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Physical Chemistry

Irving Langmuir Award In Chemical Physics

by Jyllian Kemsley
February 24, 2014 | A version of this story appeared in Volume 92, Issue 8

Credit: Courtesy of Mark Johnson
This is a photo of Mark A. Johnson.
Credit: Courtesy of Mark Johnson

Sponsored by General Electric Global Research and the ACS Division of Physical Chemistry

Talking about his own graduate experience, Mark A. Johnson says he valued the encouragement to explore. “It’s very seductive when you are allowed to fail and then succeed and feel the satisfaction from that,” he says. “It’s very inefficient, but what came out of it was a real joy in doing basic ­research.”

Now the Arthur T. Kemp Professor of Chemistry at Yale University, Johnson tries to maintain that environment in his own research group as it studies fundamental properties of frozen clusters. In particular, Johnson is known for examining hydrated protons and electrons and how water’s hydrogen-bonding network accommodates them. The team uses hybrid spectroscopic techniques that combine the ability to get mass and formula from mass spectrometry with structural information from optical spectroscopy.

In the case of the hydrated electron, researchers had thought that the electron would be surrounded by water molecules that were all the same. Instead, Johnson’s group found that the electron is closely associated with a single water molecule that has both O–H bonds pointed toward the electron.

For the hydrated proton, Johnson’s group determined that the proton switches between H3O+ and H5O2+. “This shuttling behavior lies at the heart of the broad spectra of aqueous acids and the anomalously large rate of water-mediated proton transport,” says Richard N. Zare, a chemistry professor at Stanford University who was Johnson’s graduate adviser.

Johnson’s current work on water moves beyond the frozen realm to look at what happens when the clusters melt. It’s taken a lot of work to get the temperature control necessary to do the experiments, he says. The goal of the work is to systematically warm up water clusters “and then look at how they start to percolate as the molecules switch between sites, giving us a first look at the microscopic mechanics underlying the remarkably broad vibrational spectrum of liquid water,” Johnson explains.

Research on other ion-cluster systems has revealed key elements of reactivity, such as in studies of NO+ in water. In the atmosphere, NO+ and water react to form HONO, which in turn leads to HO∂ and O3. But HONO production critically depends on the geometry of water molecules around NO+ and their ability to promote charge delocalization, Johnson’s group found.

Johnson’s techniques are being applied to a broadening range of systems. “Tools that he originally crafted to address problems in the chemical physics domain have evolved into a powerful analytical platform” that can provide new information on polypeptides, supramolecular architectures, and highly unstable intermediates in homogeneous catalysis, says Kenneth Jordan, a chemistry professor at the University of Pittsburgh whose research includes theoretical studies of water clusters. Johnson “has extended his cryogenic-ion chemistry approach to provide a general means with which to obtain highly resolved vibrational spectra for essentially any species produced using a wide array of ionization methods.”

Johnson, 54, received a Ph.D. from Stanford in 1983, then did postdoctoral research with W. Carl Lineberger at the University of Colorado, Boulder, before joining the Yale faculty in 1985.

He will present the award address before the Division of Physical Chemistry.


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