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The reaction H + C 4 H 2 : Absolute rate constant measurement and implication for atmospheric modeling of Titan

1986; American Geophysical Union; Volume: 91; Issue: A4 Linguagem: Inglês

10.1029/ja091ia04p04585

2156-2202

D. F. Nava, Mark B. Mitchell, L. J. Stief,

Astrophysics and Star Formation Studies

Diacetylene (C 4 H 2 ), an intermediary product in the photodecomposition of Titan's methane, has been suggested to be the catalyst in processes converting H atoms into molecular hydrogen for subsequent escape from Titan's atmosphere. This role for C 4 H 2 has importance concerning formation and survival of the unsaturated species in the atmosphere of Titan. The reaction kinetics of H + C 4 H 2 are also of significance in incomplete combustion processes forming higher unsaturated hydrocarbons from acetylene and its radicals in the C 2 H 2 /O/H system. The absolute rate constant for the reaction H + C 4 H 2 has been measured over the temperature interval 210–423 K, using the technique of flash photolysis‐resonance fluorescence. At each of the five temperatures employed in this study the results were independent of variations in [C 4 H 2 ], total pressure (Argon or Nitrogen), and flash intensity (i.e., initial [H]). The rate constant results for 210 ≤ T ≤ 423 K are k = (1.39±0.25) × 10 −10 exp (−1184±44/T) cm³ s −1 , where the error quoted is one standard deviation. This represents the first temperature study of this reaction. The Arrhenius parameters at the high pressure limit determined here for H + C 4 H 2 are contrasted with those for the corresponding reactions of H with C 2 H 2 and C 3 H 4 . Implications of the kinetic results, particularly those at low temperatures, are considered for models of the atmospheric hydrocarbon chemistry of Titan. The rate of this reaction, relative to that of the analogous, but slower, reaction of H + C 2 H 2 , appears to make H + C 4 H 2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.