A featured paper in the December 5th 2014 issue of Science demonstrates – for the first time – a computational method to successfully a priori predict pressure-dependent chemical reaction rates. In short, this work provides an accurate means of treating pressure dependence, which is required to accurately predict the aggregate reaction rate. Unlike previous models that rely on empirical parameters, this work relies entirely on simulation results. The authors report strong agreement with experimental results. The work was performed by researchers at Sandia and Argonne national laboratories (specifically Ahren W. Jasper, Kenley M. Pelzer, James A. Miller, Eugene Kamarchik, Lawrence B. Harding, and Stephen J. Klippenstein). It represents an important breakthrough in combustion and atmospheric chemistry that is expected to benefit auto and engine manufacturers, oil and gas utilities and other industries that employ combustion models.
A key step, Jasper (the lead author) said, was the development of a model for the collisional energy and angular momentum transfer function that reproduced detailed features predicted by the trajectories and was simple enough to be used in practical reaction rate calculations. “Finding a way to accurately compute and represent the energy and angular momentum transfer from these vibrationally-excited molecules proved to be the final piece needed to solve the problem”.
“The overall theoretical model is rather complex, involving many separate unrelated calculations, and it is remarkable how accurately one can now treat all aspects of the problem in developing such completely a priori predictions”, Klippenstein said.
Following is the paper abstract (courtesy Science) from Predictive a priori pressure-dependent kinetics:
The ability to predict the pressure dependence of chemical reaction rates would be a great boon to kinetic modeling of processes such as combustion and atmospheric chemistry. This pressure dependence is intimately related to the rate of collision-induced transitions in energy E and angular momentum J. We present a scheme for predicting this pressure dependence based on coupling trajectory-based determinations of moments of the E,J-resolved collisional transfer rates with the two-dimensional master equation. This completely a priori procedure provides a means for proceeding beyond the empiricism of prior work. The requisite microcanonical dissociation rates are obtained from ab initio transition state theory. Predictions for the CH4 = CH3 + H and C2H3 = C2H2 + H reaction systems are in excellent agreement with experiment.
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