Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions

Abstract

High-level ab initio molecular orbital theory is used to obtain benchmark values for the ferricenium/ferrocene (Fc(+)/Fc) couple, the IUPAC recommended reference electrode for nonaqueous solution. The gas-phase ionization energy of ferrocene is calculated using the high-level composite method, G3(MP2)-RAD, and two higher-level variants of this method. These latter methods incorporate corrections for core correlation and, in the case of the highest level considered, use (RO)CCSD(T)/6-311+G(d,p) in place of (RO)CCSD(T)/6-31G(d) as the base level of theory. All methods provide good agreement with one another and the corresponding experimental values. Solvation energies have been calculated using PCM, CPCM, SMD, and COSMO-RS. Using G3(MP2)-RAD-Full-TZ gas-phase energies and COSMO-RS solvation energies, the absolute redox potentials of the Fc(+)/Fc couple have been calculated as 4.988, 4.927, and 5.043 V in acetonitrile, 1,2-dichloroethane, and dimethylsulfoxide solutions, respectively.

DFT predictions of the oxidation potential of organic dyes for opto-electronic devices

“…(2) is usually determined by using a thermodynamic cycle (Fig. 2) in which the free energy of reaction (1) is separated into the sum of the gas-phase free energy and the difference of solvation free energy between the oxidized and reduced species [12,9]:…”

“…[49,50,11,51]; that choice was dictated by the fact that electron densities and optimum geometries of some of the investigated dyes exhibit a large dependence on the dielectric environment [30]. For the same reason we have not tested the SMD solvation model [48], a method designed to compute absolute solvation energies starting from gas-phase optimization/Hessian calculations, although there is evidence that SMD is capable of very accurate predictions of redox free energy for small molecules [9,10].…”

“…[53] which assumes the electron in vacuum at 0 K as the zero energy level, appears to be the most acceptable and it is indeed widely used in computations [53,12,9]. By adopting the electron convention, according to which the electron is equivalent to an element thus having zero free energy and enthalpy of formation, and evaluating the integrated thermal capacity and entropy by FermiDirac statistics, the free energy of the electron turns out to be À0:038 eV at 298.15 K [54][55][56].…”

“…Post Hartree-Fock correlated methods proved to be very accurate in predicting oxidation potentials [9,10] but, because of the large size of the molecules which have found successful employment in technological devices, density functional theory (DFT) is the only approach which can provide sufficiently reliable results at a reasonable computational cost.…”

“…We have also included spiropyran compounds (9)(10)(11) for which there is a renewed interest because of their potential applications in optical recording media, photo switching devices, and nonlinear optics due to their reversible isomerization to merocyanine in response to light, to redox potentials, and to mechanical stress [26][27][28][29].…”