engleski

Calculation of redox potential in ferrocene derivatives. The effect of the exact exchange fraction in hybrid DFT methods.

The B3LYP method, the most popular hybrid density functional, is regularly applied in state-ofthe- art computational models for predicting redox potentials. Benchmark calculations of absolute reduction potential of ferricinium/ferrocene couple, the IUPAC-proposed reference in nonaqueous solution, include the SMD-B3LYP/6-31G(d)/LanL2TZf protocol. We used this procedure to calculate redox potential for a series of ferrocene derivatives. In a number of cases a significant discrepancy between experimental and calculated data was detected. The cases include substituents bonded to ferrocene system via C, S, N, B, or P atom. Most of them are monosubstituted, some are disubstituted, including ansa-derivatives. The same error appeared in a large number of ferrocene derivatives collected throughout the literature. This DFT method overestimates the stability of oxidized ferrocene species (ferricinium radical). Therefore, the calculated value of redox potential for specific ferrocene derivative is too low, and the compound appears/pretends as an extremely strong reducing agent. The deviation amounts to 1000 mV, which corresponds to energy equivalent of 100 kJ/mol. Three variables were assessed to detect a possible origin of the observed failure: functional, basis set, and solvation model. It comes out that the Hartree-Fock exchange fraction in hybrid DFT methods is the main source of the error.

redox potential, ferrocene, DFT

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