W. Ronald Fawcett

Studies of electron transfer through self-assembled monolayers using impedance spectroscopy

An investigation of the kinetics of reduction of Ru(NH 3 ) 6 3+ has been carried out at single crystal electrodes modified by alkanethiols of varying chain lengths. Careful study of the impedance of the modified electrode system in the absence of the reaction has shown that there is a highly resistive path in parallel to the capacitance due to the self-assembled monolayer. Analysis of the kinetic data and extrapolation of the Tafel plots show that the standard rate constant for the Ru(NH 3 ) 6 3+ /2 + system is 1.7 cm s -1 an unmodified Au surface with a transfer coefficient close to 0.5. These parameters are expected for a very fast simple heterogeneous electron transfer reaction. The present results are compared with whose reported previously in the literature.

Monte Carlo, density functional theory, and Poisson-Boltzmann theory study of the structure of an electrolyte near an electrode

Monte Carlo (MC) and density functional theory (DFT) results are reported for an electrolyte, consisting of charged hard spheres of diameter 3 A with the solvent modeled as a dielectric continuum, near a charged flat uniformly charged electrode. These results are more interesting than the earlier MC results of Torrie and Valleau [J. Chem. Phys. 73, 5807 (1980); J. Phys. Chem. 86, 3251 (1982)] for 4.25 A spheres because the popular Gouy–Chapman (GC) theory is less successful for this system. The DFT results are in good agreement with the MC results. Both the MC and DFT results show particularly interesting features when the counterions are divalent. For such divalent counterions, the diffuse layer potential passes through a maximum magnitude, then declines, and ultimately has a sign that is opposite to that of the electrode charge. The consequences of this behavior are discussed. In contrast, the well-known GC theory consistently overestimates the magnitude of the diffuse layer potential, does not have any...

The Ionic Work Function and its Role in Estimating Absolute Electrode Potentials

The experimental determination of the ionic work function is briefly described. Data for the proton, alkali metal ions, and halide ions in water, originally published by Randles (Randles, J. E. B. Trans Faraday Soc. 1956, 52, 1573) are recalculated on the basis of up-to-date thermodynamic tables. These calculations are extended to data for the same ions in four nonaqueous solvents, namely, methanol, ethanol, acetonitrile, and dimethyl sulfoxide. The ionic work function data are compared with estimates of the absolute Gibbs energy of solvation obtained by an extrathermodynamic route for the same ions. The work function data for the proton are used to estimate the absolute potential of the standard hydrogen electrode in each solvent. The results obtained here are compared with those published earlier by Trasatti (Trasatti, S. Electrochim. Acta 1987, 32, 843) and more recently by Kelly et al. (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066. Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2007, 111, 408). A comparison of the ionic work function with the absolute Gibbs solvation energy permits an estimation of the surface potential of the solvent. The results show that the surface potential of water is small and positive whereas the surface potential of the nonaqueous solvents considered is negative. The sign of the surface potential is consistent with the known structure of each solvent.

Solvent-induced frequency shifts in the infrared spectrum of acetone in organic solvents

The solvent-induced frequency shifts (SIFS) for the C≡N stretching vibration (ν 2 ) in acetonitrile and deuterated acetonitrile have been determined in a wide variety of organic solvents. It is shown that this band is shifted to higher energies in the presence of solvents which are more stronger Lewis acids than acetonitrile itself. On the other hand, in the presence of solvents which are stronger Lewis bases, the ν 2 band is shifted to lower energies. This is taken to be evidence that the electronegative end of the Lewis base interacts with the methyl group in acetonitrile. Supporting evidence for this conclusion is reported on the basis of shifts in the symmetrical (ν 1 ) and antisymmetrical (ν 5 ) CD3 stretching bands

Solvent Effects on Simple Electron Transfer Reactions. A Comparison of Results for Homogeneous and Heterogeneous Systems

Abstract Solvent effects on the rate constants for both homogeneous and heterogeneous electron transfer reactions have been analyzed on the basis of current models which consider the role of dynamic relaxation processes in determining the magnitude of the pre-exponential factor. A statistical method for separating the effects of the solvent longitudinal relaxation time τL from those of the solvent permittivity parameter γ is described and applied to 15 sets of experimental data for which results are available in at least four solvents. The degree to which the explained variation in the logarithm of the rate constant could be attributed to either of these effects varied all the way from 0 to 100% depending on the degree of reaction adiabaticity and the relative sizes of the inner and outer sphere components of the Gibbs energy of activation. Data for the limiting cases in which there is no τl dependence in the pre-exponential factor or in which the pre-exponential factor is proportional to τL−1 were analyzed further to obtain the size-distance parameter and the components of the pre-exponential factor relevant to the encounter pre-equilibrium model. These parameters have been discussed with respect to current developments in electron transfer theory. Problems in estimating the longitudinal relaxation time in the solvent, required for the analysis, are also considered.

A SNIFTIRS study of the adsorption of pyridine at the Au(111) electrode-solution interface

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been applied to study coordination of the pyridine molecules to the Au(111) electrode surface. The IR spectra have been recorded using both p- and s-polarized radiation. The ratio of the integrated band intensities for the spectra recorded with p- and s-polarized light was then used to study changes in the surface coordination of pyridine molecules. We have derived an expression describing the dependence of this ratio on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface, and: (i) C2v axis of the pyridine molecule, (ii) the direction in plane of the molecule and normal to the C2v axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface.

Infrared Reflection-Absorption Spectroscopy of the Electrode/Electrolyte Solution Interface: Optical Considerations

Calculations based on the classical Fresnel equations have been carried out to investigate the effect of various experimental parameters on infrared spectra obtained for molecules adsorbed at the electrode/solution interface. The effects considered include the nature of window material, the variation in the angle of incidence, the incident beam diameter, the instrument focus, and the presence of an absorbing bulk electrolyte solution. It is shown that, under certain circumstances, the resultant spectra are enhanced with respect to those obtained in the absence of an IR window and bulk adsorbing phase. The significance of the present results with respect to practical experiments is discussed.

Discreteness-of-charge effects at an electrode covered with a self-assembled monolayer containing a simple redox couple

Abstract The electrostatic description of an electrode modified by a self-assembled monolayer containing redox centers is considered. The model assumes that there is a Stern layer containing hydrophobically bonded water molecules between the monolayer and the electrolyte solution, and also considers the stabilizing effects of the adjacent metal and solution in estimating the local potential at the chargeable groups in the monolayer. The local potential and interfacial capacity are estimated as a function of electrode potential, and the results are compared with those from a simpler model.

Role of the solvent in the kinetics of heterogeneous electron and ion transfer reactions

Abstract Solvent effects studied in simple heterogeneous redox reactions, and for amalgam formation reactions have been reviewed. It is emphasized that on the basis of a regression analysis of the data, solvent effects relating to the pre-exponential factor of the electron transfer rate constant can be separated from those for the exponential term. Analysis of data in non-Debye solvents such as the alcohols shows that the parameters relating to the second dielectric relaxation process are more important than those for the first in determining the magnitude of the rate constant. Examination of solvent effects for amalgam formation reactions reveals that they are quite different from those observed in electron transfer processes. As a result, it is concluded that the rate determining step in the amalgam formation reaction is ion transfer, not electron transfer, and that this rate limiting step is often located in the inner part of the double layer. Data recently obtained for the electroreduction of Cu(I) in the nitrile solvents are presented and discussed with respect to solvent trends found for other systems.

A sniftirs study of the diffuse double layer at single crystal platinum electrodes in acetonitrile

Abstract In situ reflection infrared spectroscopy with electrochemical modulation has been used to investigate the structure of the double layer for the system: Pt(hkl)/acetonitrile. The electrolytes used were tetraethylammonium perchlorate and sodium perchlorate. It has been found that acetonitrile is preferentially chemisorbed on the surface either through the non-bonding electrons on the nitrogen atom at potentials positive of the point of zero charge, or on its side in a rehybridized form at negative potentials. The transition between these orientations can be followed by the present technique. The experiments were also used to study the accumulation of solvated cations in the double layer. The spectroscopic data are discussed with respect to the orientation of the solvent and location of the electrolyte ions in the double layer.