Revisiting the Hydrated Proton

Previously inaccessible spectroscopic details are providing a fuller picture of the physical nature of hydrated protons than has been possible before. The study--by chemistry professors Michael A. Duncan of the University of Georgia, Athens; Mark A. Johnson of Yale University; Kenneth D. Jordan of the University of Pittsburgh; and coworkers--shows that the spectroscopic properties of hydrated protons are, to a previously unrecognized degree, extremely sensitive to changes in the clusters of water molecules that surround them (Science 2005, 308, 1765).

"The results are nothing short of remarkable, showing what are easily the most dramatic spectroscopic changes ever observed with cluster size," says chemistry professor Timothy S. Zwier of Purdue University. "The study finds infrared (IR) absorption frequencies that oscillate back and forth between those characteristic of a localized and a delocalized proton"--that is, a hydrated proton whose charge is either retained closely or spread out in the water cluster surrounding the proton. "These results are the sort that will change the way we as chemists look at acidic aqueous solutions."

Acid solutions contain excess protons "that are intimately associated with one or more water molecules but are typically viewed as single hydronium ions [H₃O⁺], the quintessential carriers of positive charge in acidic aqueous solution," Zwier says. "Research is still seeking a clearer molecular-scale picture of what this species is and how it interacts with water molecules around it. Because the hydronium ion is so intimately involved with its aqueous environment, the spectroscopic signature of an excess proton in water and especially its molecular-scale interpretation have remained amazingly elusive."

Duncan, Johnson, Jordan, and coworkers addressed this problem by using a tabletop laser to generate IR radiation with sufficient power to probe a critical but previously inaccessible low-energy region of the hydrated proton's vibrational spectrum. They used a tagging technique to cool the clusters sufficiently to identify their minimum-energy structures.

The researchers "recorded IR spectra over a very wide spectral range that made it possible to follow the absorptions of the hydronium ion and its first solvation shell directly," Zwier says, "and carried out extensive ab initio calculations to compare with the experimental data and facilitate proper interpretation. This work builds on a long history of previous studies, none of which could piece together enough spectral coverage to put together a coherent physical picture of the structural changes induced by addition of each water molecule to the cluster."

Until now, the IR spectroscopy of the hydrated proton has yielded diffuse bands, making it difficult to understand the internal structure of acid solutions and the way excess positive charge is spread in them. "When we scanned the low-frequency range of the spectra," Johnson says, "we didn't know if we were going to see the same kind of diffuse bands that had been observed before or sharp molecular-like features that have vibrational normal modes associated with them. We found that they are sharp and that they are highly responsive to the environment of local water molecules around the proton."

THE IR SPECTRA of the clusters reveal sharp bands that "display huge shifts throughout the 1,000- to 3,000-cm^­1 region as water molecules are added sequentially," Jordan adds. "The size-selective vibrational spectra then directly reveal the absorptions associated with particular hydration configurations. The cluster studies are thus telling us important and essential features of how condensed-phase water works."

Professor of chemical physics Mitchio Okumura of California Institute of Technology comments that the researchers "find striking new spectral features that reveal details of the solvent environment of the excess proton"--that is, clear spectral signatures of H₃O⁺ (called the "Eigen structure" of the hydrated proton) and H₅O₂⁺ (the "Zundel structure"). "They find large variations in the structure of the hydrated proton as waters are added one by one, as the proton shifts between the two forms," he says. "These are fascinating and important new results in this field."

Earlier work in this area has been done by several groups, Okumura notes. The earliest and somewhat forgotten study, he says, was in 1977, by chemist Harold Schwarz of Brookhaven National Laboratory, who first observed the Eigen structure's IR absorption band at 2,660 cm^­1. In the 1980s, chemistry professor Yuan T. Lee (now at Academia Sinica, in Taipei, Taiwan) and coworkers studied the IR spectroscopy of mass-selected proton clusters and obtained the first IR spectra of H₃O⁺ and H₅O₂⁺ clusters containing one or more water molecules. In 2003, the spectrum of proton motion in the Zundel cation was obtained with a free-electron laser by Knut R. Asmis of the Institute for Experimental Physics at the Free University of Berlin, chemistry professor Daniel M. Neumark of the University of California, Berkeley, and coworkers.

The new study takes a step beyond these earlier studies by accessing the important low-energy part of the IR spectrum of large clusters of the hydrated proton and revealing new details about the hydrated proton's structure and charge delocalization properties. "We are getting a much more sophisticated idea of how to use vibrational spectroscopy to identify what kind of structural arrangements are important," Johnson says.