Understanding the Gas Phase Chemistry of Alkanes with First-Principles Calculations

Alkyl radicals are key intermediates in multiple industrially important reactions, including the dehydrogenation of alkanes. Because of their diverse chemistry, alkyl radicals form various products via a number of competing reactions in the gas phase. Using Density Functional Theory (DFT) and accurate ab initio electronic structure calculations (CBS-QB3), we investigated the thermodynamics and kinetics of gas phase alkyl radical reactions. Specifically, we investigated the hydrogen abstraction, radical recombination, and alkene formation reactions of light alkyl radicals (C₁-C₈). We show that the hydrogen abstraction Gibbs energies are correlated with the relative Gibbs energies of the corresponding radicals. On the basis of the reaction energy calculations, we identified that the competition between radical recombination reactions and alkene formation reactions is governed by the stability of the alkene products, with the alkene formation being preferred when more substituted alkenes are formed. It was found that the radical recombination is preferred over alkene formation at 298 K, but at high temperatures (773 K) alkene formation becomes highly preferred. Importantly, owing to the competition of different reactions, we demonstrate a systematic methodology to select a computational method to investigate the complex chemistry of alkyl radicals. Overall, this study provides a rich database of reaction energies involving alkyl radicals and identifies their thermodynamic preference that can aid in the design of more efficient processes for the chemical conversion of alkanes.