February 2, 2004
Rule-breaking molecule could lead to non-metal magnets
WEST LAFAYETTE, Ind. Purdue University scientists have uncovered an unusual material that could lead to non-metallic magnets, which might be lighter, cheaper and easier to fabricate than magnets made of metal.
A team of researchers, including Paul G. Wenthold, has analyzed a radical hydrocarbon molecule whose electrons behave differently than they should, according to well-known principles. The compound is not the only molecule that exhibits such odd behavior in its surrounding cloud of electrons, but it is the first to be discovered that does not include a transition metal.
"In that respect, this is a unique exception to the electron-behavior rule, and it might help chemists think more clearly about where other exceptions lie," said Wenthold, an assistant professor of chemistry in Purdues School of Science. "Designing materials with novel properties depends on understanding the forces at work inside their molecules, and understanding the structure of this exceptional molecule could lead to new tools for material design."
The research, which Wenthold conducted with Anna I. Krylov of the University of Southern California and members of both their research groups, appears in todays (2/ 2) issue of Angewandte Chemie International Edition, a major European chemistry journal. The team deduced the structure of the compound using advanced techniques, including mass spectrometry.
Radical molecules, which contain unpaired electrons and are thus more reactive than molecules without them, have gained household notoriety primarily because so-called "free radicals" in the bloodstream can damage healthy cells. While the molecule Wentholds team has investigated is not found in the body and has no household name it is referred to only by its chemical description, 5-dehydro-1,3-quinodimethane it has a property that would raise the eyebrows of any observant student in a first-year chemistry course. The surprise stems from the uncommon way its three unpaired electrons arrange themselves around the nuclei in the molecules atoms an arrangement that students learn is virtually fundamental.
"Its called Hunds Rule," Wenthold explained. "It says that unpaired electrons line up facing the same direction when they arrange themselves around the molecular center. You might think of them as three-ring binders lying flat on shelves: You want to be able to read the labels on all of their spines, so you lay each binder flat with its spine pointing outward."
Paired electrons, he explained, would resemble two binders stacked one atop another; if their spines were both facing the same way, the top face of the upper binder would not form a flat surface, and it would tend to slide off the lower binder. None of a radicals unpaired electrons is constrained by this need to face the opposite direction, as they all have their own "shelves," or quantum energy levels.
"Nonetheless, one of the three unpaired electrons in our molecule faces the opposite direction," Wenthold said. "Since this is the first time weve ever seen this happen in an organic triradical, it opens up a few new possibilities for materials designers."
Krylov said the possibilities might include the building blocks for molecular magnets.
"People are already trying to build magnets from materials other than metals, such as the polymers that form plastic," she said. "Since magnetism is related to the behavior of unpaired electrons, this compound could be used as a building block for such polymers, leading to non-metallic magnets. It could extend a materials scientists options."
The National Science Foundation (NSF)s Tyrone Mitchell said that non-metallic magnets might have significant advantages over metal ones.
"Non-metal magnets would have several conceivable advantages," said Mitchell, who is program director in the NSFs chemistry division. "If we can find ways to magnetize hydrocarbons, for example, they would weigh less than metallic magnets, making them attractive to the space program and other commercial applications in which weight is always a concern. And since the raw materials would also be cheaper and easier to fabricate than metal substances, such magnets could conceivably save money in the long run."
Wenthold and Krylov cautioned that such possibilities are only speculation for the moment, and for now the major significance of the find is the fundamental knowledge it provides.
"We still have a lot to learn about molecules such as this one," Wenthold said. "We have a long list of steps that will follow this one, such as comparing this molecules properties with one that does not have its unpaired electrons facing different directions. But the unique property this substance exhibits will be of interest in its own right, even before we come up with any actual applications for it. It is one thing to discover magnets designing them is far more difficult and requires an understanding of what makes them magnets in the first place."
This research was sponsored in part by the National Science Foundation.
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5-Dehydro-1,3-quinodimethane: A Hydrocarbon with an Open-Shell Doublet Ground State
Paul G. Wenthold, and Anna I. Krylov
We report experimental and theoretical studies of the organic triradical, 5-dehydro-1,3-quinodimethane (5-dehydro-m-xylylene, DMX, Figure 1), a hydrocarbon with an unprecedented electronic ground state of three low-spin coupled unpaired electrons, that is, an "open-shell doublet." Although low-spin, open-shell states occur in molecules containing transition metals, the ground states of organic molecules are rarely of this type. Organic biradicals can have open-shell singlet ground states, depending on the orbital structure, but DMX is the first example of an organic triradical with an open-shell doublet ground state.