Characterization of organic free radicals. Structure and magnetic properties

2 Characterization of organic free radicals. Structure and magnetic properties. 

Organic free radicals are key intermediates in a number of reactions of biological significance. For istance, there is strong evidence that the biosynthesis of several natural substances and many enzymatic reactions involve amino acid radicals[126-129], and radiation damage to DNA is known to proceed through a number of base-centered radicals[130-132]. Furthermore organic free radicals can be exploited as spin probes in the study of macromolecular systems by means of EPR spectroscopy [133]. 

While a full characterization of these intermediates would provide significant mechanistic information, this task is not simple from an experimental point of view since usually only spectroscopic techniques can be enq loyed and the relationships between spectroscopic parameters and structural features is quite indirect. Furthermore, the measured quantities often result from the superposition of different contributions which are very difficult, when not impossible, to separate. In such circumstances, quantum mechanical computations can provide an invaluable support to experiment since they are able to selectively switch a number of interactions on and off, thus allowing an unbiased evaluation of the effect of different contributions. 

The following description of the theoretical characterization of organic radicals can be considered as a template both for the methodological approach to 

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