Scientists Learn Something New About One Of The Most Studied Chemical Reactions

With funding from the National Science Foundation, experimentalists from Stanford University and theorists from the University of Durham, UK, report in the March 7 issue of the journal Nature a surprising new observation: As a result of the collision, a tiny proportion of H2 molecules is flying off in an unexpected direction.

``These experiments are done really to understand chemistry in a deep, profound way,`` says Stanford`s Richard N. Zare, the Marguerite Blake Wilbur Professor in Natural Science. ``It is to get the mystery of how chemical reactions occur.``

When a hydrogen atom (H) collides with a hydrogen molecule (H2), the collision breaks the original bond between two hydrogen atoms in H2 and forms a new bond as one of those hydrogen atoms bonds to the incoming H. This reaction usually occurs when the three H atoms lie in a straight line.

To follow this hydrogen-exchange reaction, scientists shot hydrogen atoms at molecular heavy hydrogen (D2), so that the HD reaction product could be distinguished easily from the initial H2 reactant. Using a sophisticated laser technique developed at Stanford, Zare and colleagues measured the spread of the reaction product, HD, in space. Usually, the newly formed hydrogen molecule departed in the opposite direction as the motion of the incoming atom. But to the surprise of the scientists, they found that a tiny fraction of the newly formed HD product moved in the ``wrong`` direction. Instead of bouncing back toward the incoming H atoms, some HD traveled in the opposite direction.

Zare and colleagues further noticed that this reaction occurs slightly later than the immediate hydrogen-exchange reaction. These findings suggest more than one mechanism by which H and D2 come together and react. ``We don`t know what the delay cause is, but it is some form of trapping,`` Zare says.

Experiments that follow this tiny part of the reaction are extremely difficult to perform. Free hydrogen atoms don`t exist naturally and need to be created for the experiment. ``H atoms don`t come in bottles - you have to make them,`` Zare says.

Zare uses laser photochemistry to create free hydrogen. Newly created H atoms react with molecules in a jet of D2 to create HD. Using the laser system and a method called ``photoloc,`` Zare can record the appearance and ``travel direction`` of HD as well as the speed at which the newly formed HD molecule vibrates and rotates.

With a new simulation method, Stuart S. Althorpe and colleagues at the University of Durham use computers to ``observe`` the reaction at the quantum mechanics level. It is the first time scientists simulated the hydrogen reaction from start to finish calculating the scatter of the reaction products into space at such a high level of sophistication.

``The theory done in Durham is a breakthrough by itself,`` Zare says.

Due to the uncertainty principle of quantum mechanics, the simulation can`t determine any specific position of an HD product. Instead, it calculates a cloud of the most likely positions only.

``Quantum mechanics doesn`t allow us to `photograph` individual molecules,`` Althorpe says. ``The act of photography would alter the behavior of the molecules and change the outcome of the reaction.`` But with the simulation, Althorpe and Zare could use experimental measurements to infer what atoms are doing over the whole reaction time.

Other authors of the Nature paper include Felix Fernandez-Alonso, who led the experimental work at Stanford but is now at the Instituto di Struttura della Materia - Consiglio Nazionale delle Ricerche in Italy; Brian D. Bean, James D. Ayers and Andrew E. Pomerantz, all of Stanford; and Eckart Wrede of the University of Durham.

More than a thousand papers have been written about the hydrogen exchange reaction, but this is the first study combining theory and experiment that points to a new time-delayed reaction mechanism.

``This result is very surprising for a system people had thought is now pretty well understood,`` says Nobel laureate Dudley Herschbach of Harvard University. Despite 70 years of research, chemists haven`t achieved full correspondence between theory and experiments. ``We are still discovering fresh, unsuspected aspects that apply for all chemical reactivity,`` Herschbach says.

The purported existence of time-delayed mechanisms is a controversial area of chemistry. The quality of the theory always left a bit of uncertainty, says George Schatz of Northwestern University in Evanston, Ill. But ``now all seems to work and it provides an extraordinarily intricate picture of what happens when H hits D2,`` Schatz says.