Calculating Low Pressure Chemical Activation Atmospheric Hydrocarbon Synthesis Rates

Details Calculating Low Pressure Chemical Activation Atmospheric Hydrocarbon Synthesis Rates Calculating Low Pressure Chemical Activation Atmospheric Hydrocarbon Synthesis Rates

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.4910s&link_type=abstract

American Astronomical Society, DPS meeting #40, #49.10; Bulletin of the American Astronomical Society, Vol. 40, p.486

Statistics

Computation

Scientific paper

The formation of larger hydrocarbons, polyacetylenes, aromatics, and nitriles in the reducing atmospheres of the outer planets and moons occurs by recombination and addition reactions of smaller fragments at low temperatures and pressures, difficult to reach in the laboratory. However, because C-C bonds are usually weaker than C-H bonds, cleavage reactions can occur which interrupt this synthesis. The competing dissociation and recombination channels in these chemical activation reactions will be energy and pressure dependent. (Similar competition can occur during photolysis.) The rate constants can be determined computationally using statistical rate theory with parameters available from experiments and quantum calculations.
We have applied this approach to generate temperature and pressure dependent rate constant expressions for several reactions that help determine atmospheric concentrations of larger hydrocarbon species in jovian atmospheres. Attention must be paid to isomers and H tunneling. Examples include:
H + C3H7 -> CH3 + C2H5
C2H3 + C2H5 -> C4H8, H + C4H7, or CH3 + C3H5 (allyl isomer)
H + C3H5 -> C3H6 or CH3 + C2H3
CH3 + CN -> H + CH2CN
In addition, if photolysis is followed by internal conversion of electronic energy to vibrationally excited ground state molecules, the same approach may apply to computing C-C scission product yields. Product experiments to test this are underway.
Research supported by grants from the NASA Outer Planets Research Program and Planetary Atmospheres Program.

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