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I calculated the redox potential with Gaussian for hydroquinone with different combinations of functional/basis sets always values like 5.4V (B3LYP/6-311G+(2d,p) and SMD model). The experimental value is about 1.1V according to [1], so I'm missing by a decent amount.

Has somebody an idea what I did wrong?

These are the energies I calculated:

SCFE (g) (molecule) =   -382.81331295
SCFE (solv) (molecule) =  -382.82970187
GibbsCorr (molecule) =    0.077174
SCFE (g) (species) =     -382.53186782
SCFE (solv) (species) = -382.63171676
GibbsCorr (species) =   0.078470

and an example job file for the optimization of the charged species:

%rwf=bench_40_spec_b3lyp_tight.rwf
%NoSave
%chk=bench_40_spec_b3lyp_tight.chk
#p opt=(calcfc,tight,recalcfc=3) b3lyp/6-311+g(2d,p) nosymm scf=qc

040 Tight Opt Gas

1 2
 C                 -0.68613672   -1.20444633   -0.00000200
 C                  0.69506215   -1.19541581   -0.00000235
 C                  1.39161904    0.00718044    0.00000031
 C                  0.68613843    1.20444943   -0.00000167
 C                 -0.69505913    1.19541474   -0.00000053
 C                 -1.39161802   -0.00718091    0.00000159
 O                 -2.75490670   -0.05822072   -0.00000096
 H                 -3.11159203    0.83776787    0.00004645
 H                 -1.23719750    2.13172433   -0.00000924
 H                  1.23422725    2.13274296   -0.00001200
 O                  2.75490286    0.05821954    0.00000081
 H                  3.11158561   -0.83777105    0.00003365
 H                  1.23719782   -2.13172346   -0.00000066
 H                 -1.23422866   -2.13274050   -0.00000041

I used the method linked in here: Redox Method

References:

  1. Neugebauer, H.; Bohle, F.; Bursch, M.; Hansen, A.; Grimme, S. Benchmark Study of Electrochemical Redox Potentials Calculated with Semiempirical and DFT Methods. J. Phys. Chem. A 2020, 124 (35), 7166–7176. DOI: 10.1021/acs.jpca.0c05052.
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  • $\begingroup$ It would be helpful to know what you specifically you got for the potential using the energies in the question. One thing to note is the linked paper subtracts the potential for a reference electrode to obtain its results. I can't find in the paper what value that should have, but it may make up for most of the discrepancy. Wikipedia has an estimate of 4.44V for the absolute potential of the standard hydrogen electrode. $\endgroup$
    – Tyberius ♦
    Jan 12, 2022 at 22:19
  • $\begingroup$ I got 5.4V, edited in the post. $\endgroup$
    – Andrea
    Jan 12, 2022 at 22:32

1 Answer 1

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Your reference paper says that it subtracts the "absolute potential of the reference electrode" from the calculated values. I can't get to the citation that explains what reference electrode they are referring to, but the most common is the Standard hydrogen electrode, which is estimated to have an absolute potential of $\pu{4.44V}$.

Subtracting this from the potential of $\pu{5.4V}$ that you calculated gives $\pu{0.96V}$. This is only off by ~$\pu{0.1V}$, which is well in line with the errors reported in the paper for DFT and semiempirical methods.

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  • $\begingroup$ Thank you very much for your support! Do you got an idea why Grimme et al. subtract die reference electrode and the paper from yale university (linked in the other thread) do not need to be? $\endgroup$
    – Andrea
    Jan 12, 2022 at 23:23
  • $\begingroup$ @Andrea I wonder why you clicked "accept" but didn't issue a vote to the question? See this. $\endgroup$
    – Nike Dattani
    Jan 12, 2022 at 23:57
  • $\begingroup$ @Andrea I think the tutorial is wrong. Ferrocene (the molecule considered) should have a potential of 0.4 V vs the standard hydrogen electrode (SHE). Even adding back in the absolute potential, it's much too small to match the 5.52 V they find. The experimental reference they mention doesn't seem to list the value of 5.3 V anywhere, just the 0.4 V against the SHE. $\endgroup$
    – Tyberius ♦
    Jan 13, 2022 at 0:37

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