Molecular Chemistry takes a New Twist

In a discovery that could have implications for biological research, new calculations show that a basic tenet of chemistry is wrong. The new finding indicates that in some molecules quantum forces trump the traditional explanation. The find focuses on ethane, which has two carbon atoms bonded to each other. On the other end of the carbon-carbon bond the methyl group, spins like a three-pronged turnstile, says chemist Frank Weinhold of the University of Wisconsin-Madison. Scientist Vojislava Pophristic spent 5 years crunching calculations with a supercomputer to figure out what underlies the stability of ethane's staggered conformation. First, she mathematically modeled ethane and removed the parts of the calculations that relate to the steric effect. The ethane molecule remained staggered. She then looked at the other known influence on ethane twisting which is known as hyperconjugation. When Pophristic blocked the electron jumping by placing a hypothetical screen between the two methyl groups, ethane's structure finally assumed the eclipsed form. Hyperconjugation, not steric effects, makes staggered ethane stable, they conclude. Researchers can no longer assume that steric effects play the major role in determining stable forms.
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