Energy Flow In Molecules Can Affect Reaction Rates, Chemists Say

December 04, 1997

CHAMPAIGN, Ill. -- The transfer of vibrational energy within a molecule -- long thought to occur nearly instantaneously -- can actually take place so slowly that overall reaction rates are affected, researchers at the University of Illinois say.

Using quantum mechanics, chemical physics professor Peter Wolynes and postdoctoral research associate David Leitner have developed a theory to account for energy flow within large molecules. They recently applied their theory to the kinetics of a well-studied chemical reaction -- the isomerization of the light-sensitive molecule stilbene.

"The most commonly accepted unimolecular reaction rate theories assume that intramolecular energy flow occurs so rapidly that it doesn't affect reaction rates," said Wolynes, who holds the James R. Eiszner Chair in chemistry at the U. of I. "However, there is a finite rate to energy flow, and in certain isomerization reactions -- where the structural rearrangement takes place at very low energies -- the quantum energy flow can indeed be slow enough to modify the reaction rates."

In the past, chemists have tried to simulate energy transfer in molecules by using classical mechanics, "but the artificially high rates they obtained didn't match the experimental data," Leitner said. "Because molecules are quantum mechanical objects, you have to use quantum mechanics to accurately describe them, particularly for processes occurring at low energies. In the case of stilbene, this was the first time the energy transfer rates were calculated reliably enough to show that they really do matter."

Stilbene is a large molecule that possesses a carbon double bond that rotates when light is absorbed. This torsional mode allows the molecule to undergo an isomerization reaction that transforms it from trans-stilbene to cis-stilbene. Because the stilbene reaction has been extensively studied by both theorists and experimentalists, it provided an ideal test for the new theory.

"Our theory -- which we call Local Random Matrix Theory -- emphasizes the local nature of energy flow in the vibrational space of a molecule," Leitner said. "Energy flows through certain preferred paths because some of the vibrational modes couple much more favorably than others. Our theory provides a statistical description of these couplings and introduces selection rules for energy transfer in the vibrational space, yielding a sequential structure for energy flow."

Predictions derived from the theory for vibrational flow rates in stilbene "compare well with those directly measured in the laboratory," Wolynes said, "and our calculations for the resulting reaction rates also compare favorably with the measured rates. These calculations show that the process of transferring energy within the stilbene molecule is, in fact, slow enough to influence the reaction rate, thereby bringing theory and experimental observations into full agreement."Wolynes and Leitner will describe their theory in the Dec. 12 issue of Chemical Physics Letters.

University of Illinois at Urbana-Champaign

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