Isomerization Barriers in Hydrocarbon and Functionalized Radicals, as Predicted by Composite Ab Initio Methods

摘要

Kinetic modeling of fuels often relies on automatic mechanism generation; these methods frequently utilize Evans-Polanyi relationships to calculate kinetic parameters from thermodynamic data. This approach efficiently produces a variety of kinetic information for the thousands of elementary reaction steps involved in combustion of a given fuel. Both mechanism generation and kinetic modeling approaches will have important roles in predicting alternative fuel chemistry. It is imperative that these automatically-generated kinetic parameters be accurate, especially for the isomerizations (internal hydrogen-atom transfers) that dictate much of low-temperature combustion. Composite ab initio methods (e.g., G3MP2B3) have promise in determining quantitatively useful kinetic information and were used to explore the H-atom transfers possible for a variety of alkyl, alkenyl, cycloalkyl, and oxoallylic (derived from aldehydes and ketones) radicals. For alkyl radicals, a significant correlation between enthalpies of activation and enthalpies of reaction was demonstrated (reinforcing the idea of a valid Evans-Polanyi relationship); such scaling behavior was not seen in the functionalized radicals. The non-Evans-Polanyi behavior of the alkenyl and oxoallylic radicals is significant and has implications for automatic mechanism generation approaches to alternative fuels.