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.
-end-


University of Illinois at Urbana-Champaign

Related Quantum Mechanics Articles from Brightsurf:

Theoreticians show which quantum systems are suitable for quantum simulations
A joint research group led by Prof. Jens Eisert of Freie Universit├Ąt Berlin and Helmholtz-Zentrum Berlin (HZB) has shown a way to simulate the quantum physical properties of complex solid state systems.

A new interpretation of quantum mechanics suggests reality does not depend on the measurer
For 100 years scientists have disagreed on how to interpret quantum mechanics.

New evidence for quantum fluctuations near a quantum critical point in a superconductor
A study has found evidence for quantum fluctuations near a quantum critical point in a superconductor.

Simulating quantum 'time travel' disproves butterfly effect in quantum realm
Using a quantum computer to simulate time travel, researchers have demonstrated that, in the quantum realm, there is no 'butterfly effect.' In the research, information--qubits, or quantum bits--'time travel' into the simulated past.

Orbital engineering of quantum confinement in high-Al-content AlGaN quantum well
Recently, professor Kang's group focus on the limitation of quantum confine band offset model, the hole states delocalization in high-Al-content AlGaN quantum well are understood in terms of orbital intercoupling.

A Metal-like Quantum Gas: A pathbreaking platform for quantum simulation
Coherent and ultrafast laser excitation creates an exotic matter phase with spatially overlapping electronic wave-functions under nanometric control in an artificial micro-crystal of ultracold atoms.

Fluid mechanics mystery solved
An environmental engineering professor has solved a decades-old mystery regarding the behavior of fluids, a field of study with widespread medical, industrial and environmental applications.

Quantum leap: Photon discovery is a major step toward at-scale quantum technologies
A team of physicists at the University of Bristol has developed the first integrated photon source with the potential to deliver large-scale quantum photonics.

USTC realizes the first quantum-entangling-measurements-enhanced quantum orienteering
Researchers enhanced the performance of quantum orienteering with entangling measurements via photonic quantum walks.

A convex-optimization-based quantum process tomography method for reconstructing quantum channels
Researchers from SJTU have developed a convex-optimization-based quantum process tomography method for reconstructing quantum channels, and have shown the validity to seawater channels and general channels, enabling a more precise and robust estimation of the elements of the process matrix with less demands on preliminary resources.

Read More: Quantum Mechanics News and Quantum Mechanics Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.