Grotthuss-type mechanism

The most important property of PVPA is conductivity based on hydrogen bonding, which allovt proton transport via a Grotthuss type mechanism (structural diffusion). The conductivity is the key property of PVPA, which is successfully used to obtain polymer electrolyte membranes by copolymerization or blending with appropriate materials. The applications of these materials have greatly expanded and now have many roles in the modern industrial economy. 

Biochemical ion-transfer reactions, and especially proton-transfer reactions, are enormously important for life. They are used in catalysis of acid-base reactions and enzymatic catalysis, or to establish concentration gradients in living cells. Many biomolecules exhibit proton-transport chaimels through which protons may be transported. These ehannels are designed by the molecule s structure, and generally filled with a small amount of water molecules. The water in this eonfined system is used to transport the proton via a Grotthuss-type mechanism. 

Noda et al. [ 168] reported the details of Bronsted acid-based ionic liquids consisting of a monoprotonic acid and an organic base, in particular solid bis(trifluorometha-nesulfonyl)amide (HTFSI) and solid imidazole (Im) mixed at various molar ratios to form liquid fractions. Studies of the conductivity, H NMR chemical shift, selfdiffusion coefficient, and electrochemical polarization results indicated that, for the Im excess compositions, the proton conductivity increased with an increasing Im molar fraction, with rapid proton-exchange reactions taking place between the protonated Im cation and Im. Proton conduction was found to occur via a combination of Grotthuss- and vehicle-type mechanisms. Recently, Nakamoto [169] reported the... 

Recent experimental results on the acid-base neutralization reaction between HPTS and the carboxylic bases mono-, di- and trichloracetate have revealed the underlying mechanisms of proton transfer of the loose complexes [138]. It turns out that a sequential, von Grotthuss-type of hopping occurs through a water molecule bridging the HPTS photoacid and the carboxylic base. Figure 14.9 shows the transient spectra obtained with a solution of 20 mM HPTS in D2O with 1 M of monochloroacetate OAc-Cl added. At early pulse delays about 20% of HPTS has released its deuteron, as is indicated by the appearance of the HPTS photobase marker band at 1435 cm i within the time resolution. A vibrational marker band at 1850 cmr indicates the transient existence of hydrated deuterons. Comparison with literature values for hydrated proton species with well-defined surroundings... 

Figure 14.8 Proton transfer mechanism ofthe loose complexes R0H---(H20) ---B with a sequential, von Grotthuss-type, hopping of protons through water bridges. For H PTS and monochloroacetate the first transfer to the water bridge forming the hydronium ion HjO+ is ultrafast, and the second transferto the base is slower. (Adapted from Ref [136].)...

As discussed in the physical model, the conductivity is caused by both the vehicle and Grotthuss mechanisms. The main difference between the conductivity of the vapor and liquid-equilibrated membranes stems from their respective water contents. A percolation-type equation is used for the conductivity although, at higher water contents, the conductivity should level out and ideally approach that of liquid water at infinite dilution (i.e., the polymer is dissolved in an infinite amount of water). The temperature dependence of the conductivity is due to the change in the equilibrium constant for the dissociation of the sulfonic acid sites and the activation energies for the Grotthuss mechanism and vehicle mechanisms [39]. Finally, the expressions given in Table 5.2 are valid for pure water and the... 

What then is the structural feature most characteristic of a proton conductor system It is generally believed that a proton is transferred through a solid in one of two distinct ways by a vehicular mechanism, whereby the proton rides on a carrier molecule of type NH4 or HjO ion, or by a Grotthuss mechanism, in which the proton jumps from a donor to a suitably placed acceptor molecule (typically, from H30 to H2O, or from H2O to OH ). How then is such a process sensed in a conventional diffraction experiment The nature of the diffraction method is such that we obtain a time- and space-average of the unit-cell content within the characteristic coherence length of the diffraction process (typically, hundreds of Angstroms), and over the duration of the experiment (days to weeks). This follows from the extremely short photon-electron and neutron-nucleus interaction times ( 10 s), which are significantly shorter than the characteristic time of the fastest of the dynamical processes in the structure ( 10 s for the vibration of a covalently bonded atom). It follows then that some type of structural... 

The fourth part concerns interpretation of three main conductivity mechanisms the electron-like type, the proton jump in statically or dynamically disordered solids and the quasi-liquid state in hydrated materials giving rise to either vehicle or Grotthuss mechanism. 

It should be kept in mind that conventional AIMD simulation techniques, both BOMD and CPMD, are not able to describe all types of dynamics encountered in chemistry. One thing they lack is an ability to handle dynamics that can only be explained with frill quantum mechanics. Proton tunneling and ion dispersion, for example, are purely quantum effects that can play a fundamentally important role in biological systems, in polymer electrolyte fuel cells, and in many other water-containing systems. In fact, the commonly accepted mobility mechanism is the so-called stmctural diffusion or Grotthuss mechanism, in which solvation stmctures diffuse through the hydrogen bond network via sequential proton transfer reactions. 

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