Chemical kinetic mechanisms for gas-phase processes (including combustion, pyrolysis, partial oxidation, or the atmospheric oxidation of organics) will often contain hundreds of species and thousands of reactions. The size and complexity of such models, and the need to ensure that important pathways are not left out, have inspired the use of computer tools to generate such large chemical mechanisms automatically. But the models produced by existing computerized mechanism generation codes, as well as a great many large mechanisms generated by hand, do not include pressure-dependence in a general way. This is due to the difficulty of computing the large number of k(T, P) estimates required.Here we present a fast, automated method for computing k(T, P) on-the-fly during automated mechanism generation. It uses as its principal inputs the same high-pressure-limit rate estimation rules and group-additivity thermochemistry estimates employed by existing computerized mechanism-generation codes, and automatically identifies the important chemically activated intermediates and pathways. We demonstrate the usefulness of this approach on a series of pressure-dependent reactions through cycloalkyl radical intermediates, including systems with over 90 isomers and 200 accessible product channels. We test the accuracy of these computer-generated

Autocatalytic, lower-temperature (≤1100 K) methane pyrolysis has defied mechanistic explanation for almost three decades. The most recent attempt (by Dean in 1990) invoked the chemically activated addition of an allyl radical to acetylene, leading to a cyclopentadiene/cyclopentadienyl chain-branching system that prompted the observed autocatalysis. However, newer, more accurate thermochemical data for the cyclopentadienyl radical render that explanation untenable. A new model for methane pyrolysis is constructed here, using a novel mechanism generation approach that automatically computes any needed rate constants k(T,P) for chemically or thermally activated pressure-dependent reactions. The computer-generated mechanism accurately predicts the observed autocatalysis and concentration profiles without any adjustable parameters. Radical-forming reverse disproportionation reactionswhich involve propyne, allene, and fulveneaccount for at least half of the experimentally observed autocatalytic effect. Many of these reverse disproportionations were neglected in previous studies. The cyclopentadienyl radical is also important, but it is formed primarily by the chemically activated reaction of propargyl with acetylene. New rate estimates for unimolecular ring-closure reactions of unsaturated radicals are also presented. This approach is the first to incorporate pressure-dependent reactions generally and systematically during computerized mechanism construction. It successfully identifies complex but critical chemical-reaction pathways and autocatalytic loops missed by experienced kineticists.

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