Hydrated hydronium ions

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. 

First of all, what was considered were bare hydronium H3+0 ions with three equivalent protons, a hydrated hydronium ion with three strongly bound water molecules (i.e., Eigen cluster H904+), and the symmetric H502+ complex in which a proton is shared between two water molecules (i.e., the Zundel ion). Many intermediates or more-complex states of the hydrated proton, H+(H20) , may also exist. All clusters have a finite lifetime and transform between each other during charge transport. Due to the variation of the relative abundance of these three basic states, proton transfer may occur via different pathways. 

The ternary reactions (32) and (33) sequentially build up the NO+ hydrates until N0+(H20)3 is produced, which then undergoes a binary reaction to give the hydrated hydronium ions. Further increases in the size of the hydrates is considered to be via direct clustering ... 

Of course the hydronium ion, H3O+, will be associated with additional water molecules as well through H-bonding. For simplicity we will just write H+, with the understanding that it refers infact to hydrated hydronium ions in aqueous solution.) Note the reaction is not highly favoured, in neutral solution (no excess H or OH-) there will only be molar hydronium ions, in other words only about 2 of every billion water molecules will be protonated ... 

The above-described features are reproduced in a high level quantum-molecular-dynamics simulation of an excess proton in water [30, 31]. In accordance with results from several other groups, this finds the excess proton either as part of a dimer (H5O2+, Zundel -ion) or as part of a hydrated hydronium ion (H9O4+, Eigen -ion) (Fig. 23.3). 

Table 11. Temperature dependence of electron-ion recombination rate constants for hydrated hydronium ions Leu et al. [48] in units of 10 cm s ).

The very large aj rate constants for molecular ion-electron recombination have also been reported for cluster ion studies, performed at low total gas pressures. Table 11 shows the results of measurements performed by Leu et al. [48] on gaseous ion-electron recombination involving the hydrated hydronium ion [H30 (H20)n]. Significant increases in the recombination rate constant are seen for even a few cluster water molecules, with a value for the H30 (H20)5 cluster at 205K as high as 7.5 x 10 cm s obtained. 

Simple coordination and ion-association, as with ( Onium ), FeCl4. Onium stands for one of the following cation types hydrated hydronium ion, (H20)3H+, a rather labile cation requiring stabilization by solvation with an oxygen-containing solvent a substituted ammonium ion, R NH(4 >, where R is an alkyl or aralkyl group and n may vary from 1 to 3 a substituted phosphonium ion R4P+ ... 

A FIGURE 16.2 Ball-and-stick models and Lewis structures for two hydrated hydronium ions. 

In bulk water, protons exist in interchangeable charged clusters of water the hydronium ion the hydrated hydronium ion (the Eigen cluster)... 

The distribution of hydrated hydronium ions observed in the Cl mass spectrum is determined by the equilibritrm constants for the reactions described with Eq. (215) and the temperature and pressure in the ion source. Hydronium ions do not react further with water vapour, but if samples with higher proton affinity than water are present, they will react according to proton transfer reactions such as ... 

Further termolecular reactions produce the hydrated hydronium ions H30+(H20) , which are observed to be the major positive ions in this altitude region. 

Figure 5 Observed variations with the number of water molecules in the rate coefficient for proton-transfer reactions between hydrated hydronium ions and various molecules B at room temperature. The measurements were taken using the flowing-afterglow technique. Adapted with permission from Bohme DK, Mackay Gl and Tanner SD (1979) Journal of the American Chemical Society 101 3724-3730.

The mechanism by which hydrated hydronium ions form in aradioactive or electrical discharge source Is described in Chapter 3. 

The ratios of [H30 + (H20)1 to [H3O + ] calculated using this procedure are shown in Table 3.1 for several different E/N values. These calculated values suggest that at 120 Td the proportion of H3O + (H2O) in the drift tube is relatively low. However, at smaller values of E/N the contribution of H3O + (H2O) grows substantially and below 100 Td hydrated hydronium becomes the dominant ion. Descending to still lower E/N values than those shown in Table 3.1 will see larger hydrated hydronium ions become the dominant species. As discussed in Chapter 2, proton transfer from H3O + (H2O) ean be much slower than from... 

The 4-hydrated ion is, in a sense, the solution structure, and therefore is of paramount importance. The behavior of the energy hyper-surface, described above, is not only odd in and of itself, but is at substantial variance with the equivalent behavior of the 4-hydrated proton (or the 3-hydrated hydronium ion). The Polarization odel computations on this moiety found a local minimum in the energy hyner-surface which corresponds to the roughly pyramidal structure shown in Figure 7 a and b. ( We also show, for completeness, in Figure 8 the extended structure which corresponded to the absolutely lowest energy which we could find for the 4-hydrated proton. This structure corresponds closely to... 

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