We provide a theoretical frame for in situ scanning tunneling microscopy (STM) of adsorbate molecules with low-lying redox levels strongly coupled to the environmental nuclear motion. The STM process is viewed as a coherent two-step electron transfer (ET) involving electron exchange between the local redox level and the manifolds of electronic levels in the substrate and tip. The notion coherence is here taken to imply that the intermediate electron or hole state after the first ET step does not fully relax vibrationally before the second ET step. These views and the theoretical formalism are appropriate to in situ STM of large transition metal complexes and redox metalloproteins. The formalism offers two kinds of spectroscopic features. One is the relation between the tunnel current and the bias voltage at fixed overvoltage of either the tip or the substrate relative to a reference electrode. The other one is the tunnel current dependence on the overvoltage, at fixed bias voltage. Recent data on tunneling current patterns for adsorbed or covalently tethered metalloporphyrins and the blue single-copper protein azurin are discussed in terms of the formalism.

The objective of this study is to suggest an interaction mechanism for the influence of static magnetic fields on electrochemical processes occurring at a ferromagnetic electrode immersed in a paramagnetic electrolytic solution. The hypothesis is that the magnetic field will cause a transport of all ions due to the difference in the magnetic susceptibility in the solution at the electrode surface. The ion transport induced by the magnetic field is directed from electrode into solution. Experimentally, the effects of static magnetic fields on electrochemical systems were observed only within systems consisting of ferromagnetic electrodes immersed in paramagnetic solutions. The results showed that the magnetic field caused an anodic polarization for the ferric/ferrous system and a cathodic polarization for the nickel/nickel-ion and the cobalt/cobalt-ion system. The results were obtained by the open-circuit potential and the potentiostatic/galvanostatic methods. The suggested interaction mechanism is magnetoconvection, which predicts that to obtain any magnetic field effect, there has to exist a gradient of paramagnetic ion concentration in the solution at the electrode surface. Theoretically, it is shown that the magnetic field tends to cause an additional convective transfer of all components of the solution, which will be generated in the vicinity of the electrode surface. Further, both the experimental results and the suggested mechanism show that the magnetic field effect increases with increasing magnetic flux density and magnetic susceptibility of the solution and decreases with increasing temperature and stirring rate. The evidence presented here show that the proposed hypothesis and the proposed interaction mechanism are verified.

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An approach is suggested in order to investigate the mechanism of interfacial electron-transfer processes with reagents of nonspherical form and complicated charge distribution. Chelate chromium(III) ethylenediaminetetraacetate (EDTA) complexes are regarded as a good model system. A series of SCF quantum chemical calculations at the ZINDO/1 level were performed for the systems Cr(EDTA)(H2O) n - (n = 0, 1, 2, 3, and 4). A quinquedentate complex Cr(EDTA)H2O- was found to be energetically more favorable compared to Cr(EDTA)-. The interaction of Cr(EDTA)- with a cadmium electrode (modeled as a 19-atomic planar cluster) was investigated as well. The analysis of both the potential energy surfaces and the partial charge transfer allows the explanation of the absence of “specific” interaction between Cr(EDTA)- complexes and mercury-like metals observed experimentally. A way to estimate the inner-sphere contribution (E in) to reorganization energy is proposed. The inner-sphere asymmetry (nonequality of E in values for reduction and oxidation) was found, which plays a significant role in the theoretical analysis of the activation energy. To obtain estimates of the solvent reorganization energy, the shape of reagents was approximated by ellipsoids. The detailed atomic charge distribution in oxidized and reduced complexes was employed to provide a “microscopic” description of electrical double layer effects. Additional ab initio SCF calculations with several basis sets of different quality were performed for the analysis of the charge distributions. The activation energies for several orientations of Cr(EDTA)- and Cr(EDTA)H2O- relative to the metal surface were calculated for two values of the electrode charge densities. It is concluded that a sufficient interpretation of relevant experimental data is not possible without calculations of the reaction pre-exponent.