Solvent Cage Effects: A Comparison of Geminate and Nongeminate Radical Cage Pair Combination Efficiencies.

This study measured and compared the combination efficiencies (FcP) of geminate radical cage pairs to nongeminate (collisional) radical cage pairs (Fc'). For the [Cp'(CO)3Mo·, ·Mo(CO)3Cp'] radical cage pair, Fc' was found to be smaller than FcP in solutions having the same viscosity. It is proposed that the difference in FcP and Fc' arises because the radicals in the collisional cage pair are less likely to have the correct orbital orientation for radical-radical combination to occur, whereas photochemically generated geminate cage pairs are more likely to have the correct orbital orientation. As predicted, FcP and Fc' both increase when the microviscosity of a solution increases.

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types.

This study reports a method for correlating the radical recombination efficiencies (FcP) of geminate radical cage pairs to the properties of the solvent. Although bulk viscosity (macroviscosity) is typically used to predict or interpret radical recombination efficiencies, the work reported here shows that microviscosity is a much better parameter. The use of microviscosity is valid over a range of different solvent system types, including nonpolar, aromatic, polar, and hydrogen bonding solvents. In addition, the relationship of FcP to microviscosity holds for solvent systems containing mixtures of these solvent types. The microviscosities of the solvent systems were straightforwardly determined by measuring the diffusion coefficient of an appropriate probe by NMR DOSY spectroscopy. By using solvent mixtures, selective solvation was shown to not affect the correlation between FcP and microviscosity. In addition, neither solvent polarity nor radical rotation affects the correlation between FcP and the microviscosity.

Radical Cage Effects: Comparison of Solvent Bulk Viscosity and Microviscosity in Predicting the Recombination Efficiencies of Radical Cage Pairs.

This study reports the results of experiments that probed how solvents affect the recombination efficiency (FcP) of geminate radical cage pairs. The macroviscosity of solvents has traditionally been used to make quantitative predictions about FcP, but experiments reported here show that FcP varies dramatically for solvent systems with identical macroviscosities. Experiments show that FcP correlates with the solvent microviscosity: five different solvent systems (consisting of a solvent and a structurally similar viscogen) were examined, and FcP was the same for all five solvent systems at any particular microviscosity. The translational diffusion coefficient of the radicals (measured by DOSY) in the solvent system was used to define the microviscosity of the solvent system.