Ionization energy and reduction potential in ferrocene derivatives. The
effect of the exact exchange fraction in hybrid-DFT methods.
Abstract
Hybrid density functionals have been regularly applied in
state-of-the-art computational models for predicting reduction
potentials. Benchmark calculations of the absolute reduction potential
of ferricenium/ferrocene couple, the IUPAC-proposed reference in
nonaqueous solution, include the B3LYP/6-31G(d)/LanL2TZf protocol. We
used this procedure to calculate ionization energies and reduction
potentials for a comprehensive set of ferrocene derivatives. The
protocol works very well for a number of derivatives. However, a
significant discrepancy (> 1 V) between experimental and
calculated data was detected for selected cases. Three variables were
assessed to detect an origin of the observed failure: density
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. The accidental errors were observed for other hybrid
models like PBE0, BHandHLYP, and M06-2X. Therefore, hybrid DFT methods
should be used with caution, or pure functionals (BLYP or M06L) may be
used instead.