Hydrated proton structures

The largest protonated cluster of water molecules yet definitively characterized is the discrete unit lHi306l formed serendipitously when the cage compound [(CyHin)3(NH)2Cll Cl was crystallized from a 10% aqueous hydrochloric acid solution. The structure of the cage cation is shown in Fig. 14.14 and the unit cell contains 4 [C9H,8)3(NH)2aiCUHnOfiiai- The hydrated proton features a short. symmetrical O-H-0 bond at the centre of symmetry und 4 longer unsymmetrical O-H - 0 bonds to 4... 

Applying this to our electrochemical system could qualitatively explain some of the observed effects. Assuming that there is only a weak interaction between metal and the perchloric acid hydrate, protons being part of the clathrate structure may not be in a favorable position for a charge transfer reaction at the interface. This could result in small pre-exponential factors. The electrode potential, however, may... 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

For comparison purposes, the proton mobility. Do (for Nafion solvated with water), which is closely related to the self-diffusion coefficient of water, is also plotted. At low degrees of hydration, where only hydrated protons (e.g., H3O+) are mobile, it has a tendency to fall below the water diffusion coefficient (this effect is even more pronounced in other polymers), which may be due to the stiffening of the water structure within the regions that contain excess protons, as discussed in Section 3.1.1. . Interestingly, the proton mobility in Nafion solvated with methanol (Da(MeOH) in Figure 14a) is even lower than the methanol self-diffusion (Z ieon). This may... 

The hydrated protonated ester, SH+(H20)p, and the transition state, have the same charge, and are structurally closely similar, and it is assumed that the ratio of their activity coefficients is medium-independent124. Thus, by taking logarithms and rearranging we have... 

Proton NMR spectroscopy and dielectic constant measurements provide evidence about the motion of the water molecules in crystal structures, as reviewed by Davidson and Ripmeester (1984). At very low temperatures (<50 K) molecular motion is frozen in so that hydrate lattices become rigid. The hydrate proton NMR analysis suggests that the first-order contribution to motion is due to reorientation of water molecules in the structure the second-order contribution is due to translational diffusion at these low temperatures. 

OXOANION SELECTIVITY WITH PROTONATED AZACRYPTATE HOSTS THE INFLUENCE OF HYDRATION ON STRUCTURE AND STABILITY... 

Architecture of Hydrates and Local Structure of Acetic Acid Aqueous Solution Ab Initio Calculations and Car-Parrinello Molecular Dynamics (CPMD) Simulations on Hydrogen-Bonding Rings, Network, and Intra-Hydrate Protonation in Multi-Hydrates of Acetic Acid Monomer... 

Both the experimental and theoretical studies indicate that the interactions between acetic acid and water molecules are more competitive in dilute aqueous solution. However to our knowledge, the specific interactions between acetic acid and water molecules are still not well understood, especially in such as the nature of hydrogen bonding, the bonds networking, the rule in architecture of larger hydration compounds, deprotonation of acetic acid in solution, stability of the hydrated proton, the local structure of its aqueous solution, and so on. In the present work, we have performed ab initio calculations on multi-hydrates (rich water hydration compounds) of acetic acid, and ab initio Car-Parrinello molecular dynamics (CPMD) [20] simulations on acetic acid monomer and water system (at dilute aqueous solution condition) to find something helpful for interpreting the nature of acetic acid aqueous solution. 

These bonds are analogous to the classical example bifluoride ion [F-H-F] [48,388]. They occur in the crystal structures with hydrated hydroxyl ions and with hydrated protons [112, 1131. These latter crystals are referred to as pseudo-... 

Figure 1. Structures for the hydrated proton in water (a) trigonal pyramidal (b) tetrahedral...

Further, the coarse grained nature of the algorithm will allow the extension of modeling of proton transport in bulk water to PFSA membranes because hydrated protons form similar Zundel-ion-like stractme and Eigen-ion-like structure with the oxygen of the sulfonate groups, which can be easily integrated into the RMD formalism. 

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