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Abstract: The adsorption of 4-nitrobenzenediazonium tetrafluoroborate (NBD) in acidic aqueous solution was investigated by scanning electrochemical microscopy (SECM) on polycrystalline gold and glassy carbon (GC) substrates. The feedback mode of SECM was used to study the local properties of each substrate before and after immersion in the aryldiazonium solution, by performing approach curves with two redox mediators, and . The fitting of the approach curves with analytical approximate equation was used to evaluate the apparent heterogeneous rate constant k for redox system at NBD-adsorbed GC (k =2.6×10−4 cms−1). [Copyright &y& Elsevier]

Abstract: This paper examines the role of reactant mass transfer in the preparation of monolayers derived by the chemisorption of thiols on gold and related systems. As a starting point, equations are derived that relate the total impingement of thiol molecules, specifically n-hexanethiol, onto a gold surface for two substrate/deposition vessel geometries. The first model is based on a construct that delivers the reactant to the surface by linear diffusion. The second geometry mimics the case in which hemispherical mass transport is a critical component in the diffusional delivery of reactants. The relationships describe the effects of thiol concentration and immersion time solely on the impingement of the reactant on the gold surface with respect to both deposition geometries. With these predictive equations as guides, adlayer formation is followed by using infrared reflection-absorption spectroscopy, contact angle measurements, and assessments of heterogeneous electron-transfer kinetics. This work demonstrates that, in addition to pretreatment protocols of the substrate surface, the mode of reactant mass transport must be carefully considered and controlled in order to repetitively yield adlayers with comparable structures and, therefore, interfacial properties. Both the total impingement and the rate of impingement of the reactant are found to be major contributors, which implicates the role of nucleation and growth in ultimately dictating subtle, but important differences in the interfacial properties of the adlayer. [Copyright &y& Elsevier]

The oxidation of the CrIII hydroxy complex, TptBu,MeCr(pz′)(OH), 1, (TptBu,Me=hydrotris(3-tert-butyl-5-methylpyrazolyl)borate; pz′=3-tert-butyl-5-methylpyrazolyl anion) was studied in methylene chloride at a glassy carbon electrode. There are two oxidation peaks, Ia and IIa, that are totally irreversible and CrIV oxo complex 2 is the product at both peaks. Peak Ia grows with respect to IIa as the scan rate is reduced and the voltammograms were quantitatively accounted for by a CEirr mechanism involving two interconverting forms of 1, 1a giving peak Ia and 1b giving peak IIa. The reduction of 2 is detected on the return scan along with a small amount of 3 that arises from hydrogen-atom abstraction by 2 from hydrogen-atom donors in the medium. The processes giving rise to peaks Ia and IIa have the characteristics of dissociative electron- transfer reactions. It is suggested that they may be concerted electron-proton transfer reactions. It is speculated that the postulated minor form of 1, 1a, may be an isomer having axial OH rather than the equatorial OH found in the favored isomer, 1b. [Copyright &y& Elsevier]

Electrochemical impedance spectroscopy has been used to determine the heterogeneous electron-transfer kinetic parameters for the reduction of the following nitrobenzene derivatives in acetonitrile containing 0.10 M tetrabutylammonium hexafluorophosphate at a mercury working electrode: 2-methylnitrobenzene(1), 2,6-dimethylnitrobenzene(2), 2,4,6-trimethylnitrobenzene(3), 2,3,5,6-tetramethylnitrobenzene (4), pentamethylnitrobenzene (5), 2,4,6-triisopropylnitrobenzene (6) and 2,4,6-tri-tert-butylnitrobenzene (7). The results are discussed together with previously published results for nitromethane (8), nitroethane (9), 2-nitropropane (10) and 2-methyl-2-nitropropane (11). In general, a decrease in the standard electron-transfer rate constant, ks, is seen on going from 1 to 7 and this trend continues for 8–11. Double layer effects are shown to be of minor importance. A previous explanation, based on increasing outer reorganization energies on going from 1 to 11 is called into question by the fact that the Gibbs energies of solvation of the radical anions of 1, 3 and 8 have been found to be almost identical. Arguments are presented supporting the idea that the outer reorganization energy should scale linearly with the Gibbs energy of solvation. A previously presented model, used to explain the effect of electrolyte cation size on the values of ks, has been extended to include the effect of reactant size. It is inferred that the decrease in ks on going from 8 (CH3NO2) to 11 ((CH3)3CNO2) results at least partly from the increased size of the alkyl group, which causes the average distance of closest approach to increase from 8 to 11 resulting in an increased tunneling distance and smaller rate constants. This suggests that the “true” values of ks for 8–11, corrected for this distance effect, are roughly constant. Finally, semi-empirical molecular orbital methods (AM1; 6 and 7) and density functional theory (B3LYP; 1–5; 8–11) were used to estimate the contributions of the inner reorganization energy for 1–11. These contributions are small for 1 but steadily increase on going to 5. The inner reorganization contribution for 8–11 is quite substantial, ca. 4–5 kcal/mol (1 kcal/mol = 4.184 kJ/mol). This suggests that the inner reorganization term is a very significant factor underlying the sharp decrease in ks on going from 1 to 11. The principal structural change causing the calculated larger values of inner reorganization energy for 1–5 is the significant turning of the nitro group out of the plane of the ring in the neutral, a feature that is resisted in the radical anion. For 8–11, pyramidalization at nitrogen in the radical anion is the most significant structural change. [Copyright &y& Elsevier]

• Theoretical models of heterogeneous electron transfer can predict electrochemical responses at low temperature. • Kinetic and thermodynamic data can be extracted from various electrochemical methods at low temperature. • Chemically-coupled electron-transfer reactions can be better investigated by using low-temperature electrochemistry. This paper aims at providing theoretical basis on electrochemical processes performed at low temperature (T) according to three different aspects. First, the effect of T -decrease is treated in terms of thermodynamics, mass-transfer, and kinetics of electron transfer (ET) heterogeneous reactions. In particular, predictions of ET kinetics at low temperature are discussed in the frame of Marcus-Hush's model. The second part is focused on the changes associated to temperature decrease for different electrochemical methods including cyclic voltammetry (CV), chronoamperometry (CA), AC Impedance (ACI) and AC voltammetry (ACV). This section gives keys to extract electrochemical data from low- T experimental curves. In the third part, theoretical aspects of low- T cyclic voltammetry for multiple or chemically-coupled electron-transfer reactions are discussed. CVs of typical molecular mechanisms (EE, EC, ECE...) are provided in order to better visualize the impact of temperature on the redox behavior. [ABSTRACT FROM AUTHOR]