Electron transfer, activation control slow

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The transformations of SbClj caused by a one-electron transfer from an aromatic compound have been described earlier. If the pure Lewis acid SbClj is used, its reactivity is very difficult to control, and single-electron oxidation as well as chlorination of various aromatic donors can occur readily (Mori et al. 1998). Meanwhile, in the case of EtjO SbClg", the slow release of the active monomer SbClj occurs. In the case of SbClj as such, the 2SbCl5 — C Sb—CI2—SbCl4 dimerization occurs (Cotton and Wilkinson 1988, p. 395). The dimeric form may lead to the following electrophilic chlorination ... 

The biofilm subsists on the oxidation of an organic substrate, S (mM), which is delivered to the biofilm matrix via diffusive mass transport. The substrate is supplied at a constant concentration to a large, well-mixed anodic chamber. This allowed us to assume that the bulk concentration of substrate is constant because of the size of the chamber and the relatively slow consumption rate of substrate by the biofilm. The concentration at the biofilm surface is equal to the bulk concentration because of mixing in the anodic chamber and because of simplification of the model. The substrate utilization rate is controlled by both the substrate (electron donor) concentration and the eleetron acceptor concentration, through multiplicative Monod substrate utilization equations [37, 38]. Equations 9.1 and 9.2 simply state that the biofilm can only metabolize in the presence of both an electron donor and an electron acceptor. The lack of either one prevents biofilm metabolic activity. In our model, we assume that there are two possible electron transfer pathways thus, there are two substrate utilization equations. For diffusion-based EET, substrate utilization is given by ... 

Activation energy must be supplied before these processes and the magnitudes depend on the properties of the reactants, products and intermediates for most physical and chemical reactions. If any one of the reactions listed is slower than the activation, thermal and electrical output will control the cell s current voltage characteristics. The slow step may occur before or after the electron transfer step and may not involve charged particles. If the products or reactants of the non-electrical reaction participate in electron transfer, then potential will affect the kinetics by its effect on reactant activity. This potential dependent for the non-current producing reactions is due to chemisorption prior to electron transfer and a surface reaction following the ion discharge. 

Kochi and co-workers reported the photochemical addition of various stilbenes to chloroanil 53, which is controlled by the charge-transfer (CT) activation of the precursor electron-donor/acceptor (EDA) complex. The [2-1-2]-cycloaddition products 54 were established by an x-ray structure of the trans-oxetane formed selectively in high yields. Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Patern6-BUchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies that involves quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate, which appears on a nanosecond/microsecond time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse k = 10 s ) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

Activation polarization can be a slow step in the electrical reaction for which an activation energy in the form of potential is reqnired for the reaction to proceed. When a certain step in a half cell reaction controls the rate of electron flow, the reaction is said to be under activation charge transfer control and activation polarization occurs. For example, consider the reduction of hydrogen ions ... 

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