A New Molecular Dynamic Frontier

A provocative new study shows that, contrary to previous experimental results, exciting a stretching mode of a bond in a molecule during a particular simple chemical reaction does not break the excited bond. In fact, the reaction rate slows, and unexpected products form. The result lays open a vast new territory waiting to be explored: one of energy distribution and transfer during molecular dynamics.

Researcher Kopin Liu and colleagues Hiroshi Kawamata and Weiqing Zhang of Academia Sinica, in Taiwan, show that exciting the C–H stretching mode in the exothermic reaction F + CHD3 not only leads to unexpected products DF + CHD₂ but also slows the reaction rate (Science 2009, 325, 303).

Most experiments that selectively excite the stretching mode of a particular bond in simple chemical reactions have been on endothermic reactions, and the process breaks the excited bond. For example, exciting the CD bond during the reaction CH₃D + Cl leads to the products CH₃ + DCl.

Although the precise mechanism of this new exothermic reaction has yet to be elucidated, the authors posit that exciting the C–H stretching mode enacts a host of controls on the reaction, including, perhaps, blocking energetic channels that would have ordinarily permitted the breaking of the C–H bond, transferring energy to other parts of the system, and slowing the reaction.

The new work is "beautiful," says University of Wisconsin, Madison, chemistry professor F. Fleming Crim, whose lab has been at the forefront of selective bond excitation chemistry. "These incisive experiments reveal the complexity of reactions of polyatomic molecules," Crim says. "Experiments such as Kopin's move us toward that goal and call on theory to help build models of this more complex behavior."