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June 4, 2001
Volume 79, Number 23
CENEAR 79 23 pp. 10
ISSN 0009-2347
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Steric Effects Don't Explain Ethane Conformation


Every organic chemistry student learns that ethane prefers the staggered conformation over the eclipsed conformation because of steric effects. It turns out, however, that steric effects are only of secondary importance. In fact, by themselves steric effects favor the eclipsed conformation.

FAVORITISM Ethane's C–H bond orbitals overlap more favorably in staggered (top) than the eclipsed (bottom) conformation. The bonding orbitals are colored blue; the antibonding orbitals, yellow.
New calculations by chemistry professor Lionel Goodman and graduate student Vojislava T. Pophristic of Rutgers University show instead that hyperconjugation, a quantum-mechanical effect involving the transfer of electrons from an occupied orbital to an unoccupied orbital, causes ethane to adopt its preferred conformation [Nature, 411, 565 (2001)].

"We worked with a hypothetical molecule without steric repulsions," Pophristic says. "If one removes steric repulsions, then the molecule remains in the staggered conformation. However, if one removes hyperconjugative interactions, the molecule assumes the eclipsed form, which means that it is the presence of hyperconjugative interactions that keeps the molecule in the staggered conformation."

Goodman and Pophristic had been studying the origins of potential energy barriers to internal rotation of methyl groups in molecules such as dimethyl ether and acetone when they decided to perform the calculations on ethane. "We thought maybe we ought to go back to square one and ask why ethane has the staggered structure at all," Goodman says.

Understanding molecular structural preferences is a "cornerstone of modern electronic structure theory," Goodman says. "If you have that wrong for a molecule like ethane, there's a big red flag for our understanding of structural preferences for more complex molecules," he says.

"Improved understanding of the electronic origins of molecular twisting may also spur overdue efforts to incorporate more realistic quantum-mechanical effects into molecular modeling and protein folding calculations," Frank A. Weinhold, a theoretical chemist at the University of Wisconsin, Madison, writes in an accompanying commentary [Nature, 411, 539 (2001)].

Goodman and Pophristic expect their conclusions to be important in analyzing molecular phenomena such as polymer rheology and protein folding.

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