Spectral Signatures of Hydrated Proton Vibrations in Water Clusters

Abstract

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 (H3O+) and Zundel (H5O2+) 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.

Finite-Temperature Effects on the Stability and Infrared Spectra of HCl(H2O)6 Clusters

“…The dynamical nature of the Eigen-ion band in the dissociated cluster confirms the broad nature of such feature also in a confined geometry. 47 We hope our study will stimulate further efforts to identify spectroscopically larger HCl-water clusters than obtained so far.…”

Can the Four-Coordinated, Penta-Valent Oxygen in Hydroxide Water Clusters Be Detected through Experimental Vibrational Spectroscopy?

“…Our results suggest that single photon argon tagged action-spectrum experiments 7 and multiphoton action-spectrum experiments 28 may lead to different spectral results due to the higher internal temperature generated in a multiphoton experiment. The higher internal temperature leads to the sampling of different portions of the potential energy and hence different spectroscopic results.…”

IR Spectrum of the H5O2+ Cation in the Context of Proton Disolvates L−H+−L

“…Band c probably originates from an overtone of ν as +ν s in the condensedphase spectra. The weak band that appears in the same frequency region in the gas-phase spectrum of the bare ion is due to the δH 2 The disappearance of the δ(H 2 O) band of H 5 O 2 + in condensedphase spectra appears to be related to the disappearance of stretching vibrations of O-C-O, COO, POO, PdO, etc., groups in L-H + -L complexes. This phenomenon is poorly understood but may to be related to the way these vibrations are modulated by the stochastic motion of the bridging proton.…”