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Physical Chemistry

Arnie Rheingold

UC San Diego chemist set out to learn crystallography--that was 6,000 structures ago

April 26, 2004 | A version of this story appeared in Volume 82, Issue 17


To say that crystallography has come a long way in the past quarter century is an understatement. Structures that once took weeks to resolve can now be done in a few hours. One person who has witnessed this change is Arnold L. Rheingold, a chemistry professor at the University of California, San Diego.

Rheingold is probably the most prolific crystallographer around today. His group, besides solving the structures of their own compounds, has worked with many of the best known inorganic and organometallic researchers worldwide to produce some 6,000 crystal structures that have appeared in nearly 1,500 research papers. The most recent edition of Thomson ISI's Essential Science Indicators lists Rheingold as the fifth most-cited chemist during the past decade, with 800 of his papers cited more than 10,600 times.

Rheingold's foray into crystallography had a rather humble beginning. He received a Ph.D. degree in inorganic chemistry from the University of Maryland in 1969. The next year, he began his academic career as an assistant professor at the State University of New York, Plattsburgh. During the '70s, Rheingold developed a research program synthesizing inorganic clusters of transition metals with heavier group 15 elements, particularly arsenic and antimony.

"We were the first to look at isolobal replacement of a CH group with an arsenic atom, where a ring of five arsenic atoms could become like a cyclopentadienyl group," he says. "We explored in great detail what was later called inorganometallic chemistry: mimicking organometallic chemistry but without carbon."

At the time, not many chemists were equipped to do crystallography, nor did they have the patience for the time it took, he relates. "I was making compounds that I couldn't characterize without crystallography. And I was running out of friends who were crystallographers who could do structures for me. It became clear that I better learn how to do it myself."

In 1980, Rheingold took a sabbatical leave and went to SUNY Buffalo to learn crystallography from chemistry professor Melvyn R. Churchill. The next year, Rheingold ended up at the University of Delaware, where the chemistry department was setting up a unit in crystallography and asked him to run it. He gave up his tenured position at Plattsburgh to take a staff position as crystallographer--a bold gamble. Rheingold later moved into a faculty position, but he had to re-earn tenure. He became full professor again in 1987 and remained at Delaware until January 2003, when he moved to UC San Diego.

At Delaware, requests for Rheingold to solve structures began to come in from all over the world at a rapid pace. "I slowly transitioned from being a synthetic inorganic chemist to an inorganic chemist who did crystallography to now having mostly left the synthesis behind," he notes.

Besides his own work, some of the more interesting structure determinations include work in the early 1990s on compounds with Si=Pt and Si=Ru bonds from chemistry professor T. Don Tilley, formerly at UC San Diego and now at UC Berkeley. "These were the first compounds containing metal-silicon double bonds," Rheingold says. "This was difficult crystallography on air-sensitive materials."

Rheingold's most cited paper is one with chemistry professor Robert H. Crabtree of Yale University that describes dihydrogen bonding--an unconventional type of hydrogen bonding between the hydrogen atoms of NH or OH and a transition-metal hydride or boron hydride [Acc. Chem. Res., 29, 348 (1996)].

"This is a wonderful example of what one can learn by close inspection of crystal lattices," Rheingold observes. "Most people do crystallography with the focus on a molecule. You get the molecular structure, and you think you are done. But I insist on looking at the entire crystal structure. How are the components of the lattice arranged? What are the interactions with other molecules? If we had not done that, we would have missed seeing this unusual type of hydrogen bonding."

The biggest changes in crystallography over the years have been computer interfacing and the advent of the charge-coupled device (CCD) detector that replaced scintillation counters, he says. The revolution began in the mid-1970s with automated processing and data collection. "Now that we are into the third generation of CCD detectors, we have the ability to work with crystals one-tenth the size of what they used to be," Rheingold adds. "And we are getting much better data at the same time."

Rheingold revels in helping enthusiastic young researchers just starting out. He also would like to make crystallography a more routine presence on college campuses. "I would like to see a diffractometer in every chemistry department," he says.

One of his current efforts is a 10-day summer school, "Crystallography for Organic Chemists," to be held at UC San Diego in August. It's funded by a grant from the American Chemical Society's Petroleum Research Fund.

"Organic chemists don't often use X-ray diffraction methods for routine characterization. Instead, they usually rely on spectroscopic methods that are not always definitive. I believe that their inhibitions arise from a lack of experience and a fear that they may make costly mistakes. The course is designed to help chemists to become better acquainted with the process, rewards, and pleasures of obtaining and using crystallographic data."

Rheingold's group--one graduate student, one postdoc, and a couple of people on sabbatical leave--uses two diffractometers and is looking for a third to keep up with the workload. "We are doing four or five structures a day--about 1,000 structures per year," he says. "I think we could do six or seven structures a day."



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