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Computers uncover new chemical bond

U. ILLINOIS (US) — Computer modeling has helped chemists identify a previously unknown type of chemical bond.

The University of Illinois research team dubbed the new state “recoupled pair bonding.”

“This phenomena has implications for all of chemistry,” says Thom Dunning, who co-leads the research with research chemist David Woon. Their findings are reported in the Journal of Physical Chemistry A.

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“Chemists have long known that the chemistry of the main group elements nitrogen through fluorine is different than that of the elements in succeeding rows of the periodic table. The question was: What makes phosphorus different than nitrogen or sulfur different than oxygen? It appears that this new type of bond is a major cause of this anomaly,” adds Dunning, who is also the director of the National Center for Supercomputing Applications.

One of the main anomalies that recoupled pair bonding addresses is hypervalency—the large class of molecules that form more than the expected number of bonds.

The Octet Rule states that atoms want to have eight electrons in their valence shells. When you know how many electrons an atom has in its outer shell, you know how it will gain, lose, or share electrons to form complete octets, which tells you the number of bonds the atom can form.

Hypervalent molecules, however, form more than the expected number of bonds, appearing to stuff their valence shells with more than eight electrons.

The greenhouse gas sulfur hexafluoride (SF6) is one example; another is chlorine trifluoride (ClF3), a gas used in rocket fuel, nuclear fuel processing, and etching operations in the semiconductor industry.

“It’s really common behavior,” says graduate student Jeff Leiding. “There’s been this entire class of compounds that has been poorly understood in terms of how they bond and how they react.”

Through the years, scientists had proposed several theories to explain hypervalency, but none provided a satisfactory explanation. Then Woon and Dunning’s group conducted computational studies, using supercomputers at NCSA, of the ground and low-lying excited states of the sulfur fluorine species: SF, SF2, SF3, SF4, SF5, and SF6.

They found molecules can form this previously unknown type of chemical bond.

Imagine two atoms, one with two electrons in an outer orbital and the other with just one. Normally, the paired electrons wouldn’t participate in a bond. In order for a bond to form, the pair must be split apart.

Some atoms, like fluorine, are able to force that split. One electron from the original pair is “recoupled” by the fluorine, forming a recoupled pair bond with the electron in the singly occupied fluorine orbital. The other previously paired electron is now available to form another bond. Two bonds can form where you would expect none.

The team has even found recoupled pair bonding in “normal valent” molecules such as SF2, albeit in excited rather than ground states. In addition, much of the chemistry of the early elements in each row (even in carbon) can be explained by recoupled pair bonding.

“The nice thing about our theory,” Woon says, “is that it is a predictive theory. When we studied compounds of phosphorous and chlorine, we had much better intuition about how they would behave based on what we’d learned about recoupled pair bonding in sulfur compounds.”

In addition to further exploration of recoupled pair bonding, the team plans to rewrite the book on bonding.

“We looked at the general chemistry textbooks to see how they are teaching bonding in general and said . . . ‘Aaargh!’ ” Woon says.

More news from the University of Illinois: www.ncsa.illinois.edu/News/

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