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Caught in Flight

November 24, 1999

Chemists are very interested in unusual molecules that are made from atoms of a single element. For example, fullerenes
("buckyballs") and nanotubes, made of pure carbon, are generating a lot of excitement among materials scientists. If all were as it should be, the element phosphorus should be more similar to carbon than any other member of the periodic table. Yet although an entire series of "phosphorus-only molecules" have been simulated on computers, reality looks rather dismal; neutral molecules that contain more than four phosphorus atoms are decidedly scarce. Chemist Helmut Schwarz and his colleagues could not let this rest. They have now been able to detect the tiniest amounts of a compound consisting of six phosphorus atoms. The techniques that the researchers used to track down their elusive compound are as meticulous as those used by analysts who must detect the minutest quantities of pesticides in water samples, for example. Schröder, Schwarz and Jutzi did indeed turn to an instrument that is also used for trace analysis: the mass spectrometer. Within this device, molecules are electrically charged in flight,deflected by a magnetic field, and identified by the resulting curvature of their flight path.

In the case of the new phosphorus particle, however, this was not enough. It was possible to electrically charge a special precursor molecule and to get it to fall apart into a fragment made of six phosphorus atoms, and other unimportant particles. However, the phosphorus molecule did not get electrically charged in this process, flying straight ahead instead of being forced into the circular path that is vital for detection. The chemists were only able to identify the elusive phosphorus particle by examining the fragments it generated when it, in its turn, was blasted by a burst of high energy.

The new compound most likely does not resemble the most well known carbon molecule, benzene, which likewise contains six atoms. The researchers are betting it is a cage-like molecule. It is difficult to say whether this will lead to chemistry as rich as that of the fullerenes - the phosphorus cage probably rearranges to form other compounds too rapidly. However: the mysterious fullerenes were also first detected in a mass spectrometer. The synthesis of larger quantities only happened much later.



WILEY-VCH Verlag GmbH