Complex chemical processes that occur during combustion and in the atmosphere involve a large number of elementary reactions. Models of these processes require knowing the rate of every relevant reaction, captured by the rate constant k, which traditionally has been determined experimentally. The vast number of reactions involved makes this approach daunting, but as theoretical methods have become increasingly accurate, they may provide a practical route for some reactions where k depends solely on temperature. For an important subset of reactions, kalso depends on pressure; accurate a priori calculation of this dependence has proved elusive. On page 1212 of this issue, Jasper et al. (1) have developed a methodology that overcomes the difficulties and provides a general approach for the accurate prediction of rate constants for pressure-dependent reactions.
Report
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.
Feed: Science: Current Issue
Posted on: Friday, 5 December 2014 11:00 AM
Author: Ahren W. Jasper
Subject: [Report] Predictive a priori pressure-dependent kinetics
A purely theoretical method for predicting gas-phase chemical reaction rates shows strong agreement with experiment. [Also see Perspective by Pilling] Authors: Ahren W. Jasper, Kenley M. Pelzer, James A. Miller, Eugene Kamarchik, Lawrence B. Harding, Stephen J. Klippenstein
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