarity of the solvent [133,134]. 

Solvent Agxx Myy BP86 A z iso Agx. B3LYP Agjy Ag iso Exptl. iso 

All values in ppm. Solvent optimized geometries were used. Reproduced with permission from [133]. Copyright 2006, American Chemical Society. 

the models were straightforwardly enhanced by explicit water and methanol molecules that were attached to the oxygen of the nitroxide group (Fig. 7). Upon recalculating these supermolecules inside a COSMO cavity, accurate results were found in comparison to the experimental values. Thus, it was concluded that the COSMO model enables one to aceurately study the solvent effeet on the g-tensors of organic radicals if possible hydrogen bonds with the solvent are explicitly treated. 

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