Determining Reaction Paths by Evaluating Kinetic Isotopic Effects with Density Functional Theory: Example of Methane...

This study demonstrates the determination of reaction pathways by evaluating the carbon kinetic isotopic effect and interpreting isotopic fractionations based on quantum chemical calculations. The reaction under investigation is methane thermogenesis from the decomposition of kerogen, which is a geochemical reaction under temperatures below 150 °C and lasts for tens of millions of years. Investigating its mechanism requires theoretical simulations because lab experiments at practical time-lengths require elevated temperatures, which introduce unwanted side reactions. Density functional theory and kinetic simulations were conducted on isotopic fractionations with the use of two possible pathways (free-radical versus carbonium), and the results were compared to field data sets. Different molecular sizes of kerogen were investigated to account for the hindrance of translation and rotation in modeling a reactant in the solid phase. Both pathways have low reaction barriers, implying that the reaction rates are limited by the concentration of active species (hydrated protons and free radicals). The results support the carbonium pathway and rule out the free-radical pathway as the ¹³CH₄ from the latter would be 30‰ more depleted than the observed data. Additionally, simulations were conducted on hydrocarbon isotope fractionation of the carbonium pathway with consideration of hydrogen exchange between methane and water, successively reproducing the observed abundances of deuterium-containing isotopologues (¹³CH₃D, ¹³CH₃D, and ¹²CH₂D₂).