Partners for a noble element.

Partners for a noble element

The noble gases--helium, neon, argon and xenon--stand apart from other elements. As their title implies, these atoms resist forming chemical bonds. Carrying a full complement of electrons, they are generally content to lead solitary lives. Since 1962, however, chemists have found elemental companions so irresistible that stable, neutral molecules incorporating each of neon, argon and xenon have been synthesized. The one holdout is helium. Now a team of theorists in California and West Germany have calculated that the combination of helium with beryllium and oxygen (HeBeO) is likely to be stable. "Helium can form strong chemical bonds in ions and may even be bound in the ground state of a neutral molecule,' the researchers conclude in the Sept. 30 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

The computations, the most extensive ever performed for helium, consisted of determining from quantum mechanical principles the likelihood that helium would bond with a number of other elements. Gernot Frenking of the Molecular Research Institute in Palo Alto, Calif., and his colleagues started by looking at positively charged ions (cations) containing helium and elements such as carbon. They found, for instance, that a doubly charged species consisting of two helium and two carbon atoms is surprisingly stable.

The cation calculations provided clues indicating that stable, neutral molecules containing helium could exist. Step by step, the researchers were led to HeBeO. "All this computation would not have been possible without access to a supercomputer,' says Frenking. Even on a supercomputer, the calculations for a single candidate often took several hours.

According to Frenking's calculations, HeBeO is barely stable. The energy required to disrupt its bonds is much smaller than the energy required to split, say, a carbon atom from a hydrogen atom. Nevertheless, the compound would probably hold together under the right conditions.

"You never know that strange, new characteristics this compound may have,' says Frenking, "but my guess is that you won't find many practical applications for it.' No one is ever likely to bottle significant amounts of HeBeO and be able to carry it around. "This compound is important just for knowledge on chemical bonding,' he says, and it opens up the "new and really fascinating' field of helium chemistry.

It may be possible to synthesize HeBeO, says Frenking, by heating beryllium oxide, a polymeric solid, until it dissociates into pairs of beryllium and oxygen atoms, then trapping the pairs in liquid helium. A group of researchers at the IBM Research Center in San Jose, Calif., is already trying a somewhat different approach: shining laser light on a beryllium oxide surface in a helium atmosphere. No one has yet reported success in synthesizing the compound. The situation for helium cations looks more promising. Calculations show that several of these ions are likely to be much more stable than neutral helium-containing molecules. One research group has already detected a singly charged carbon-helium pair.

Because helium, after hydrogen, is the second most abundant element in outer space and because evidence for the presence of helium ions has been found, Frenking says that helium may play a role in the chemistry that takes place on dust particles in interstellar space. The synthesis and study of helium cations in laboratories may soon provide the data needed to monitor interstellar chemistry more effectively.

Frenking and his colleagues are also excited about a more recent set of calculations that concerns bonding between helium and various ions such as lithium. The researchers have found bonding trends and indications of chemical structures that may turn out to be useful for constructing new kinds of lasers.