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News

75-year-old molecular-chemistry reaction-rate problem solved

DOE/Argonne National Laboratory : 28 March, 2007  (Technical Article)
A 75-year-old problem in molecular chemistry has been solved by a team of researchers from Argonne and several other institutions. For the first time, theory and experiment have converged, enabling chemists to predict the rate of a chemical reaction with near-perfect accuracy.
Until now, computer-based theoretical predictions have failed to match the accuracy of experiments for any chemical reaction, including one of the simplest: the interaction of a lone hydrogen atom with a hydrogen molecule. In chemical shorthand, the reaction is written as H + H2 > H2 + H. The team studied this gas-phase hydrogen exchange reaction over a wide range of temperatures.

'The framework for understanding chemical reactivity has been known for around 75 years,' said Argonne Chemist Joe Michael, who performed precise experiments to confirm the computer model's predictions. 'But that's just the outline. It turned out the actual development of the specifics took decades.'

For most of those 75 years, computers either were unavailable or didn't have enough horsepower to solve the complex equations governing chemical reactions at the quantum level. Even with a reaction as simple as the hydrogen-exchange, the atoms go through many intermediate forms as atomic bonding changes the orbitals that electrons can occupy, which also changes the energies of the electrons.

Steven Mielke of the Pacific Northwest National Laboratory , along with several other researchers from PNNL, Washington State University, the University of Minnesota, and NASA/Ames, used a new computational approach that takes into account the dynamic rotation and vibration of the atoms involved in the reaction. To check the accuracy of their theoretical predictions, they turned to Michael.

Working with visiting scientists and using a more accurate experimental method than in previous studies, Michael developed the world's best experimental database on the reaction at temperatures ranging from -148 to 3,500 degrees F.

'The theoretical data, compared to the experimental data, were inside the error bars over this wide temperature range,' Michael said. 'It shows a complete convergence, as far as we can tell, between this detailed theory and experiment. It's really a first.'

The results were published in Physical Review Letters, and garnered attention on the PhysicsWeb site and in Physics World magazine.

'This has been a real odyssey in chemical kinetics,' Michael said. 'There have been hundreds of people, since the early 1900s, who have had a hand in this.'
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