Models proton exchange

J-M Thomasin, Ch Pagnoulle, G Caldarella, A Germain and R Jerome, Contribution of nanoclays to the barrier properties of a model proton exchange membrane for fuell cell application. Journal of Membrane Science, Vol. 270 (2006) p.50... 

In molecular mechanics and molecular dynamics studies of proteins, assig-ment of standard, non-dynamical ionization states of protein titratable groups is a common practice. This assumption seems to be well justified because proton exchange times between protein and solution usually far exceed the time range of the MD simulations. We investigated to what extent the assumed protonation state of a protein influences its molecular dynamics trajectory, and how often our titration algorithm predicted ionization states identical to those imposed on the groups, when applied to a set of structures derived from a molecular dynamics trajectory [34]. As a model we took the bovine... 

Following a period of slack, decisive improvements were made after 1990 in the area of PEMFCs. Modem models now achieve specific powers of over 600 to 800 mW/cm while using less than 0.4 mg/cm of platinum catalysts and offering a service fife of several tens of thousands of hours. These advances were basically attained by the combination of two factors (1) using new proton-exchange membranes of the Nafion type, and (2) developing ways toward much more efficient utilization of the platinum catalysts in the electrodes. 

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

This section deals with the quantitative description of the proton transfer processes (denoted by Eqs. (4) and (6) in Scheme 1), identified by the qualitative NMR experiments on the acid/base behavior of the Mo(IV), W(IV), Re(V), Tc(V), and Os(VI) systems as described in Section II. The data obtained on the signal behavior from these similar complexes were used to simulate spectra and model the proton exchange processes to finally obtain rate constants associated therewith. 

Similarly, the proton transfer on the hydroxo oxo complex is illustrated by Eqs. (13)-(15), based on the Eigen model (Scheme 4), and can again be due to protolysis, hydrolysis, or direct proton exchange... 

The semi-empirical descriptions of adsorbate/solid interactions are based on net changes in system composition and, unlike surface complexation models, do not explicitly identify the details of such interactions. Included in this group are distribution coefficients (Kp) and apparent adsorbate/proton exchange stoichiometries. Distribution coefficients are derived from the simple association reaction... 

Furthermore, although other electrostatic models for the oxide/ water interface may yield different relationships among postulated system components, it appears unlikely that either Xp nor x alone will adequately represent the postulated true adsorbate/proton exchange ratio. 

T. Koido, T. Furusawa, and K. Moriyama. An approach to modeling two-phase transport in the gas diffusion layer of a proton exchange membrane fuel cell. Journal of Power Sources 175 (2008) 127-136. 

A. (1996 a) Comparison of different site binding models for cation sorption Description of pH dependency, salt dependency and cation proton exchange. J. Colloid Interface Sd. 181 45-59... 

First-order rate constants of the proton exchange between bulk water and the different paramagnetic species assumed in Model II3 (Table 43)... 

From Table 20.6 we conclude that independent of which model we use, typical transfer times are between a few tenths of a second and a minute. Proton exchange reactions of the form (see Section 8.2) ... 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

The temperature dependence of the PMR spectra of the 00(111X 2 Dtc)3 complexes has been studied and certain unusual aspects of the spectra are accounted for by a total line-shape analysis in terms of a general two-proton-exchange model. In similar studies Siddall (573) assigned certain of the complexities of the PMR spectra of Co(RR Dtc)3 complexes to rotation about the C -N bond. 

A two-proton exchange model using a density matrix formalism has been used to analyze the unusual structure of the PMR spectra in the diamagnetic Ni(R2Dtc)2 complexes (R = n-Pr, Et,/-Pr,/-Bu, and Bz) (273). 

Regardless of how much improvement can be made in NMR techniques, conformation problems will always require the support of other techniques such as x-ray diffraction, CD, ORD, UV, proton exchange, thin-film dialysis, model building, and energy-minimization calculations. 

The systematic study of gas-phase proton exchange reactions between bases has led to the development of a new formalism describing the quantitative effects of substitution on the GBs of organic compounds200. Because of their formal simplicity and general importance, proton transfer reactions are excellent models for the study of other acid-base reactions, both in the gas phase and in solution201. 

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