C&EN: TODAY'S HEADLINES - TWO FAST STUDIES OF LIQUID WATER

LOUISA DALTON

Water can be slippery. Despite centuries of study, experimental details about its rapidly shifting hydrogen-bond network remain elusive. Two reports this week make headway by using rapid-fire femtosecond spectroscopy to probe basic water phenomena: transferring a proton and dissolving a salt.


	ADAPTED FROM SCIENCE

Researchers in Germany and Israel measured the speed of proton transfer from acid to base in water and propose a detailed mechanism [Science, 301, 349 (2003)].

This "simple and elementary process" has historically been divided into three steps, notes coauthor Matteo Rini, a Ph.D. candidate in physics under Erik T. J. Nibbering at Max Born Institute for Nonlinear Optics & Short Pulse Spectroscopy in Berlin: a diffusion step as acid and base approach each other, proton transfer, and separation. But that key second step has been hard to catch in action. Unable to measure fast enough, previous groups "could never see this intimate stage of the proton transfer," says collaborator Ehud Pines, professor of chemistry at Ben-Gurion University of the Negev, in Israel.

The group added to deuterated water the base acetate and the acid 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS)--called a photoacid because it can be transformed by light from a weak acid into a strong acid. They allowed the acid and base to hydrogen-bond with each other and with water and then excited the photoacid. They then closely followed the deuteron transfer with femtosecond mid-infrared spectroscopy.

Two things happened. Some transfers went lightning fast--as fast as or faster than the shortest duration they could measure, 150 femtoseconds. This, Pines says, is a stripped-down step two, a direct proton transfer between acid and base already hydrogen-bonded in the ground state.

But other transfers took up to 1,000 times longer. The scientists propose that these slow transfers are held up by an intermediate "encounter stage." They say that in plenty of water, acid and base first form a loose complex through a "wire" of intervening hydrogen-bonded water molecules.

What happens at this point is fuzzy. Water may partly rearrange itself out of the complex as the authors propose. Or, says David Chandler, a chemist at the University of California, Berkeley, the proton may transfer indirectly through the wire of water molecules in the encounter complex.

In the Netherlands, a separate group is observing ion solvation shells in water. This research, headed by chemistry professor Huib J. Bakker at the University of Amsterdam and the FOM Institute for Atomic & Molecular Physics, firmly establishes that only in the single layer of water molecules surrounding an ion is the hydrogen-bonding network affected, says graduate student Michel F. Kropman [Science, 301, 347 (2003)].

"This clearly contradicts the views held by some that ions cause the entire aqueous solution to either be 'more structured' or 'less structured,' " says Edward W. Castner Jr., a chemistry professor at Rutgers University. He refers to an idea from as early as the 1930s that explains bulk liquid effects of ions in water, such as increased viscosity. "If this view were correct," Kropman says, "one should see, slightly away from the ion, in the second or third shell, an effect on the arrangement and mobility of water molecules." The Dutch group observed solutions of Mg(ClO₄)₂in water using femtosecond pump-probe spectroscopy and found no effect on water molecules not directly adjacent to the ion.

It is hard to predict what such fundamental revelations about proton transfer and ion solvation in water might lead to. But insights into reactions so basic, Castner says, are "highly relevant to all chemists who are concerned with reactivity in water."