Predictive a priori pressure-dependent kinetics

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Science  05 Dec 2014:
Vol. 346, Issue 6214, pp. 1212-1215
DOI: 10.1126/science.1260856

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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.

Theoretical chemistry can withstand the pressure

Theoretical methods can predict the chemical consequences of a discrete molecular collision in exquisite detail. However, practical chemistry, whether in a flame, in Earth's atmosphere, or in an industrial reactor, involves billions of trillions of such collisions. Predicting the aggregate reaction rate requires an accurate means of treating the pressure dependence. Jasper et al. present such a method, which shows strong agreement with experimental measurements (see the Perspective by Pilling). Unlike past approaches that require parameters derived from empirical fits to data, the new technique relies strictly on simulations.

Science, this issue p. 1212; see also p. 1183

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