Published September 13, 2013
Jochen Autschbach, professor of chemistry, is co-author of a paper that appeared Sept. 12 in Science Express, an online archive that compiles selected articles that will appear in forthcoming print editions of the leading journal Science.
The research deals with dirhodium metal complexes—powerful catalysts that facilitate a wide variety of useful chemical reactions. Since the 1970s, scientists have sought to understand precisely what happens during these reactions, but their efforts have been hindered by the fact that the catalysts react with other molecules to produce unstable intermediate compounds that exist for such brief periods of time that detailed study has been impossible.
The Science Express article reports a major breakthrough: that scientists have managed to isolate and study the properties of a dirhodium carbene intermediate that is stable for hours at 0°C.
Autschbach is one of nine co-authors on the study, which was led by John Berry at the University of Wisconsin-Madison and Huw Davies at Emory University. Davies previously collaborated with Autschbach while working as a UB faculty member from 1995 to 2008.
“We’ve provided the first solid fundamental data on these compounds,” says Berry, who headed the effort to synthesize the dirhodium carbene intermediate. “People have thought about it for 40 years, but this is the first time that we can actually see it and say this is definitely what’s going on.”
Knowledge of the properties of the dirhodium carbene intermediate, and proof of its existence, will help scientists better understand how dirhodium catalysts work, and how to improve them and extend their range of applications.
Autschbach’s involvement in the study began with an email from Berry and Davies in 2011. The message asked if Autschbach could help verify that a compound the researchers had synthesized was indeed the long-sought dirhodium carbene intermediate.
If the compound was the right one, one of its carbon atoms would be expected to produce a characteristic nuclear magnetic resonance (NMR) signal.
The problem was that NMR signals fall along a spectrum. The team was unsure where along the spectrum they needed to look to spot the signal.
So they turned to Autschbach, who used quantum chemistry computer programs he developed or co-developed to predict the location.
“A few weeks later, I got an email saying (paraphrasing), ‘Ah. We’ve seen it after refining our low-temperature NMR technique.’ So that was exciting, very good news,” Autschbach tells the UB Reporter. The NMR signal of the synthesized compounds matched his calculations.
Later, Autschbach made additional predictions that helped the team verify that another expected feature of the intermediate’s NMR signal was present.
“This is a really nice success story for computational chemistry,” Autschbach says. “The programs we used to make the calculations were developed in a time frame from 14 to six years ago, and at the time of development, it was very basic science—we didn’t know how they would be used. We and others subsequently applied them to explain many intriguing experimental results that were published in the literature.”
The programs are in widespread use today, and with the new Science Express study, “we were able to use the programs to make these very useful predictions,” Autschbach says. “People have been looking for this dirhodium carbene intermediate for decades.”
Nuclear magnetic resonance (NMR) spectroscopy is a technique that scientists use to indirectly observe the structure and properties of chemical compounds.
As UB chemistry professor Jochen Autschbach explains, many of the nuclei of atoms in molecules behave like tiny magnets, not unlike a compass needle, and NMR spectroscopy exploits the presence of such nuclear “magnetic moments.”
The method calls on scientists to place chemical solutions or solids inside a magnetic chamber, which causes the nuclear magnetic moments to align themselves with the chamber’s magnetic field.
Once this step is complete, researchers can glean information about the chemical compounds by applying energy to change the orientation of the moments.
“The energies that are required to change the orientations are very sensitive to the chemical environment of the atom, so the amount that is applied can tell us a lot about the chemical bonds and their arrangement around certain atoms,” Autschbach says.