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Fourier transform analysis of ramped square-wave voltammograms indicates the availability of a novel form of kinetic selectivity for surface-confined electron-transfer processes. Thus, for all the even harmonic components, quasi-reversible processes are sensitive to the surface coverage, the reversible potential, the electron-transfer rate constant (k(0')), and the electron-transfer coefficient (alpha), as well as to the amplitude (DeltaE) and frequency (f) of the square wave and dc scan rate. Additionally, it is insensitive to background capacitance current. In contrast, reversible processes and background currents are predicted to be absent from the even harmonics and only detectable in the odd harmonic components. The square-wave voltammetry of the surface-confined quasi-reversible azurin process azurin[Cu(II)] + e(-) right arrow over left arrow azurin[Cu(I)] at a paraffin-impregnated graphite electrode has been employed as a model system to test theoretical predictions. Most voltammetric characteristics of the even harmonic components obtained from the Fourier analysis are consistent with electrode kinetic values of k(0') = 90 s(-1) and alpha = 0.48, although some nonideality possibly due to kinetic dispersion also is apparent. Conditions also have been determined under which a readily generated waveform constructed from the Fourier series of sine waves produces voltammograms that are essentially indistinguishable from those predicted when an ideal square wave is employed.

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Ferrocene-based tris(1-pyrazolyl)borate ligands 1R-Li and 1R-Tl have been synthesized and used to generate a variety of heterotrinuclear transition metal complexes, 3R-M [R = H, SiMe(3), cyclohexyl, (cyclohexyl)methyl, phenyl; M(II) = Mn, Fe, Co, Ni, Cu, Zn]. The poor solubility of 3H-M is greatly enhanced by the introduction of large organic substituents into the 4-positions of all pyrazolyl rings. The unsubstituted ligand 1H-Li and the trinuclear complex 3Cym-Cu [Cym = (cyclohexyl)methyl] have been investigated by X-ray crystallography. 1H-Li, which represents the first example of a structurally characterized lithium tris(1-pyrazolyl)borate, forms centrosymmetric dimers in the solid state. A severe Jahn-Teller distortion was observed for the (Bpz(3))(2)Cu fragment in 3Cym-Cu. Compared to the parent compounds [(HBpz(3))(2)M], the presence of uncharged ferrocenyl substituents in 3R-M tends to shift the M(2+)/M(3+) redox potential to significantly more cathodic values. The opposite is true if the ferrocenyl fragments are in their cationic state, which results in an anodic shift of the M(2+)/M(3+) transition. Most interestingly, the two ferrocenyl fragments in 3R-Cu appear to be electronically communicating.

The electrochemistry of the water oxidation catalyst, Rb(8)K(2)[{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)] (Rb(8)K(2)-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A'/A to H/H') that are readily detected under dc voltammetric conditions. The six most positive processes (A'/A to F/F'), each involve a one electron ruthenium based charge transfer step (A'/A, B'/B are Ru(IV/V) oxidation and C/C' to F/F' are Ru(IV/III) reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B'/B, E/E', and F/F' processes are quasi-reversible, while the others are close to reversible. A third Ru(IV/V) oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb(8)K(2)-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G') assigned to Ru(III/II) reduction and the multiple electron transfer reduction process (H/H') that arise from the tungstate polyoxometalate framework. A significant shift of E°' to a more positive potential value for process H/H' was observed on removal of K(+) (~100 mV in H(2)SO(4) and ~50 mV in HCl).

In this review article, the basic principles of Fourier transformed large amplitude ac voltammetry (FTACV), as developed by the Monash Electrochemistry Group, are surveyed. Special attention is paid to the key features associated with the higher order harmonic components of FTACV that are not available in dc transient voltammetry, including (1) enhanced kinetic sensitivity; (2) rejection of background charging current and (3) insensitivity to homogenous catalysis coupled to electron transfer. Examples are then given to demonstrate the high sensitivity offered by FTACV in studies of surface-confined processes and electrode kinetic determinations, and the ability to extract thermodynamic information relevant to the electron transfer step and kinetic data for the coupled catalytic process separately under catalytic turnover conditions. IntroductionIn December 2014, I (Dr. Zhang) was invited to visit Japanese electrochemistry groups at Polarography, Vol.61, No.1, (2015) from our group. Earlier works that employed small amplitude ac voltammetry also are not Basic principles and key features of FTACVFTACV was developed to overcome some major drawbacks associated with dc voltammetry. To appreciate the basic principles of the technique, it is informative to survey the origins of these major drawbacks, and understand how they are overcome by FTACV.In dc cyclic voltammetry, a ramped (or staircase) potential waveform is applied to the working electrode, and the resulting current is recorded as a function of applied potential (Fig. 1). Dc voltammetry is understandably the most popular voltammetric method because it is both simple and very informative. 6) For example, from simple inspection of the dc voltammogram shown in Fig. 1, one can recognize that a redox active species present in solution in the reduced state can be oxidized to a stable product on the relevant timescale since the ratio of the oxidation and reduction peak currents is close to 22Review of Polarography, Vol.61, No.1, (2015) 1 : In dc cyclic voltammetry, a decrease in the rate of an electrode process leads to the increase of the separation between oxidation and reduction peak potentials. Therefore, the peak-to-peak separation or ΔE p value is commonly used to quantify the electrode kinetics. 7) However, ΔE p is also influenced by R u . 7) Thus, only when the effect of R u is negligible, can the electrode kinetics be determined directly from the ΔE p value. When the effect of R u is significant, the electrode kinetics may be determined using numerical simulations that take the effect of this term into account. In principle, to measure the kinetics of fast electrode processes using dc cyclic voltammetry, one can increase the scan rate to shorten the timescale of the measurement to achieve an outcome where the electrode process becomes relatively slow with respect to the measurement timescale. However, when the scan rate is very high, the contribution from the background double layer charging current to the