Cornell Research Team Makes Fundamental Discovery About Nature Of Hydrogen Combustion

Because of growing concerns about future energy sources based on fossil fuels -- concerns that range from supply to global warming -- considerable experimental and theoretical research is underway to better understand energy conversion processes, particularly those involving hydrogen. The element, which is readily formed through the electrolysis of water and leaves only water as its combustion byproduct, is viewed as an attractive future material for use in heating and the internal combustion engine.

"The process we are studying is one of the most fundamental steps in combustion," says principal investigator Floyd Davis, associate professor of chemistry and chemical biology at Cornell. "Of course, ultimately the goal is to develop more efficient ways to convert energy from one form to another, but what we are doing is more fundamental: to study how energy is released in this reaction."

The researchers' report appears in the latest issue (Nov. 3) of the journal Science . Other authors are Davis's graduate students, Brian R. Strazisar and Cheng Lin. Their experimental report and a separate theoretical research paper by another group are discussed in an accompanying article by George C. Schatz, a Northwestern University chemist. The importance of the two papers, Schatz says, is that they "present important new results concerning a topic of longstanding interest to chemists, namely the relationship between reagent and product vibrational motions and the dynamics of chemical reactions."

Davis explains that during the combustion of hydrogen in air, energy is released in the form of heat, which to the observer appears as a hot flame. But at a submicroscopic level, the energy is released when the newly formed water molecules are produced in excited vibrational and rotational levels. The question, Davis says, is "how is this energy distributed?" The question, he notes, is important in the development of theoretical models to understand how energy flows in chemical reactions.

To find the answer, Davis and his coworkers used a technique called crossed molecular beams in which two separate molecular beams cross at right angles. Four lasers were used to measure the angular and velocity distributions of newly formed chemical products.

"A chemical reaction results from successful collisions between molecules," says Davis. "If the orientation of the reactants [the colliding molecules] is correct, they can pass through a critical configuration known as the transition state, allowing products to be formed."

In the reaction studied by Davis and his colleagues, the oxygen atom in a hydroxyl radical (OH) grabs an atom from deuterium (D2), forming water called monodeuterated water (HOD) and a deuterium atom. A hydroxyl radical is an oxygen atom connected by a single bond to a hydrogen atom. Deuterium, or heavy hydrogen, is an isotope of hydrogen, or a hydrogen atom with a neutron added. In all, the reaction involved four atoms and six dimensions.

"By observing where the products are scattered, and by measuring their velocities, we can learn about the details of the reactive encounter," says Davis.

He likens the effect to a game of baseball. When a batter hits a line drive, it is because the bat has struck the ball head-on. A pop fly results when the bat hits the ball slightly below its center. By watching where the "baseball" goes, researchers can learn about its collision with the "bat." In nature, molecules bump into each other constantly, but in most cases collisions don't lead to a reaction. Thus researchers are interested in how the probability of a reaction depends on the details of the collision.

A closely related issue is how energy is distributed into newly formed molecules following chemical reactions. In the reaction involving the formation of HOD+D, the Cornell group found that the HOD is primarily formed with two quanta of vibrational energy in the oxygen-deuterium (OD) bond. A quantum is the smallest "package" of energy that can be observed.

"The significance of this measurement is that since the 1980s, theory predicted that the energy would be deposited nearly statistically into all of the vibrational modes of HOD," says Davis. Recently, though, new calculations predicted that energy would instead appear only in certain HOD vibrational modes. The Cornell experimental measurements are in nearly total agreement with these theoretical results. "This is a breakthrough in our understanding of the way a reaction occurs," says Davis. The Science article is titled "Mode-Specific Energy disposal in the Four-Atom Reaction

OH + D₂ --> HOD + D."

Related Web site:

o Davis Group Molecular Reaction Dynamics:http://www.chem.cornell.edu/hfd1/index.html