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Science Concentrates

February 2, 2004 | A version of this story appeared in Volume 82, Issue 5

Triradical breaks the rules

The first example of an organic triradical with three un-paired electrons in an "open-shell" doublet ground state has been reported by the research groups of chemists Paul G. Wenthold of Purdue University and Anna I. Krylov of the University of Southern California [Angew. Chem. Int. Ed., 43, 742 (2004)]. Three antiferromagnetically coupled unpaired electrons are encountered in transition-metal complexes, Wenthold tells C&EN, but this state is unprecedented in a hydrocarbon and is contrary to the Aufbau principle and Hund's rules, which govern electron occupancy of molecular orbitals. The researchers deduced the electronic configuration of the 5-dehydro-m-xylylene triradical (shown) from electronic structure calculations and mass spectrometry studies on its biradical anion and other model compounds. The anion was prepared in situ by reacting tris(trimethylsilyl)-m-xylene with F­ and F2. The researchers rationalize the doublet ground state by thinking of the triradical as the superposition of two biradical structures: m-xylylene and -3-didehydrotoluene. This interpretation is a "nice combination" of calculations and gas-phase experiments "to demystify an ostensibly very complicated structure," notes Dean J. Tantillo of the University of California, Davis.


Testosterone and Alzheimer's

In the right form, testosterone may provide protection against Alzheimer's disease. The hormone circulates in the body either bound to albumin or sex hormone binding globulin (SHBG) or in the unbound or "free" form. As a man ages, his SHBG level rises but his levels of free and total testosterone fall. One way to describe changes is with the free androgen index (FAI), the ratio of total testosterone to SHBG. In a study that tracked the health of a group of men for several years, the National Institute on Aging's Susan M. Resnick and colleagues found that men whose FAI is lower than average are more likely to develop Alzheimer's [Neurology, 62, 188 (2004)]. The researchers found no correlation between total testosterone or SHBG levels and the likelihood of getting the disease. In a study of lean older men, an Italian team led by the University of Cagliari's Gian B. Melis also found that those with Alzheimer's disease have a lower than average FAI [Neurology, 62, 301 (2004)]. Total testosterone was similar in men with and without the disease. Unlike the Americans, the Italians found higher SHBG levels in the Alzheimer's patients.


Ionic liquids fall into columns

A new class of ionic liquids that self-assemble into liquid-crystal columnar structures could potentially be used as electrolytes in nanoscale batteries, according to its discoverers. University of Tokyo chemistry professor Takashi Kato and coworkers synthesized two ionic liquids (shown), each consisting of two incompatible parts: an imidazolium-based ionic part and an insulating tris(alkoxy)phenyl part that contains either octyl or dodecyl chains [J. Am. Chem. Soc., 126, 994 (2004)]. The imidazolium part forms a one-dimensional ionic path inside the column with the alkoxy aromatic part on the outside. The compounds exhibit hexagonal columnar phases over a wide range of temperatures, including room temperature. Even with their 1-D order, the ionic liquids retain the fluidity of solvents, Kato tells C&EN. The researchers measured the 1-D ionic conductivities of the liquids and showed that incorporation of lithium salts, such as LiBF4, in the columnar materials enhances their ionic conductivities and anisotropy. "These materials would be useful for transportation of ions, energy, and information at the nanometer level," the authors suggest.


A bonanza from Mars, hopefully

Engineers continue to work on fixing the software bug that seems likely to keep NASA's Mars rover Spirit out of commission for several weeks. Meanwhile, halfway around the red planet, the mission's second rover, Opportunity, was readying itself to roll off its lander and start exploring the martian surface, possibly as early as Feb 1. Opportunity, having fortuitously dropped into a tiny crater, has already sent back striking images of layered rocks, which could be layers of volcanic ash or sediments deposited by water that might once have flowed on Mars, scientists say. They are particularly interested in the possibility of finding hematite, an iron oxide frequently formed in the presence of water. In 1998, the Mars Global Surveyor orbiter spotted large fields of hematite around the martian equator, where Opportunity now sits.


First isolable, covalent selenium azide

A stable organoselenium compound containing a covalently bound azido group has been isolated and characterized by chemistry professor Thomas M. Klapötke, research scientist Burkhard Krumm, and crystallographer Kurt Polborn of the University of Munich, in Germany [J. Am. Chem. Soc. , 126, 710 (2004)]. Klapötke's group has prepared and studied a number of main-group azides over the years, including ionic selenonium azides of the type R3SeN3. The Munich researchers also have been intrigued by published reports on the in situ organoselenium reagent C6H5SeN3, which has been reported to add azidoselenenyl groups to double bonds, although it's too unstable to be isolated or fully characterized. These developments led them to synthesize the selenenyl azide shown (red = Se, blue = N) by substituting the chlorine atom of 2-(CH3)2NCH2C6H4SeCl with an azido group using NaN3 or AgN3. Spectroscopic data and the X-ray crystal structure, supported by the calculated structure, indicate that the amino nitrogen coordinates to selenium, forming a heterocyclic zwitterion with an unusual three-atom, four-electron N­Se­N bond system. The selenenyl azide is "an attractive candidate" for applications in organic synthesis, they add.



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