Hydronium ion hydration

Jinnouchi and Okazaki performed AIMD studies of the first-electron transfer reaction with 1 hydronium ion, 9 water molecules, and 12 Pt atoms at 350 K as shown in Fig. 13.108 The adsorbed water molecules and the hydronium ion hydrated the adsorbed oxygen atoms, and proton transfer through the constructed hydrogen bonds frequently occurred. When the conformation of these species satisfied certain conditions, the oxygen dissociation with the proton transfer reaction was induced and three OH were generated on the platinum surface (Fig. 14). The authors concluded that the oxygen dissociation tendency is one of the dominant factors for the reactivity of the cathode catalyst. This work demonstrates the power of AIMD that does not require specific assumption in order to describe charge transfer. 

The degree of hydration of hydronium has relevance to structural diffusion as it requires the presence of Eigen ion (HsO + 3 H2O), which is discussed in detail in Section III. Figure 14 supports the experimental observation of low proton conductivity at low water contents due in part to the reduction of stractural diffusion because the probability of finding HsO surrounded by sufficient H2O molecules is lower. Figure 14 shows that at intermediate water contents, the probabilities for hydronium ions hydrated with three or more H2O molecules are higher in Nafion than in... 

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

Two ions are thus formed protons or hydrogen ions, H, and hydroxyl ions, OH. Free protons are immediately hydrated to form hydronium ions, HjO ... 

In studies of the hydration and dehydration of pteridine and the methylpteridines, but not levelled out as solutions were made more acid. This was explained by assuming that hydronium ion catalysis of the reactions proceeded only by the formation of the cations of HY+ and HX+, respectively. This effect is strikingly shown by 1,3,8-triazanaphthalene, for which the pH-rate profile of is V-shaped between pH 6.82 and 10.29 but levels out and remains constant from pH 5.3 down to, at least, 2.4. ... 

Hydronium ion, HjO+, is a structural unit in solid perchloric acid hydrate, HCKVHjO, as shown by nuclear magnetic resonance studies. 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

Because at equilibrium virtually all the HCl molecules have donated their protons to water, HCl is classified as a strong acid. The proton transfer reaction essentially goes to completion. The H30+ ion is called the hydronium ion. It is strongly hydrated in solution, and there is some evidence that a better representation of the species is H904+ (or even larger clusters of water molecules attached to a proton). A hydrogen ion in water is sometimes represented as H + (aq), but we must remember that H+ does not exist by itself in water and that H CC is a better representation. 

Hydration of compounds 2, 3, 4, 5 was found to be first order both in substrate and in hydronium ion (4-10). Furthermore, a careful kinetic study of compounds 2c-g and the sulfur analog 4 revealed that the hydration rate at constant ionic strength was dependent on the buffer concentration and hence was general acid catalyzed. 

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

A hydrated hydrogen ion, called the hydronium ion, contains only one water of hydration H+(H20) or H30+... 

Let us now extend the long-period hydronium ice-like model for the IHP on Pt(lll) to explain the observations in electrolytes other than sulphate. In acid chloride, both the observations and the model carry-over directly from the case of sulphate. In fluoride, perchlorate, bicarbonate and hydroxide, in Which the anomalous features shift considerably in both potential and appearance (especially in the basic media) from sulphate, another model is needed. Both (bi)sulphate and chloride are large weakly hydrated anions, and in the double-layer model of Figures 4-5, they interact strongly with both the hydronium ions and the Pt surface. The contact adsorption... 

In aquatic chemistry, the unitary proton level of the proton dissociation reaction is expressed by the logarithm of the reciprocal of the proton dissociation constant i.e. p = - log K here, a higher level of proton dissociation corresponds with a lower pK. When the pKy of the adsorbed protons is lower than the pH of the solution, the protons in the adsorbed hydronium ions desorb, leave acidic vacant proton levels in adsorbed water molecules, and form hydrated protons in the aqueous solution. Fig. 9-22 shows the occupied and vacant proton levels for the acidic and basic dissociations of adsorbed hydronium ions and of adsorbed water molecules on the interface of semiconductor electrodes. 

Fig. 9-22. Unitary proton levels of hydrated and adsorbed hydronium ions (acidic proton) and of hydrated and adsorbed water molecules (basic proton) the left side is the occupied proton level (the real potential of acidic protons), and the right side is the vacant proton level. Hi/HjO) = unitary occupied proton level of adsorbed hydronium ions (acidic proton level) H20.d = unitary vacant proton level of adsorbed hydronium ions (acidic proton level) and unitary occupied proton level of adsorbed water molecules (basic proton level) OH = unitary vacant proton level of adsorbed water molecules (basic proton level) (pHi, ) = hydrated proton level at iso-electric point pR...

These studies showed that sulfonate groups surrounding the hydronium ion at low X sterically hinder the hydration of fhe hydronium ion. The interfacial structure of sulfonafe pendanfs in fhe membrane was studied by analyzing structural and dynamical parameters such as density of the hydrated polymer radial distribution functions of wafer, ionomers, and protons water coordination numbers of side chains and diffusion coefficients of water and protons. The diffusion coefficienf of wafer agreed well with experimental data for hydronium ions, fhe diffusion coefficienf was found to be 6-10 times smaller than the value for bulk wafer. 

Figure 27. Defective structure of solid trifluoromethane-sulfonic acid hydrate (CF3S0sH H20)4 found using ab initio molecular dynamics (AIMD see Section 2.2.3 for a description of the technique), showing two hydronium ions hydro-gen-bonded to sulfonate groups (as found in the perfect structure) but, more importantly, two shared protons (one between two sulfonate groups and the other as part of a Zundel ion see text). Note that the energy of the defective structure is only --30 kj/mol higher than that of the perfect structure.

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