Molecules & Matter

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Container Molecules Focus of Dr. Travis Holman's Work

By Katherine Morrissey

Dr. Travis Holman, a professor in the Department of Chemistry at Georgetown University, sees his lab as a place to play, to stretch the boundaries of molecular chemistry, to build new molecular structures, and to see what can be done with them.

“In chemistry, we make, we measure, we theorize,” says Dr. Holman. “What defines us as chemists is an ability to make new things, new forms of matter. Seemingly every day in our lab we make new molecules and materials that, as far as we’re aware, don’t exist anywhere else in the universe. That’s the reality of chemistry, and that’s one of the reasons I’m so passionate about it.”

Dr. Holman uses synthetic chemistry in his lab to build molecular containers. Although they are incredibly small, and impossible to see without powerful scientific equipment like X-ray devices, Dr. Holman and other chemists actually view these containers as large molecular structures, nanocapsules carefully designed to hold smaller molecules or sets of molecules (see Figure 1).

A chemist uses chemical reactions to produce new molecules. Controlling these reactions, however, can be a very difficult process. This is where the molecular containers come in.

“Say you have two different sets of molecules moving around in a solution,” explains Dr. Holman. “You know that point A on one molecule will react with point B on another in a certain way, but for that reaction to occur, they need to hit each other in just the right way, from just the right angle.”

Scenarios like this mean that chemists are frequently forced to produce as much, or more, of compounds they don’t want as the ones they do. The more chemists can control the space they work in, the better they’re able to induce the reactions they’re looking for. This allows them to produce greater amounts of specific molecules in a much more efficient manner.

“One unique thing we’re working on at Georgetown is synthesizing molecular capsules in which parts of the container itself actually participate in the chemical reaction,” Dr. Holman explains. “This way, chemical reactions can be forced to occur within the capsules, in a very controlled space, or not at all. So, the capsules could conceivably select the molecules that are to react.”

Dr. Holman was recently recognized for his work with a Career Award from the National Science Foundation. This award is funding a second area of Dr. Holman’s research, his work with container-based materials. Similar to his research with molecular capsules, one goal in this project is to create materials that can store, contain, and condense volatile compounds, such as fuel gases like hydrogen and methane. One of the problems with using hydrogen or methane as a fuel source in automobiles is the ability to safely store large amounts of these gases. Storage is problematic due to the need for large, high-pressure containers, which add to the difficulty in transporting usable amounts. But what if there were a way to condense the space the molecules are in, shrinking it down safely by containing the gas molecules in a material, one possibly derived from molecular capsules? Dr. Holman and the members of his lab are testing a variety of ways in which specially constructed, so-called “porous” materials can help hold volatile compounds, making them easier to transport.

“What we’re working on here won’t immediately solve the larger issues, like the gas storage problem,” says Dr. Holman. “The materials are too expensive. We’re testing different methods of storage, creating new ones, and helping the scientific community to build its knowledge base.”

The lab is also working to design materials with cavities and pores that can be used to conduct chemical reactions. These intricate molecular structures look somewhat like mesh and can operate like a sieve, a sponge, or a mixing bowl, depending on their construction. One type functions like a molecular sifting device, allowing some varieties of molecules to flow through it and another to stay behind. A second form is designed to simply absorb and hold molecules, and a third contains reactive agents for molecules to encounter, functioning like Dr. Holman’s capsules to control and encourage specific molecular reactions (see Figure 2).

Synthetic chemists struggle with efficiently forming container molecules, and cyclic molecules in general. So, in a third project, he and the members of his lab try to strategically allow the container molecules to synthesize themselves, via a process known as self-assembly. They explore many possibilities in their search for solutions to these synthetic chemistry problems. For Dr. Holman, half of the fun is trying different combinations of molecules and seeing what new molecular structures the reactions produce (see Figure 3).

This exploration of the unknown also allows students the space to work on their own projects and explore their own questions. They have the help and guidance of Dr. Holman, but no one knows the final outcome or what new molecular structures will appear from the experiments. The adventure is a major part of the learning process.

In addition to his research, Dr. Holman teaches a variety of courses, from a large, lecture style Organic Chemistry course, to smaller, graduate level courses in X-Ray Crystallography. He is also the manager of the Department of Chemistry’s X-ray diffraction facility.

For more information on Dr. Holman’s X-Ray diffraction work, see the video in this issue.

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