Density functional theory investigation of the redox properties of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes

The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L(3)UCl with L = C(5)H(5) (Cp), C(5)H(4)Me (MeCp), C(5)H(4)SiMe(3) (TMSCp), C(5)H(4)(t)Bu ((t)BuCp), C(5)Me(5) (Cp*), and C(4)Me(4)P (tmp), has been investigated using relativistic density functional theory calculations, with the solvent being taken into account using the conductor-like screening model. A very good linear correlation (r(2) = 0.99) has been obtained between the computed electron affinities of the L(3)UCl complexes and the experimental half-wave reduction potentials E(1/2) related to the U(IV)/U(III) redox systems. From a computational point of view, our study confirms the crucial importance of spin-orbit coupling and solvent corrections and the use of an extended basis set in order to achieve the best experiment-theory agreement. Considering oxidation of the uranium(IV) complexes, the instability of the uranium(V) derivatives [L(3)UCl](+) is revealed, in agreement with experimental electrochemical findings. The driving roles of both the electron-donating ability of the L ligand and the U 5f orbitals on the redox properties of the complexes are brought to light. Interestingly, we found and explained the excellent correlation between variations of the uranium Hirschfeld charges following U(IV)/U(III) electron capture and E(1/2). In addition, this work allowed one to estimate theoretically the half-wave reduction potential of [Cp*(3)UCl].     

Dissolved carbon dioxide effect on the behavior of carbon steel in a simulated solution at different temperatures and immersion times

Dissolved carbon dioxide effect on the behavior of carbon steel (0.4%carbon), in a simulated solution at different temperatures and immersion times, has been investigated using different techniques as potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy. The observation of the steel surface was done by scanning electron microscopy. All measurements reveal that the corrosion resistance is strongly dependent on both the temperature and immersion times. The corrosion resistance of carbon steel decreases in the solution considered as the temperature increases from 20?°C to 50?°C. Potentiodynamic polarization curves showed that cathodic and anodic current densities increase with the increased temperature. Besides, the study concluded that the addition of CO₂ gas to the simulated solution affects negatively the corrosion resistance in one hand and that, on the other hand, the higher the time of immersion the higher is the resistance to corrosion.     

Applying the Powell's Symmetrical Technique to Conjugate Gradient Methods with the Generalized Conjugacy Condition

This article proposes new conjugate gradient method for unconstrained optimization by applying the Powell symmetrical technique in a defined sense. Using the Wolfe line search conditions, the global convergence property of the method is also obtained based on the spectral analysis of the conjugate gradient iteration matrix and the Zoutendijk condition for steepest descent methods. Preliminary numerical results for a set of 86 unconstrained optimization test problems verify the performance of the algorithm and show that the Generalized Descent Symmetrical Hestenes-Stiefel algorithm is competitive with the Fletcher-Reeves (FR) and Polak-Ribiére-Polyak (PRP⁺) algorithms.