Criteria stable free radicals

Kazmaier, R. M., Moffat, K. A., Georges, M. K., Veregin, R. R. N., Hamer, G. K., Free-Radical Rolymerization for Narrow-Rolydispersity Resins. Semiempirical Molecular Orbital Calculations as a Criterion for Selecting Stable Free-Radical Reversible Terminators, MacromolecM 1995, 28, 1841-1846. 

A first simultaneous EPR and visible spectropho-tometric study is reported on the interaction of the stable free radical l,l-diphenyl-2-picrylhydrazyl (DPPH) dissolved in ethanol with thioglycolic acid (HSCH2CO2H, TGA). The results of the kinetic studies at room temperature allow us to assume 1 1 stoichiometry of the reaction between DPPH and TGA giving l,l-diphenyl-2-picrylhydrazine (DPPH) and thioglycolic disulfide. The linear plots of EPR and UV-visible responses versus the quantity of added TGA are used to find the DPPH molar absorptivity at 520 nm to be 12350 3% Imol cm which may be used as a criterion for the purity of the material itself. It was also found that the paramagnetic and optical properties of a 30-year old sample gave results suggesting that in the solid state DPPH is a fairly stable material. 

The criterion of reasonablenesses not so vague as it might appear. The fact iiat a particular structure seems reasonable to us means that wc ha e previously encountered a compound whose properties are pretty well accounted for by a structure of that type the structure must, therefore, represent a fairly stable kind of arrangement of atoms and electrons. For example, each of the contributing structures for the allyl radical appears quite reasonable because we have encountered compounds, alkenes and free radicals, that possess the features of this structure. 

In a general sense, a stable compound is resistant to degradation by heat, oxidation, or hydrolysis. By any criterion, the simple transition metal alkyls (neutral compounds which have only alkyl ligands on the metal atom) are very unstable. Dimethylmanganese, for example, is a spontaneously flammable solid which explodes on impact. Most other simple transition metal alkyls seem to be thermodynamically unstable relative to alkyl radicals and the free metal, although quantitative data are not available. 

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