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The surface potential, together with the accompanying electrostatic fields, have long been recognized by electrochemists as playing a central role in the properties of adsorbates at metal-solution interfaces. This appreciation has arisen in large part through the ease by which the surface potential in electrochemical systems (as controlled by the electrode potential) can be varied by external means. Similar potential-induced effects can be anticipated to also play a role in adsorbate properties at metal-ultrahigh vacuum (uhv) interfaces, especially in the presence of dipolar or ionizable coadsorbates. The recognition as well as understanding of such effects in the latter case, however, has been hampered somewhat by the more limited control of the surface potential attainable in uhv systems, together with the relative paucity of the required work-function data. Even so, some intriguing information on potential-induced effects upon the thermodynamic and spectroscopic adsorbate properties in uhv environments has been obtained by dosing components of electrochemical interfaces at metal-uhv surfaces., Prepared for publication in Surface Science.

The prospects of utilizing atomic-resolution scanning tunneling microscopy (STM) as a real-space in-situ probe of molecular transformations on electrochemical surfaces are illustrated for the specific case of sulfide electrooxidation on Au(111) in aqueous solution. At relatively low electrode potentials, sulfide adsorption yields ordered ( 3 x 3)R30 deg layers. The onset of sulfide electrooxidation produces marked changes in the adsorbate structure, arrays of rectangular close-packed structures being formed which consist predominantly of S8 rings. The ring dimensions, including the S-S bond distances, differ significantly from bulk-phase polysulfur, being affected apparently by the gold substrate structure. Further reaction yields multilayer Sx structures exhibiting greater disorder. The likely scope of reactive surface electrochemistry amenable to STM is noted., Prepared for publication in the Journal of Physical Chemistry.

The utilization of scanning tunneling microscopy (STM) with high-quality atomic resolution for elucidating complex electrochemical surface reconstructions is illustrated for the Au(100)-aqueous interface. The reconstruction, triggered by negative surface electronic charges, exhibits typically a (5x27) symmetry involving quasi-hexagonal surface packing. The detailed atomic arrangements within the unit cell, including the spatial relationship of the reconstructed top layer to the underlying substrate, can be deduced from STM images featuring adjoining (5x27) and (lxl) domains. A number of subtly different superstructures could also be discerned; these are seen to arise from the need for the observed ribbon-like reconstructed domains to circumnavigate surface defects. The virtues of atomic-resolution STM for obtaining detailed local information on surface atomic arrangements in complex nonuniform systems are pointed out, along with its applicability (on an equal footing) to electrochemical as well as vacuum surface science., Prepared for publication in Physical Review B.

Some selected concepts involving electronic and dynamical factors in the electron-transfer chemistry of inorganic and organometallic systems are outlined and illustrated by recent results primarily from the author's laboratory, with some emphasis placed on the redox properties of electrochemical interfaces in comparison with molecular reagents. Three interrelated topics are considered. The first concerns the control of electron-transfer rates by an interplay between donor-acceptor electronic coupling and nuclear dynamics. The role of the solvating medium in limiting the barrier-crossing frequency ( solvent friction effects) is described, and the diagnostic capabilities (and limitations) of this phenomenon for probing the degree of electronic coupling by tuning the reaction dynamics are noted. Secondly, the behavioral differences anticipated between the kinetic properties of molecular redox reagents and electrode surfaces are discussed. A formalism is described for this purpose which intercompares homogeneous-phase and heterogeneous rate data on a unified basis, and is utilized in some illustrative experimental comparisons. Thirdly, the infrared spectroelectrochemical properties of high-nuclearity platinum carbonyl clusters in nonaqueous solvents are outlined in comparison with the potential-dependent properties of monocrystalline platinum electrode analogs., Sponsored in part by the National Science Foundation.

Coverage-dependent adlayer structures of iodine on Au(111) in air or organic solvents are reported in comparison with those obtained under potential control in aqueous electrochemical environments by means of atomic-resolution scanning tunnelling microscopy (STM). The well-known (3 x 3) R30 deg structure (I = 0.33) was only observed at the electrochemical interface at low potentials. Two higher coverage adlattices were evident from the STM images. The first comprises a (5x3) structure (1=0.40), with a pair of iodine rows compressed by 20% in the R30 deg substrate direction. The second high-coverage phase (THETA | = 0.44) consists of a hexagonal iodine overlayer compressed and rotated a few degrees from the R30 deg direction, giving rise to a long-range (- 20 Anstrom) corrugation in the STM image. The virtues of quantitative atomic- resolution STM for deducing such complex adlayer structures are pointed out.

The effect of substituting 12CO by 13CO on the infrared spectra of (Pt24(CO)30)n and (Pt26(CO)32)n (n = 0 to -6) in dichloromethane has been studied in order to explore the nature of the ligand substitution kinetics and dynamic dipole-dipole coupling for these high-nuclearity clusters in relation to metal surfaces. The chargeable nature of the platinum clusters, as scrutinized by FTIR spectroelectrochemical tactics, offers interesting comparisons with the potential-dependent dipole coupling observed for mixed 13CO/12CO adlayers at the Pt(111)-dichloromethane interface, also reported here. Unlike the electrochemical surface, CO ligand exchange on (Pt24(CO)30)(2-) and (Pt26(CO)32) (2-) is slow and non-random, incomplete 12CO/13CO substitution occurring even on long (> several hours) timescales. Substantial deviations from first-order substitution kinetics are also observed, especially for (Pt24(CO)30)(2-) where only 65% of the CO ligands are observed to undergo exchange. (Author)

The predicted alterations in the solvent-dependent reaction dynamics of electron-transfer processes brought about by the presence of reactant vibrational (inner-shell) activation in addition to overdamped solvent motion are examined by using a slightly modified from of the theoretical treatment due to Sumi, Nadler, and Marcus. Allowance is made for the occurrence of nuclear tunneling involving the high-frequency vibrational modes, and for barrier crossing driven by solvent inertial polarization as well as vibrational activation. The latter enables the emergence of the zero-friction (transition-state theory, TST) limit to be included along with providing a unified description in the absence and presence of vibrational activation. Attention is focussed on the calculated dependencies of the reaction rate constant, and barrier-crossing frequency, upon the overdamped solvent relaxation time, over a range of parameters and in a format which can be compared directly with experimental solvent-dependent kinetic data.

Atomic relaxation at the stepped surfaces Au(311), (533), and (221) in aqueous solution is examined by atomic-resolution scanning tunneling microscopy (STM). The former two surfaces yield ordered (1 x 1) domains at electrode potentials corresponding to near-zero or positive electronic charges. Substantial surface relaxation is observed, however, at more negative potentials, featuring displacements of the step edge and neighboring atomic rows. The Au(221) surface exhibits a marked relaxation at positive as well as negative electronic charges. The so-far underutilized value of STM for exploring atomic structure at stepped surfaces is demonstrated by these findings.

In-situ infrared spectra are reported in the C-O stretching (nu co) region for saturated CO adlayers on Pt (111) in four nonaqueous solvents; acetonitrile, dimethylformamide, dichloromethane and tetrahydrofuran, each containing tetraalkylammonium cations, over wide electrode potential ranges, ca -2.0 to 1.5 V vs SCE. Notably, the spectral features (nu co) Frequencies, CO site occupancies, etc) are sensitive only to the applied potential, being virtually independent of the solvating medium. Comparisons with spectra obtained in the presence of different cations, and in corresponding aqueous electrochemical and ultrahigh vacuum environments, demonstrate that the adlayer properties are controlled in each case by the surface potential drop across the adsorbate layer.

Rate parameters are reported for the electroreduction of eight CoIII(NH3)5X complexes at ordered Pt(111) surfaces coated with iodine adlayers whose structures are characterized by scanning tunneling microscopy (STM) in order to explore possible correlations between the outer-sphere electron- transfer kinetics and the spatially resolved adlattice properties as revealed by STM. The sixth ligands: acetates, and three cyclic organic carboxylates, were selected so to vary the reactant charge, and hence the magnitude of electrostatic double-layer effects, and to examine the effect of potential organic mediators. The ordered Pt(111) surfaces were prepared by flame annealing, followed by cooling in a stream of nitrogen over iodine crystals.

Solvent-dependent formal potentials, Ef, and reaction entropies, for four reversible Co(III)/(II) couples featuring macrobicyclic cage ligands have been obtained. These are utilized, together with corresponding data for other Co(III)/(II) and Ru(III)/(II) amine couples and for M(III)/(II) polypyridine couples, to unravel effects on the redox thermodynamics arising from specific ligand-solvent and other interactions. The solvent dependencies of Ef (vs. ferrocenium-ferrocene) for each redox couple are fitted by using a multiparameter approach, incorporating various well-known empirical quantities describing solvent basicity, acidity, polarity, and internal ordering. For most amine couples, solvent basicity appears to play a prevailing role, associated with interactions involving the amine hydrogens, although solvent polarity also exerts a substantial, albeit less solvent-dependent, influence upon EfD.

In-situ scanning tunneling microscopy (STM) has been combined with infrared reflection-absorption spectroscopy (IRRAS) to yield detailed atomic- level adlayer structures for saturated coverages of CO on ordered Rh(111) in aqueous solutions. Two distinctly different structures were obtained in CO- containing 0.1 M NaC104, depending on the electrode potential. At higher potentials, ca -0.1 to 0.3 V vs SCE, atomic-resolution STM images were obtained that indicated the presence of a (2 x 2)-3CO unit cell, having a CO coverage in agreement with electrochemical and IRRAS measurements. The corresponding in-situ infrared spectra indicate the presence of two atop (or near-atop) and one twofold bridging CO in the unit cell. A real-space structure is suggested that is related to the corresponding (2 x 2)-3CO adlattice on Rh(111) in uhv as deduced previously by low-energy electron diffraction and vibrational spectroscopy.

Cyclic voltammograms at microdisk electrodes that account for the effects of solution resistivity rho(Omega), double layer capacitance, C(dl), and electrochemical kinetics have been simulated by using the explicit Hopscotch algorithm and a conformally mapped space-grid. The latter procedure, in particular, provides an efficient means of simulating voltammograms at slower potential scan rates where radial, rather than linear, diffusion predominates. Simulated voltammetric waveshapes as a function of scan rate are reported for systematic variations in RHO(Omega), C(dl), and the standard electrochemical rate constant ks, both separately and together. The waveshape description utilizes both the anodic-cathodic potential peak separation, Delta E(p), and the halfwave potential for the forward scan in relation to the voltammograms where the influence of radial diffusion is sufficient to yield largely sigmoidal, rather than peaked, waveshapes.

Rate constants for electron self exchange, kax, of five cobaltocenium-cobaltocene and ferrocenium-ferrocene couples in the gas phase have been measured by means of Fourier transform ion cyclotron resonance mass spectrometry in order to explore the possible effects of donor-acceptor electronic coupling on gas-phase redox reactivity. The systems studied were selected in view of the substantial variations in electronic coupling inferred on the basis of their solvent-dependent reactivities and on theoretical grounds. The sequence of kax values determined in the gas phase is roughly similar to that observed in solution, although the magnitude of the kax variations is smaller in the former case. The likely origins of these differences in gas-phase reactivity are discussed in light of the known variations in the electronic coupling matrix element H12, inner-shell reorganization energy.