Zundel cations, protonated water

As observed, the direction of electron flow from 03 to 06 (and from 08 to Ol) is the same as that of the transferring proton H5 (and HIO). This is qualitatively different from what we observed before in the proton transfer in Zundel cation [449] and water-molecule-mediated proton transfer in formamide [286], in which electrons flow in the reverse direction of proton motion. Therefore it appears that in the cychc moleciflar field like FAD, much electron carried by proton nuclei does not have to be compensated by the backward intermolecular flow but mainly by reorganization within the individual monomers. 

In Chap. 5, Oriol Vendrell et al. give a similar example the infrared spectrum of the protonated dimer or Zundel cation, H5O+. Using a time-dependent description with the MCTDH approach including all 15 internal degrees of freedom, they have been able to completely characterize the infrared spectra in the gas phase for this system as well as all its analogues where the hydrogen atoms have been replaced by deuterium atoms [153,154]. They have also performed time-dependent simulations of the dynamics of a proton between the two water molecules [213]. As explained by S. S. Xantheas, the scope of this work exceeds the simple description of this small cluster alone since hydrogen cations are ubiquitous in nature [214]. The... 

We have seen that the effect of a full or partial deuteration of the cation not only leads to line shifts but also significantly changes the intensities and modifies the assignment of the infrared signatures of the different isotopologues. This is due to the soft, anharmonic, and coupled potential of the Zundel cation, where the dynamics and spectroscopy are strongly dominated by Fermi resonances between various coupled zeroth-order vibrations. The discussed quantum dynamical calculations represent an important milestone in our understanding of the spectroscopy and dynamics of protonated water clusters and on their dramatic isotope effects [41], and could only be achieved after a full-dimensional quantum dynamical treatment of the clusters. 

Figure 23. Description of six geometric triggers required for structural diffusion (a) O -O separation must form a Zundel ion (b) O -H separation must exceed the equilibrium bond distance (c) Z0 H 0 is nearly linear in the Zundel ion (d) Lone pair of electrons in the water should point towards the proton (e) Initial H3O forms an Eigen ion (f) Eigen cation is formed around final H3O. These six geometric triggers must he satisfied along with the energetic trigger for the reaction to take place. O of H3O, gray O of H2O, black H, white.

Figure 12.15. Representative configurations from AIMD simulations of the trifluor-omethanesulfonicacid monohydrate solid showing (a) the native solid with each hydronium ionhydrogen bonded to three different CF3SO3" anions and (b) a defect state of the solid with two of the four protons existing as Zundel-like cations one with two water molecules and the other with two CF3S03 anions.

One of the best investigated, yet not fully understood, transport processes is that in water. So far, the dispute about the Zundel and Eigen ion being the transporting species has been settled partly. Many studies favour the Eigen ion the Zundel ion is considered to be a transition state during the proton transfer. Similar species have been observed in phosphonic and phosphoric acid. However, the presented studies did not put emphasis in the distinction between those two cationic species. 

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