Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity?

The radical cage effect is briefly reviewed with an emphasis on how radical mass and size affect the cage recombination efficiencies (F-cP) of radical cage pairs. The results of one study show that F-cP(-1) varies linearly wi th radical radius(-2) and radical mass(1 2), but additional studies are nee ded to determine if this result is general. The results of a new study on s olvent polarity effects are also presented. The effect of solvent polarity on the geminate recombination of polar Cp'Mo(CO)(3) (Cp' = eta (5)-C5H4CH3) radical cage pairs was studied by using a series of solvent systems that r anged from nonpolar to polar. The solvent systems consisted of a solvent an d a viscogen of similar polarity. The solvents were hexane-squalane, hexane -paraffin oil, glyme-polyglyme, THF-polyglyme, and ethanol-propylene glycol . The cage effects were identical in four of the five solvent systems (hexa ne-paraffin oil is the exception), and no trend with solvent polarity was o bserved. From these results, it is concluded that the influence of solvent polarity is either weak or absent altogether. A similar analysis of the dat a of Szwarc et al. (J. Am. Chem. Sec. 90 (1968) 278) for [H3C.,(CH3)-C-.] a nd [F3C.,(CF3)-C-.] cage pairs showed that, although there is a solvent eff ect. it cannot be attributed to solvent polarity. All of these results are consistent with a model for the cage effect in which geminate recombination of the Cp'Mo(CO)(3) radicals is faster than molecular rotation. Rotational correlation times and diffusion coefficients were measured in the solvents used herein and the results support the model. (C) 2001 Elsevier Science B .V. All rights reserved.