ALBERT

Notes: We develop a simple but general three-variable model skeleton to describe complex nonlinear behaviors in electrochemical processes taking place at either a hanging mercury drop electrode (HMDE) or a rotating-disk electrode (RDE). We apply our formalism to the reduction of indium(III) at a HMDE in the presence of thiocyanate, a reaction known to exhibit complex mixed-mode and chaotic oscillations. Besides the role of the negative Faradaic impedance in destabilizing the electrochemical system, mass transport appears to be crucial as the model explicitly takes into account, in a truncated fashion, the time-dependent relaxation of the concentration profile. We study in detail the nonlinear dynamic behavior of our model of the indium/thiocyanate system and a RDE model. The models support mixed-mode sequences that appear either as incomplete Farey sequences or as periodic-chaotic sequences, which we discuss in terms of an incomplete homoclinic scenario whose definition and properties are worked out here. Our results compare very well to the experimental observations in the indium/thiocyanate system and the electrodissolution of a rotating copper disk in phosphoric acid. This satisfactory agreement strongly suggests that diffusion relaxation is an important phenomenon in electrochemical oscillations and could be the essential third variable in many dynamical electrochemical processes.

Notes: Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial fields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electrostatic plus Pauli) repulsion, and donation and back-donation orbital components. The increasing tendency towards multifold CO coordination seen at more negative fields is due primarily to enhanced back-donation. The decreasing propensity for atop vs multifold CO binding seen in moving from the lower-left to the upper-right Periodic corner of the Pt-group elements is due to the combined effects of weaker donation, stronger back-donation, and weaker steric repulsion. The uniformly hollow-site binding seen for NO arises from markedly stronger back-donation and weaker donation than for CO. The metal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field ("Stark-tuning") slopes. Decomposition of the DFT bond frequencies shows that the redshifts observed upon chemisorption are due to donation as well as back-donation interactions; the metal-dependent trends, however, are due to a combination of several factors. While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding site and metal again reflect the interplay of several interaction components. © 2000 American Institute of Physics.

Notes: We present molecular dynamics simulations of solvent reorganization in electron-transfer reactions in water. Studying a series of solutes with the same core radius (typical for chlorine) but with varying charge from −3 to +3, the simulations show that the single-solute solvent reorganization energy depends quite strongly on the solute's charge, in contrast with the continuum Marcus theory. Due to the ion-dipole interactions, electrostriction plays an important role for charged species. The effective radius of a neutral species is comparatively larger, making the solvent reorganization energy small. Strong increases in the solvent reorganization energy occur when the solute is charged to either −1 to +1, due to the significantly smaller effective radius caused by the ion-dipole interactions. However, the effect is nonsymmetric because the center of the water dipole can approach closer to the negative species than to the positive species. Hence, the nonlinearity occurs mainly in the transition from 0 to –1. For higher charges (+3, +2, −2, −3), dielectric saturation causes a decrease in the reorganization energy with increasing charge. We also calculate the equilibrium activation energy for an outer-sphere electrochemical electron-transfer reaction of the X+e−(r harp over l)X− type, with varying of the core radius of the X species. The deviations from Marcus theory are relatively small for large reactants, but get more significant for small reactants. This is mainly due to the fact that the changes in electrostriction have a comparatively large effect for small solutes. © 2001 American Institute of Physics.

Notes: The results of ab initio molecular dynamics simulations of liquid water and liquid water–vapor interface using the Perdew-Wang 91 (PW91) exchange-correlation functional are presented. The structural and transport properties of liquid water are comparable to the previous results using Becke-Lee-Yang-Parr (BLYP) functional and experimental data. The shape and the position of the first peak in the oxygen–oxygen radial distribution function is in good agreement with the most recent neutron diffraction data. The ab initio molecular dynamics simulation of liquid water–vapor interface, which is the first of its kind, suggests a preferred orientation of the surface water dipole towards the bulk region. © 2001 American Institute of Physics.

Notes: Some typical bifurcation sets of a generalized autonomous Van der Pol-type model are discussed as archetypes of phase diagrams occurring in nonlinear dynamical systems. The relevance of the obtained bifurcation sets is exemplified by several experimental and numerical results from the literature of oscillating chemical reactions. © 1995 American Institute of Physics.

Notes: A simple model is constructed to calculate the potential energy surface of dissociative adsorption and associative desorption reactions at the metal/solution interface. The model is based on an extension of the Anderson–Newns Hamiltonian and has three reaction coordinates; the bond length or the distance between the fragments, the distance from the surface, and the generalized solvent coordinate familiar from the classical theory of electron-transfer reactions. The properties of the three-dimensional potential energy surfaces are studied and the activation energy for dissociative adsorption is calculated as a function of the applied potential and the metal work function. In the observed trends, the absorption energy and hence the electrosorption valency of the fragments play an important role. For certain "extreme" values of the bonding or antibonding energy levels, molecular ions may become metastable and affect the reaction mechanism. © 1998 American Institute of Physics.