Lindsay E. Roy

Calculation of one-electron redox potentials revisited. Is it possible to calculate accurate potentials with density functional methods?

Density Functional calculations have been performed to calculate the one-electron oxidation potential for ferrocene and the redox couples for a series of small transition metal compounds of the first-, second-, and third-row elements. The solvation effects are incorporated via a self-consistent reaction field (SCRF), using the polarized continuum model (PCM). From our study of seven different density functionals combined with three different basis sets for ferrocene, we find that no density functional method can reproduce the redox trends from experiment when referencing our results to the experimental absolute standard hydrogen electrode (SHE) potential. In addition, including additional necessary assumptions such as solvation effects does not lead to any conclusion regarding the appropriate functional. However, we propose that if one references their transition metal compounds results to the calculated absolute half-cell potential of ferrocene, they can circumvent the additional assumptions necessary to predict a redox couple. Upon employing this method on several organometallic and inorganic complexes, we obtained very good correlation between calculated and experimental values (R(2) = 0.97), making it possible to predict trends with a high level of confidence. The hybrid functional B3LYP systematically underestimates the redox potential; however, the linear correlation between DFT and experiment is good (R(2) = 0.96) when including a baseline shift. This protocol is a powerful tool that allows theoretical chemists to predict the redox potential in solution of several transition metal complexes a priori and aids in the rational design of redox-active catalysts.

Theoretical Studies on the Redox Potentials of Fe Dinuclear Complexes as Models for Hydrogenase

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.

d-Electron mediated 4f7–4f7 exchange in Gd-rich compounds; spin density functional study of Gd2Cl3

Abstract Spin-density functional theory (SDFT) calculations of the d–f exchange coupling for the pseudo-one-dimensional (pseudo-1-D) chain compound Gd2Cl3 has been carried out using the 1-D model, Gd8Cl12(OPH3)4, by considering seven variations in the ordering of the 4 f 7 moments. The calculations indicate that this semiconducting system should exhibit antiferromagnetic ordering of the 4 f 7 moments in a pattern consistent with published neutron diffraction data. An attempt to account for the calculated magnetic energies of spin patterns using an Ising model was unsuccessful, indicating that the latter model is inappropriate. The qualitative features can be interpreted using a perturbative molecular orbital model that focuses on the influence of the 4 f 7 –d exchange interaction on the d-based molecular orbitals. Fundamental to the d-electron-mediated exchange mechanism is the intra-atomic 4 f 7 –d exchange interaction. The essence of this interaction is present in the Gd atom [4 f 7 5d 1 6s 2 ] , which is computationally investigated within SDFT. In Gd2Cl3, the d-electron-mediated f–f exchange interaction was interpreted using basic perturbation theory. Computed density of states and spin polarization information was used to support the perturbation-theoretic analysis.

Rules for Understanding and Designing Novel Molecule-Based Rare-Earth Magnetic Compounds

Results of SDFT calculations were used to construct and check features of a generally applicable qualitative approach to understanding magnetic coupling in rare-earth-rich compounds. Using fragments based on structures of metal-rich lanthanide compounds, we have investigated molecular and low-dimensional extended structures, including Gd 3 I 6 (OPH 3 ) 12 , Gd 6 I 12 Co(OPH 3 ) 6 , and Gd 2 Cl 3 . Open- d -shell clusters facilitate strong ferromagnetic coupling whereas in the closed- d -shell systems prefer antiferromagnetic coupling. The f - d exchange interaction, mediated by spin polarization of both filled and partially-filled metal-metal bonding orbitals, was described for the model system Gd 3 I 6 (OPH 3 ) 12 n+ using basic perturbation methods. This method has been successful for predicting the magnetic ground state for models of Gd[Gd 6 I 12 Fe] and Gd 2 Cl 3 .