Protonated hydrates proton transfer problem

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically [13-16], and also in a few alcohols [17,18]. Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the recombination processes are complex and depend on the solvent. The central problem in the theory of geminate ion recombination is to describe the relative motion and reaction between the two particles with opposite charges initially separated by a distance rg. In water, the primary products of solvent radiolysis are the hydrated electron e ", the hydroxyl radical OH and the hydronium cation H3O+ ... 

On the other hand, quantitative scales of Bronsted acidity available for aqueous solutions are based on measurements of equilibrium constants in proton transfer reactions like (II). The best-known scale of acidities, the scale, compares the relative proton-donating capacities of various molecules with respect to a unique base water. The problem is that in this latter case the proton transfer caimot be regarded as isolated, because the aqueous medium contributes to the energetic stabilization (hydration) of all species, reactants and products, involved in the equilibrium. 

PROBLEM 16.73 The Imine formation reaction shows the methanamine attacking acetone in the first step. The alkoxide intermediate is then protonated by an acidic ammonium species in a proton transfer process. It is known that imine formation occurs faster under slightly acidic conditions (pH 5). Why not show the acetone being protonated in the first step as we did in Carbonyl hydration and Acetal formation Select the LUMO track for this animation and examine this MO on acetone by pausing the reaction at the beginning. There are several important facts that are confirmed by this calculated image. Which atom of the molecule has the most LUMO density ... 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

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