Chemistry: probing reaction dynamics.

Chemistry: Probing reaction dynamics

Three chemists whose research involves the details of how chemical reactions occur are this year's winners of the Nobel Prize in chemistry. The Royal Swedish Academy of Sciences awarded the prize last week to Dudley R. Herschbach of Harvard University, Yuan T. Lee of the University of California at Berkeley and John C. Polanyi of the University of Toronto.

The award honors the development of two important techniques for probing what happens during the fractions of a second when different molecules collide and atoms rearrange themselves to form new molecules. Herschbach and Lee worked with molecular beams, studying the results of crossing two streams of fast-moving particles so that molecules collide under carefully controlled conditions. The spray of products provides clues about what goes on during the collisions. Polanyi measured and analyzed the extremely weak infrared radiation emitted by newly formed molecules. This allowed him to monitor the energy flow at the molecular level during a chemical reaction.

The crossed molecular beam technique is "one of the most important advances within the field of reaction dynamics,' according to the award citation. Herschbach was one of the pioneers in developing this method and used it to define the dynamics of basic reaction types.

In the reaction between potassium atoms and methyl iodide molecules, for instance, Herschbach and his colleagues showed that the product potassium iodide is formed only if a potassium atom strikes the iodide end of a methyl iodide molecule at just the right angle. This result showed for the first time that molecular orientation strongly influences how readily a chemical reaction occurs. Molecular beam experiments also led to the discovery that intermediate "reaction complexes,' temporarily created during a collision, sometimes survive for a surprisingly long time before they decay to form stable molecules.

Lee, who initially worked with Herschbach, extended molecular beam experiments to include larger and more complex molecules. He studied, for example, reactions between organic molecules and fluorine or oxygen atoms. Recent work has focused on basic reactions related to those that occur in the atmosphere or during combustion.

Lee's group at the Lawrence Berkeley Laboratory is now looking into photochemical processes. The researchers use a laser to excite molecules or atoms after they have been accelerated but before they collide. In this way, they have some control over the type of chemical reaction that occurs. They are also studying the use of laser excitation during molecular beam experiments to promote the removal of one or more specific atoms from larger molecules--a selective type of photodissociation.

Polanyi's complementary infrared-chemiluminescence technique, developed at the same time as the molecular beam method, provides information about how a product molecule gets rid of its excess energy after the high-speed collision that creates it. Spectroscopic analysis of the emitted infrared light reveals the quantum states occupied by the molecules. This gives indirect information about the system's potential energy at various stages during a reaction.

Polanyi's method, the Nobel award states, "can be considered as a first step towards the present, more sophisticated but also more complicated, laser-based methods for the study of chemical reaction dynamics.'

Photo: Herschbach

Photo: Polanyi

Photo: Lee