Proton Exchange Kinetics

Proton transfer reactions on the aqua oxo complex are described by Eq. (8) (acid catalysis or protolysis), Eq. (9) (base catalysis or hydrolysis), and Eq. (10) (direct proton exchange). 

The rate of protonation/deprotonation at the aqua oxo complex, which in principle can proceed via any of the above three pathways, is then given by Eq. (11) 

The overall rate of protonation transfer on the hydroxo oxo is given by Eq. (16) 

The observed protonation constant describing the proton transfer on the hydroxo oxo complex, is a function of the total metal complex concentration, [MJPoJoh = d[MOH]/[MMt, therefore given by Eq. (17) 

The line-broadening data as a function of pH, typically shown for the W(IV) in Figs. 13 and 14, incorporating the known pKa values (Table II), were fitted in 5 X 5 Kubo-Sack matrices describing the exchange based on the above schemes (6, 57). The experimentally determined chemical shift and linewidth data in the absence of exchange for the aqua oxo, hydroxo oxo, and dioxo species and the pH-dependent species distribution as calculated from the acid dissociation constants for the four systems were all introduced in the different matrices and the spectra were computer simulated. For each set of chosen rate con- 

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Fig. 8.3. FCS Proton exchange kinetics measurements at biological membranes, (a) Principal design of experiment. Liposomes were labeled with one FITC fluo-rophore undergoing fluorescence fluctuations due to protonation/deprotonation. (b) Collection of FCS curves of the vesicles at different pH. The FCS curves reflect singlet-triplet transitions in the microsecond time range, protonation kinetics in the 10-100 ps time range and translational diffusion in the milliseconds time range. Inset measured protonation relaxation rates vs. proton concentration, (c) Principle of the proton collecting antenna effect...

Dewey, T.G. (1994) Fractal analysis of proton exchange kinetics in lysozyme. Proc Natl Acad Sci USA, 91 (25), 12101-12104. 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

The kinetic acidity (rate constant for metal-to-metal proton exchange) also decreases down a column (Cr>Mo W) in the periodic table. This parallels the order of rates we have observed for the dinuclear elimination of methane from... 

A kinetic cis effect has been investigated by Eaton and Eaton (131 — see Table 16). The ligand exchange kinetics of reaction (9) involving various carbonylrutheni-um(II)porphyrin species [36a-36h] were derived from a H-NMRline shape analysis at various temperatures for the t-butyl protons of coordinated and uncoordinated 4-t-butylpyridine. 

J. Srmivason, et al., "High Energy Efficiency and High Power Density Proton Exchange Membrane Fuel Cells - Electrode Kinetics and Mass Transport," Journal of Power Sources, p. 36, 1991. 

Table 4. Relative H/D exchange kinetics of C2-proton of Taz peptides and compounds in 0.2 M 4-acetate buffer in D2O, pD 4.7...

The chelate formation in lithium complexes 17 or 20 contributes to stabilization. Enhancement of kinetic acidity arises from the formation of pre-complexes 16 and 19, respectively. Here, already a dipole is induced and, in addition, proton exchange can proceed intramolecularly via a five- or six-membered ring. Despite these favourable features, the acidity of alkyl carbamates 15 is lower than those of the 1-proton in butane n-BuLi does not lead to deprotonation. In order to suppress carbonyl attack, a branched amino residue NR2 such as diisopropylamino (in Cb) or 2,2,4,4-tetramethyl-l,3-oxazolidin-3-yl (in Cby) is essential. A study on the carbenoid nature of compounds 17 was undertaken by Boche and coworkers. ... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

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