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My question is straightforward and is outlined in the post title: Is it essential to include the solvent cage effect in radical reactions during the computational estimation of Gibbs free energies and activation energies?

To be more exact, I do refer to these two papers:

  1. https://doi.org/10.1002/chem.19970030208 (original formulation of correction)
  2. https://doi.org/10.1021/jp0540499 (other papers which to some point validate the aforementioned but only ionic reactions are tested)

The crux of the matter is whether it is correct to use this correction in the case of reactions involving neutral/ionic species and radicals. Given that the reactivity of the latter process is inherently greater, could the inclusion of the correction artificially enhance the feasibility of the process?

Thanks.

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From:

J. Am. Chem. Soc. 2016, 138, 30, 9389–9392 Publication Date:July 19, 2016 https://doi.org/10.1021/jacs.6b05432

"In conclusion, the experiments reported here suggest that for nonpolar solvent systems quantitative discussions pertaining to FcP should be based on microviscosity rather than bulk viscosity. In essence, if the translational diffusion coefficient for a particular radical in a cage pair is known then an accurate cage recombination efficiency is predictable, independent of the solvent system. This predictive power will be useful wherever quantitative knowledge of radical reactivity is necessary. It is noted that the method described above can be applied in reverse. That is, if FcP is known then the diffusion coefficient of the molecule can be determined; in turn, the microviscosity of the local environment around the caged molecules can be probed. This reverse application would provide a method for determining the microviscosity in complex systems such as active sites of enzymes or in heterogeneous reactions. Polar solvent systems and hydrogen-bonding solvent systems are currently being investigated in our laboratory to determine if the results obtained above with nonpolar solvents are also applicable in these types of solvents."

My reading here is they have lab and modelling work to back up that solvent cage effects do apply to radical reactions -- at least in non-polar solvents. Barry and Tyler have a few papers out on this subject, including this: https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01441

So if this doesn't answer your question, it may also be useful to reach out to them directly for a chat.

You could also just model the system with and without solvent cage effects and compare. Without wet lab results to back it up, it will be difficult to determine accuracy, but you might find one of the methods ends up with something easily identifiably false.

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