Calculation of high-level ab initio rate constants for key neutral-neutral reactions in low-temperature Titan conditions
Abstract
The ultimate source of the complex photochemistry in Titan's upper atmosphere (70-187K) is its background neutral environment which is composed primarily of N2 and CH4. Previous photochemical models show that such complex chemistry is strongly influenced by neutral-neutral reactions. Global sensitivity analyses confirm that the large errors associated with the mole fractions of various compounds in the upper atmosphere of Titan originate mainly from the uncertainties in low-temperature rate constants of "key" neutral-neutral reactions. 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 reactions in Titan, and subsequently evaluating the mole fractions using our existing one-dimensional (1D) photochemical model. 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 highly accurate theoretical rate constants will be made available to the astrochemistry community at large, and our calculated mole fractions will be used to analyze the Composite Infrared Spectrometer (CIRS) observational data which will vastly improve our current knowledge of the atmosphere of Titan.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.P21F3408B
- Keywords:
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- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGYDE: 5215 Origin of life;
- PLANETARY SCIENCES: ASTROBIOLOGYDE: 5749 Origin and evolution;
- PLANETARY SCIENCES: FLUID PLANETSDE: 6281 Titan;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS