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

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).