Exotic molecules

04.15.09 -

Understanding of the chemical bonding of many elements has been fundamentally changed by chemists at the University of Virginia, who rely on NCSA resources to verify their results.

Zap. Zapzap. Z-a-a-a-p.

That's the sound of chemist Lester Andrews and his team at work at the University of Virginia. They use a pulsed laser to create unusual, one-of-a-kind small molecules. Their work has fundamentally changed the understanding of chemical bonding in many elements, providing key insights for generations of chemists to come.

Andrews recently retired from teaching but is continuing his research. He is well known in the global chemistry community for his 40-plus years of matrix-isolation spectroscopy research, an experimental method of studying the individual molecules of chemical compounds at very low temperatures. His current work is "kind of an exotic corner" in the organometallic area, he says. Organometallic chemistry studies chemical compounds containing bonds between carbon and a metal.

Employing a method developed by Andrews and his group over the past two decades, the team uses a focused, pulsed laser to vaporize material from a solid sample and direct it toward a cryogenically cooled reaction window (four to seven degrees Kelvin) for co-deposition and reaction. Andrews notes that this method exploits two advantages of the process. First, the ablated atoms contain excess energy, which can activate reactions with small molecules. Second, collisions with matrix atoms during the condensation process relax the energetic product molecules and allow them to be trapped in the solid matrix for infrared spectroscopic study. Each chemical bond in a molecule vibrates at a frequency that is characteristic of that bond. These infrared spectroscopic studies compare vibrational energies within related chemical species, providing conclusions about the bonding in these newly observed chemical intermediates.

Andrews says the team tries "to pick things we think are going to have interesting chemical properties." During his career he has worked with every non-radioactive element in the periodic table.

Exploring the unexpected

It was an unexpected experimental result that led Andrews to his current research path. A few years ago the team reacted tungsten and methane, confident that tungsten would insert into the methane C-H bond to make a methyl metal hydride. But the reaction went further, and transferred two more hydrogen atoms from the carbon onto the tungsten to make HC--WH3, which contains a carbon-tungsten triple bond. "We didn't expect this," says Andrews.

This bond is unique in that it is not supported by large ligands, just the smallest element, hydrogen; the carbon-tungsten triple bonds in the organometallic literature are stabilized by ligands or substituents. Ligands are like armour, necessary protection to shield the bond and ensure that it holds. But ligands are also extra baggage, increasing a molecule's size and weight and making it difficult for researchers to examine the molecule at precise levels.

With the unexpected creation of a simple carbon-tungsten triple bond, Andrews says the team realized they had a way to make some fairly exotic simple molecules by using appropriate laser ablated metal atoms and small molecules. Their work with small exotic molecules further advances the understanding of chemical bonding and has been published in numerous journals, including the Journal of the American Chemical Society in 2008 and Organometallics and the Proceedings of the National Academy of Sciences in 2007.

Verifying through computation

A significant advantage of these simple molecules is that their small size lends them well to computations, enabling them to be modeled at a reasonably accurate level with straightforward approximations. Over the years the team has used Cobalt and other NCSA machines and the Gaussian 03 software. Andrews says his team likes computing at NCSA because the center keeps the Gaussian 03 software supported and updated so their jobs "sail right through." Quick job processing keeps their research moving forward, as the computations are of critical importance to understanding the chemistry.

"We make molecules, we measure spectra, we try to figure out what they are, then we do calculations to model what we think we have," says Andrews, noting that when you can figure out what you've got chemically and spectroscopically before you do modeling calculations, you have a stronger scientific case. Then, of course, the case is even stronger when experiment and computation agree on the spectroscopic properties of the new molecule.

"Our work is considerably stronger coupled with the calculations we do on NCSA's computer than it would be without NCSA's computer," he observes. "And obviously the theoretical community is interested in what we do, in part because of the calculations."

New molecular species

One of the exotic small molecules created by the Andrews team is an extension of the tungsten-methane work. "If we can make a carbon-tungsten triple bond, why not make a silicon-tungsten triple bond?" says Andrews.

So they did. Andrews said previous research by others yielded triple bonds, but they required a significant number of bulky ligands. He was able to make his molecule viable with one hydrogen atom on the silicon and three on the tungsten, which gives HSi--WH3 a simple molecular compound. Making the molecule in this manner, says Andrews, is the most straightforward way, but it also leaves the molecule in the most vulnerable situation. By trapping the molecule in an argon bed at seven degrees Kelvin, it can be measured spectroscopically, and its properties can be studied in detail computationally. But as the temperature warms, the molecule diffuses around and reacts with other molecules in the sample.

"The advantage of our work is that we have a very simple molecule. The disadvantage is that it is not chemically stable at room temperature and you can't play with it in a bottle in your hands," Andrews explains. "You can't use it for catalytic reactions and other things that the organometallic people like to do."

Andrews' research has transformed the fundamental understanding of chemical bonds. "Silicon is an exotic element. We have helped understand the bonds silicon makes. Somebody else might figure out a way to use that knowledge. There's where you'll get into new applications," he says. "Someone else will take it and add ligands, and they might use that on the way to making a better computer chip."

For more information, go to http://www.virginia.edu/chem/people/faculty/andrews/.

This project was funded by the National Science Foundation.

Team members
Lester Andrews
Xuefeng Wang
Han-Gook Cho