"Suddenly, the rules appeared to have changed, and no one could explain why," Bowman says. "We decided to roll up our sleeves and attack the problem theoretically."
Bowman and Czako drew from the computational power of the Emerson Center, specialized software and analytical techniques. They first created theoretical-computational simulations of the experiments done by Liu and others, and then described the results mathematically.
"Our calculations showed essentially an exact agreement with the experimental results," Bowman says. "When theory and experiment agree you're happy, but you still want to know why."
Determining why the reactions did not conform to the Polanyi rules was another complicated task, involving quantum mechanics and forces that govern the reaction down to the atomic level.
"As theoreticians, we're able to zoom in and look at the results of our calculations in a way that's virtually impossible in an experiment," Bowman says.
They identified a subtle interplay between the Polanyi rules and a pre-reactive long-range force of methane with chlorine. If you follow the Polanyi rules, this long-range force, or steric control, will misalign the reactants, preventing them from docking correctly and inhibiting a reaction. But if you apportion the energy in the opposite way to the rules, the misalignment is wiped out and the reaction occurs.
"This long-range force was playing a bigger role than was previously realized," Bowman says. "It can actually trump the Polanyi rules, at least in the reactions that Liu and we looked at. The Polanyi rules are certainly not all wrong, they just appear to be too simple to apply to more complex reactions."
The research was funded by the National Science Foundation and the U.S. Department of Energy.
|Contact: Beverly Clark|