C&EN: TODAY'S HEADLINES - HEAVYWEIGHTS YIELD TO COMPUTATION

MITCH JACOBY

Chemical systems that aren’t probed easily with experiments can be investigated theoretically if sufficiently accurate computational methods are available. Molecules containing transactinide atoms, for example, are notoriously tough to study because those elements are difficult to synthesize and decompose very quickly.

Malli

But now, a study conducted in Canada shows that properties of transactinide compounds can be calculated accurately using a computationally intensive theoretical method [J. Chem. Phys., 117, 10441 (2002)]. The work demonstrates that certain simplified computational techniques are inadequate for treating molecules that include superheavy elements.

Due to their large nuclear charge, the heaviest elements in the periodic table exhibit pronounced relativistic effects—sometimes causing the elements’ physical and chemical behavior to deviate from the behavior expected based on their positions in the periodic table. Gulzari L. Malli, a chemistry professor at Simon Fraser University, Burnaby, British Columbia, explains that the effect, which arises from a strong attraction between proton-rich nuclei and orbiting electrons, causes inner-shell electrons to travel at speeds close to that of light and to draw very close to the nucleus. At the same time, the effect destabilizes electrons in d and f orbitals—the ones involved in chemical bonding in these elements.

To examine these effects, Malli calculates molecular properties using a relativistic version of quantum mechanics that omits simplifications and accounts for all of a molecule’s electrons explicitly. He then compares the results to nonrelativistic calculations. Applying the method to hassium tetroxide (Hs is element 108) and the tetroxide of osmium, the element directly above Hs in the periodic table, Malli finds HsO₄’s relativistic atomization energy (a measure of a molecule’s stability) is 15.35 eV. The nonrelativistic quantity is 6.83 eV. He reports similar differences for OsO₄.

Malli also reports that a relativistic analysis of the charge on each atom indicates that HsO4 is more ionic than OsO₄, while a nonrelativistic treatment predicts the opposite. Greater ionicity is associated with stronger bonds, higher boiling points, and lower volatility. Thus, the relativistic calculations predict HsO4 to be less volatile than OsO4. The predictions that hassium should form an oxide, and that the oxide should be less volatile than OsO₄, were borne out recently in nuclear accelerator experiments [Nature, 418, 859 (2002)].

“The computational work on HsO₄ is very important,” notes Walter D. Loveland, a professor of nuclear chemistry at Oregon State University, Corvallis. The study’s distinguishing feature, according to Loveland, is the ab initio, all-electron relativistic computational technique.