Large temperature dependencies for the D, 13C and clumped kinetic isotope effects in methane oxidation by OH and Cl predicted by quantum chemical and transition state theory.

Methane emission budgets based on isotopic analysis (e.g. ¹³C-CH₄, D-CH₄, ¹³CH₃D CH₂D₂, CHD₃, and/or CD₄) correct composition for the isotopic fractionation of atmospheric oxidation reactions. They rely on a handful of laboratory measurements obtained at only a couple of temperatures. The goal of this study is to better characterize KIEs of the reactions and especially the temperature dependence of the KIEs.

As a first step we have calculated the temperature dependent reaction rates using tunneling corrected Transition State Theory. We examine the reaction of methane with Cl and OH including all possible transition states with the isotopologues: CH_4,¹³CH₄, ¹⁴CH₄, ¹³CDH₃, CDH₃, CD₂H₂, CD₃H, and CD₄. Transition State Theory has been used with M06-2X, ωB97X-D, and CAM-B3LYP level of theory, with the two basis sets 6-31++G(d,p) and 6-311++G(d,p). The KIE is calculated for all reactions and compared with literature. Results for the ¹³CH₄ + Cl reaction show that the KIE changes with -12.0 ‰ per 100 K. Whereas for ¹³CH₄ + OH the KIE changes by -1.14 ‰ from 300 to 200 K. For all isotopologues we predict that the KIE’s change significantly with temperature. Including this correction in isotopic mass balance top down emissions estimates will significantly change the results.

In future work we will examine the reaction path and molecular dynamics in detail. To do these calculations, we will perform ab initio multiple spawning (AIMS) trajectories interfaced with the TeraChem electronic structure program. This study will increase our understanding of the oxidation of methane and compare the quantum chemical understanding of isotope budgeting to observations.