'Yanking' chemical bonds with molecular wires speeds reactions

March 14, 2006

DURHAM, N.C. -- Using a chain of molecules as an infinitesimal lanyard to tug on a chemical bond about to break, Duke University chemists have found they can speed a complex chemical reaction.

Their unusual manipulative technique can reveal previously unknown details about the evolution of such two-step bond reactions, said assistant Duke chemistry professor Stephen Craig. It might ultimately aid efforts to develop new kinds of polymers that can "heal" themselves after tearing, he said.

Craig, current doctoral student Farrell Kersey and former graduate student Wayne Yount described their discoveries in a research paper published online Friday, March 3, 2006, in the Journal of the American Chemical Society (JACS). The work was funded by the National Science Foundation.

"We probed a reaction in which a bond was being made and a bond was being broken by pulling on the bond being broken with an atomic force microscope (AFM)," said Craig. An AFM detects forces or creates images of surfaces at molecular scales by mechanically probing with a flexible microscopic cantilevered tip.

In their experiments, Craig's group used an AFM tip to exert almost infinitesimally small tugs on a molecular complex made of pyridine and the metal palladium.

The researchers dangled the pyridine-palladium complex in space as if it were part of a molecular trapeze act, by attaching trapeze "wires" made of atomic chains of the molecule polyethylene glycol (PEG). One PEG chain connected the dangling pyridine-palladium to the AFM's tip. A separate PEG "wire" anchored the complex underneath onto an underlying surface substrate.

When the AFM's flexible tip pivoted upward, it pulled on the bond linking the pyridine to the palladium. "This is almost like spring-loading that bond," Craig said.

"As a bond breaks, it stretches," he said. "The distance between the atoms gets further and further. And we could infer from the behavior of this experiment that the rate of the reaction speeded up."

Since the whole array was submerged in a solution of the chemical solvent DMSO, the bond was already under pressure before the AFM began its work, he said.

"Because this solvent was present in excessive amounts, it wanted to form a bond with the palladium," he said. But the nature of that reaction requires the DMSO-palladium bond to form first before the palladium and pyridine could sever their connection, he added.

The Duke chemists sought to study how the sequence of bond forming and breaking would be affected if they artificially stretched the palladium-pyridine bond towards the breaking point.

They found that, although the pace of the reaction was accelerated, the order of bond forming and breaking did not change. "We could spring-load the bond enough so it sought to break very quickly. But the reaction still waited for the DMSO to bond to the palladium before the pyridine came off," he said.

The researchers also found that, when they repeated the experiment with a palladium-pyridine complex incorporating a modified pyridine, the response to pulling on the bond was the same even though the energy levels needed for bond-breaking were different.

These findings "are absolutely consistent with some very fundamental notions about the way energy is exchanged in chemical reactions," Craig said. "But to my knowledge it's not an experiment that anyone else has done to test whether that was the case. This could lead to a more sophisticated understanding of the way reactions happen at their most fundamental levels."

According to Craig, additional studies into the order and consequences of chemical bond-breaking might also aid the discovery of new materials. "Someone might try to design certain types of molecules that would respond to mechanical stresses by breaking in a way that's desirable," he said.

For example, he said such research might aid researchers like him who work on "self-healing polymers." Those are molecules in the early stages of development that would release chemicals to repair newly formed tears and cracks.
-end-


Duke University

Related Molecules Articles from Brightsurf:

Finally, a way to see molecules 'wobble'
Researchers at the University of Rochester and the Fresnel Institute in France have found a way to visualize those molecules in even greater detail, showing their position and orientation in 3D, and even how they wobble and oscillate.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

How molecules self-assemble into superstructures
Most technical functional units are built bit by bit according to a well-designed construction plan.

Breaking down stubborn molecules
Seawater is more than just saltwater. The ocean is a veritable soup of chemicals.

Shaping the rings of molecules
Canadian chemists discover a natural process to control the shape of 'macrocycles,' molecules of large rings of atoms, for use in pharmaceuticals and electronics.

The mysterious movement of water molecules
Water is all around us and essential for life. Nevertheless, research into its behaviour at the atomic level -- above all how it interacts with surfaces -- is thin on the ground.

Spectroscopy: A fine sense for molecules
Scientists at the Laboratory for Attosecond Physics have developed a unique laser technology for the analysis of the molecular composition of biological samples.

Looking at the good vibes of molecules
Label-free dynamic detection of biomolecules is a major challenge in live-cell microscopy.

Colliding molecules and antiparticles
A study by Marcos Barp and Felipe Arretche from Brazil published in EPJ D shows a model of the interaction between positrons and simple molecules that is in good agreement with experimental results.

Read More: Molecules News and Molecules 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.