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 with radical radius(-2) and radical mass(1 2), but additional studies are needed to determine if this result is general. The results of a new study on solvent 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 ranged from nonpolar to polar. The solvent systems consisted of a solvent and 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 (hexane-paraffin oil is the exception), and no trend with solvent polarity was observed. From these results, it is concluded that the influence of solvent polarity is either weak or absent altogether. A similar analysis of the data of Szwarc et al. (J. Am. Chem. Sec. 90 (1968) 278) for [H3C.,(CH3)-C-.] and [F3C.,(CF3)-C-.] cage pairs showed that, although there is a solvent effect. 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.