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I am interested in calculating some redox potentials of organic compounds (H,C,N,O,F,Cl,Br) in organic solvents such as MeCN. I do not need absolute agreement with experiments since this is just a first pass to identify some potential candidates for experiments, so I am looking for the best simple approach to this problem. An accuracy of +/- 0.5V would be sufficient for example, if this is doable.

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There are several protocols. Since you mention that you're mostly looking for trends, I'll give the faster method, e.g. by Jason Gilmore:

"Expanding and Testing a Computational Method for Predicting the Ground State Reduction Potentials of Organic Molecules on the Basis of Empirical Correlation to Experiment" J. Org. Chem. 2012, 77, 15, 6423–6430

  • Optimize the geometry of the neutral and charged species in gas phase (e.g., a faster optimization)
  • Optimize the geometry of the neutral and charged species in solution (e.g., using a polarizable continuum model for your solvent) with a good density functional and basis set.

The 2012 paper from Gilmore mentions optimizing in the gas phase and taking single point energies in solution.

I'd recommend at least:

  • Using a "better" basis set (e.g, from the def2 or pc- series basis sets)
  • Using dispersion correction (this likely won't make much difference but will give you better energies)
  • Using, e.g. $\omega$B97X-D3 or some newer functional (although B3LYP/6-31G* still works remarkably well)

Anyway, the main point is this: Take the difference in total energy between the optimized neutral molecule ($S_0$) and the radical cation or radical anion ($D_0$) in the solution calculation.

This is the ionization potential for that species in solution.

It's not a full redox potential - for that you'll want a full thermodynamic cycle but it yields great trends that correlate very well for many organic redox potentials in acetonitrile.

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    $\begingroup$ +10 for quite a thorough answer to an unanswered but potentially very popular question. Useful to our users without a doubt! $\endgroup$ Dec 27 '20 at 4:10
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    $\begingroup$ Interestingly enough, this paper actually reports quite a few of the compounds we would consider using. A better answer than even expected. $\endgroup$ Dec 27 '20 at 15:48

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