Summer 2004 -- Vol. 21

Research Reveals How an Acid Dissolves, Molecule by Molecule

The most precise description ever obtained by experiment of exactly how an acid compound dissolves, molecule by molecule, was published in the journal Science. In addition to shedding new light on this basic property of matter, the research is expected to have broad impacts across the fields of chemistry, biology, and physics. The research team was led by A. Welford Castleman Jr., Evan Pugh Professor of Chemistry and Physics and the Eberly Family Distinguished Chair in Science.

This new knowledge is based on experiments in which Castleman’s lab used water molecules as a solvent to dissolve the acidic molecule hydrogen bromide. “We chose to work with hydrogen bromide both because it is a good model of a typical acid and because it is of particular research interest for its role in understanding a range of situations, especially ozone depletion in the upper atmosphere and proton motion in water, which is important in a number of biological processes,” Castleman explains. Hydrogen bromide is one of the compounds whose dissolution in the upper atmosphere contributes to the formation of the ozone hole, and Castleman’s research sheds light on issues related to why such reactions occur at a rate much faster than expected.

Schematic illustration of the excited-state acid dissolution of hydrogen bromide (HBr) within the mixed cluster of HBr(H2O)4 shown at left. The first step in the mechanism is the proton transfer from the acid to a water molecule which is followed by solvent reorganization.

“While every student of freshman chemistry hears about acid dissociation and solvation in water, we have only recently begun to understand how this works at the microscopic level, and Castleman’s work is an important contribution along that route,” comments James T. Hynes, of the University of Colorado at Boulder and the French National Science Foundation’s (CNRS) director of research in the Department of Chemistry at the E´cole Normale Supérieure in Paris. “While the public is aware of ozone depletion in the Antarctic stratosphere—the ‘Ozone Hole’—significant ozone depletion also is occurring elsewhere. A striking example of this is the rapid and essentially complete ozone depletion that occurs in the spring in the Arctic, near the Earth’s surface. The acid dissociation of HBr on water ice particles is thought to play a critical role in this process,” Hynes says.

Although the dissolution of acids is one of the most fundamental chemical processes, its precise mechanism has remained a mystery for decades. Theoretical predictions about how the molecules rearrange when an acid dissolves have not been verified by experiments because the reaction, which normally occurs in a liquid, happens so rapidly that scientists have not been able to study it.

Castleman’s team overcame the experimental difficulties by taking “snapshots” of reactions that occurred in a vacuum chamber into which they injected separate gas-like streams of water and hydrogen-bromide molecules and observed the reactions that occurred at their intersection using incredibly fast lasers. “Our femto-second lasers emit very short pulses of laser light on the order of 10 to the minus 15 seconds, which is as fast as molecules vibrate,” says Sean M. Hurley, a postdoctoral scholar in Castleman’s lab and a co-author of the research paper. “We probe reactions between molecules as fast as they happen, which enables us to detect each step, and we use a time-of-flight mass spectrometer to detect the molecular products that the reactions produce.”

Scientists have speculated that hydrogen-bromide molecules, present in the upper atmosphere as a result of pesticides and flame retardants used on Earth, could be dissolved by their interaction with the water molecules on the surface of ice crystals or ultrafine particles. “When hydrogen bromide dissolves, it forms ions, which react a lot faster than a neutral molecule,” says Castleman. The formation of ions helps to explain why reactions in the upper atmosphere that involve the dissolution of acids occur so rapidly. “Ions react very quickly with other molecules in the upper atmosphere to form different compounds, many of which would not have formed if the acids had not been in the upper atmosphere in the first place,” Castleman says.

Specifically, the researchers found that the interaction of four surrounding water molecules with the hydrogen-bromide molecule tips the energy balance of the hydrogen-bromide molecule to trigger its eventual dissolution. They also found that the process is complete by the time a fifth water molecule is added to the configuration. The combined energies of the four water molecules entice the hydrogen atom’s electron to move to the bromide atom, beginning a sequence of reactions. The hydrogen atom, now a positively charged ion, immediately is captured by one of the H2O water molecules, forming an acidic H3O⁺ ion compound and leaving behind a now negatively charged and reaction-ready bromide ion, Br^-. The Castleman team’s research revealed these initial steps and all the subsequent steps in the dissolution process.

This new and more detailed description of molecular choreography is expected to aid researchers working in a variety of fields. It likely will have a particularly broad impact in chemical research, where many areas of interest involve the dissolution of acids. “This more precise understanding of how the process behaves on a molecular level could aid scientists in improving control over chemical reactions, enabling them to better achieve the desired result,” Hurley says. The research also likely will impact biological research. Among the factors that the researchers studied is the reorganization of protons while the molecules of hydrogen bromide are becoming rearranged in response to their interactions with water molecules. “This type of proton behavior is important in practically all reactions that involve water, including a broad range of biological processes,” Castleman explains. The research also revealed new information about the movement of protons and the resulting change in the way electrical charges are distributed throughout the system, which is an area of interest in physics research.

In addition to Castleman and Hurley, other members of the research team include graduate students Troy E. Dermota and Darren P. Hydutsky.

This work was sponsored jointly by the Experimental Physical Chemistry and Atmospheric Sciences Divisions of the National Science Foundation.

Barbara K. Kennedy

Back to Science Journal Summer 2004 Index

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