Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH− ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH− has a predominantly planar fourfold coordination forming a (H9O5)− complex. Occasionally this complex is transformed in a more open tetrahedral (H7O4)− structure. Proton transfer is observed only for the more waterlike (H7O4)− complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure—and not the constituent particles—of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate‐limiting step for OH− diffusion is the formation of the (H7O4)− structure, which is the solvation state showing proton transfer activity.

Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H 3 O + ) and Zundel (H 5 O 2 + ) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

https://science.sciencemag.org/content/sci/308/5729/1765.full.pdf

Spectral signatures of hydrated proton vibrations in water clusters.

The relative influence of thermal and quantum fluctuations on the proton transfer properties of the charged water complexes H5O2+ and H3O2− was investigated with the use of ab initio techniques. These small systems can be considered as prototypical representatives of strong and intermediate-strength hydrogen bonds. The shared proton in the strongly hydrogen bonded H5O2+ behaved in an essentially classical manner, whereas in the H3O2− low-barrier hydrogen bond, quantum zero-point motion played a crucial role even at room temperature. This behavior can be traced back to a small difference in the oxygen-oxygen separation and hence to the strength of the hydrogen bond.

https://science.sciencemag.org/content/sci/275/5301/817.full.pdf

On the quantum nature of the shared proton in hydrogen bonds

Inhomogeneous RF Fields: A Versatile Tool for the Study of Processes with Slow Ions

The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+·(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm^−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm^−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 "messenger." A spectrum of H7 O + 3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.

/pdf/vibrational-spectroscopy-of-the-hydrated-hydronium-cluster-2g57xz1xl5.pdf

Vibrational spectroscopy of the hydrated hydronium cluster ions H3O+·(H2O)n (n=1, 2, 3)

Using a new spectroscopic method, the gas phase IR spectra of the title cluster ions, produced in a high pressure corona discharge source, are observed between 3550 and 3800 cm-1.

J. M. Price

Papers

Vibrational spectroscopy of the hydrated hydronium cluster ions H3O+·(H2O)n (n=1, 2, 3)

Vibrational Spectroscopy of the Hydrated Hydronium Cluster Ions H3O+·(H2O)n (n = 1, 2, 3).

IR Spectroscopy of the Pentacoordinated Carbonium Ion C2H+ 7