H₂O-HO Radical Complex Unveiled

Chemists know a lot about the HO radical. Its voracious oxidizing power makes it capable of both starring in atmospheric pollutant-removing reactions and wreaking havoc in biological systems.

What's not well known, however, is what happens if HO forms a radical complex with water. Since water is ubiquitous in organisms and in the atmosphere, chemists have speculated on the importance of the chemistry of an H₂O-HO radical complex. In fact, some models suggest that the complex could be even more oxidizing than HO itself. That possibility, though, has remained relatively unexplored.

In the past few years, H₂O-HO has become the focus of a number of labs around the world. Now, two groups unveil the first spectroscopic microwave studies of the complex in the gas phase. The independent reports appeared almost simultaneously. One comes from chemistry professor Kenneth R. Leopold at the University of Minnesota, Twin Cities; his graduate students Carolyn S. Brauer, Galen Sedo, and Erik M. Grumstrup; and chemistry professors Mark D. Marshall and Helen O. Leung at Amherst College, in Massachusetts [Chem. Phys. Lett., 401, 420 (2005)]. The other is from chemistry professor Yasuki Endo at the University of Tokyo and colleagues [J. Am. Chem. Soc., 127, 1108 (2005)].

Among other things, the studies confirm the length of the weak hydrogen bond connecting H₂O and HO, and provide insights into how HO's electrons rearrange themselves energetically and spatially when the complex is formed. For example, both papers conclude that pairing HO with water removes orbital degeneracy in HO, producing two closely separated electronic states.

Such changes could profoundly affect the complex's chemistry, causing it to absorb sunlight differently, perhaps, or making it easier for other molecules to interact with it. The new observations thus represent an important step toward understanding how the complex behaves, notes Veronica Vaida, chemistry professor at the University of Colorado, Boulder, a pioneer in the study of atmospheric molecular complexes. The "very high quality, beautiful high-resolution spectra will push theoretical models to another level," she says.

Though there's an enormous body of work on free radicals and on molecular complexes, only in the past 15 years or so have experimental chemists begun to focus on radical molecular complexes. For example, chemistry professor Marsha I. Lester at the University of Pennsylvania and chemistry professor Michael Heaven at Emory University in Atlanta performed experiments on rare gas-HO complexes. Lester has also studied complexes such as OH-HCCH and OH-CH₄ in the infrared.

"To go that extra step and make a complex of a free radical is just that much harder," Leopold notes. "People have studied radical complexes by other techniques, but there has been only a small handful of examples that have been studied at the high resolution of microwave spectroscopy."

Theoretical chemists have modeled the structure and reactivity of H₂O-HO for decades. Experimental work by chemists at the Jet Propulsion Laboratory hinted at the potential importance of radical complexes when it was found that HO₂ reactivity greatly increases in the presence of water.

But H₂O-HO received little attention until Terence I. Quickenden, senior chemistry lecturer at the University of Western Australia, Crawley, and colleagues there and at the University of Otago, Dunedin, New Zealand, began infrared studies of the complex in argon matrices [J. Am. Chem. Soc., 125, 6048 (2003)].

THE NEW measurements, with their unprecedented spectral resolution, provide plenty of opportunity for examining fundamental chemistry, such as the unique stability of hydrogen bonding in such complexes.

Atmospheric chemists, in particular, find H₂O-HO interesting, as the HO radical oxidizes organic pollutants and may also play a role in the chemistry of Earth's ozone hole. Joseph S. Francisco, a chemistry professor at Purdue University who has predicted the structure of numerous molecular complexes, including ClO-H₂O and H₂O-HO₂, calls Endo's and Leopold's research "exciting," noting that the HO radical's electronic distribution is substantially changed from the uncomplexed version. "This confirms some of the theoretical things we've been finding," he says.

It's also believed that HO could form in irradiated ice and possibly affect the reflectance spectrum of icy outer solar system bodies such as Pluto and Jupiter's moon Europa. Quickenden and his colleagues, including Robert E. Johnson, engineering physics professor at the University of Virginia, have been investigating the possibility that H₂O-HO may also be formed there. "H₂O-HO has a real possibility of being the source of some of the luminescence from irradiated ices," Quickenden says.

Chemists can expect to see more research on radical molecular complexes. For example, Endo and coworkers are preparing a paper on their measurements of the rotational spectra of the H₂O-O₂ complex. They also are continuing to study other rare gas radical complexes.

Other radical complexes, such as ClO-H₂O, that may also play important roles in atmospheric chemistry are awaiting detection. "Reactions involving complexes may be much more prevalent than people thought," Francisco says.