Hydrogen abstraction rates via density functional theory

Accurate estimates of hydrogen-abstraction rate constants are critical for detailed kinetic models. These reactions usually show curved Arrhenius plots, making extrapolations from low temperature measurements to combustion and pyrolysis conditions difficult. Conventional high-accuracy quantum-chemistry techniques are not feasible for the large molecules of technological interest. Here we present rate estimates made using the less CPU intensive density functional theory (DFT) for abstractions from ethane, formaldehyde, and formic acid by CH3. Comparison with experiment indicates that pre-exponential factors and even the curvature of the Arrhenius plots are accurately predicted by transition-state theory using DFT parameters. This allows accurate rate extrapolations over a broad temperature range.