[P-PS06] Planetary Sciences

[PPS06-P22] Calculation of kinetic rate constants using high-level ab initio quantum chemical methods for planetary sciences and astrochemistry

*Shiblee Ratan Barua^1,2, Paul Romani¹ (1.NASA Goddard Space Flight Center, Greenbelt MD USA, 2.Universities Space Research Association (USRA), Columbia MD USA)

Keywords:Titan atmosphere, Low-temperature reaction kinetics, Ab initio quantum chemistry, Photochemical model, Plantery science, Astrochemistry

Existing photochemical models for the atmospheres of planetary bodies suffer from large errors when predicting the mole fraction profiles of various compounds. Global sensitivity analyses show that a major source of these model errors can be attributed to inaccurate rate coefficients for “key” radical–radical and radical–neutral reactions in low-temperature conditions. Accurate rate coefficients for key reactions thus improve the accuracy of photochemical models. Unfortunately, accurate experimental rate constants for such low-T reactions are difficult, if not impossible, to measure, and the lab data are affected by uncertainties in determining the absolute concentrations of radical species. Currently, the most common theoretical approach involves uncertainty extrapolation technique in which uncertainties in room-temperature rate constants are extrapolated to low-temperature conditions, resulting in large errors in the theoretical low-T rate constant data. To solve this existing problem, we are employing the two-transition-state (2TS) model developed by Klippenstein and coworkers to calculate high-level ab initio rate-constants for key low-T (i.e. 10–200K) reactions relevant to planetary atmospheres like that of Titan. In particular, we are investigating key reactions that have not yet been studied in the lab, and for which accurate rate coefficients are still unknown. Our calculated ab initio rate coefficients will be made available to the astrochemistry community via well-established free online kinetic databases (e.g. KIDA). These rate coefficients will be used by scientists in photochemical models to make accurate predictions of mole fraction profiles for planetary atmospheres and improve our understanding of the diverse chemistry of these bodies.