Butadiene's Bonds

Combining theory and experiment, researchers have solved a long-standing problem by confirming the effects of -electron delocalization on 1,3-butadiene bond lengths.

Organic chemistry texts use butadiene as a prototype for delocalization, which is the spreading of electron charge across neighboring bonds to improve molecular stability. The conventional wisdom is that delocalization causes butadiene's two carbon-carbon double bonds to be longer and its C−C single bond to be shorter than the corresponding bonds in electron-localized systems. But definitive structural evidence for this has been lacking.

Emeritus professor of chemistry and biochemistry Norman C. Craig of Oberlin College and coworkers have now confirmed these effects by using quantum chemistry and high-resolution infrared spectroscopy to determine, with unprecedented precision, bond lengths in the equilibrium structure of s-trans-butadiene (J. Phys. Chem. A, published online May 23, dx.doi.org/10.1021/jp060695b).

"Traditional methods of finding structures of small organic molecules give structures that are fuzzy," with bond lengths precise to about 0.01 Å, Craig says. By combining theory and experiment, he and his coworkers determined equilibrium bond lengths good to 0.001 Å. The data confirm that delocalization stretches butadiene's C=C bonds by 0.008 Å and squishes its C−C bond by 0.016 Å.

The theory-experiment combination that Craig and coworkers apply "has seen increasing use in recent years," comments professor of chemistry and biochemistry John F. Stanton of the University of Texas, Austin. The new study "has important implications in areas of physical organic chemistry, such as conjugation and bond alternation."

Robert L. Kuczkowski, professor emeritus of chemistry at the University of Michigan, agrees that the methodology has been used on other systems, "but probably none is as broadly interesting to the chemistry community as this example," he says.