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R.J. Cody, J.K. Parker, W.A. Payne, L.J. Stief (NASA/GSFC)
Cyanoacetylene (HC3N) is one of the few nitrile compounds observed by Voyager in the atmosphere of Titan. The three body reaction H + HC3N + M arrow products (1) constitutes one of the loss processes for cyanoacetylene in the photochemical models of the Titan atmosphere; e.g. the 1984 model of Yung, Allen and Pinto. The rate constant for this reaction has never been measured. The models use a value equal to that for the reaction H + C2H2 + M.
We have measured the rate constant for Reaction 1 at T = 298 K over a pressure range of 0.5 - 2.0 Torr with helium as the bath gas. The experimental technique is discharge fast flow with mass spectrometric detection and monitoring of the first order decay of HC3N. The H atom, which is the excess reactant by a factor of 100-660, is generated via the fast reaction F + H2 arrow H + HF. For each pressure, first order rate constants (kfirst) were measured for [H]= 1.2 - 13 x 1013 molecule cm-3. The bimolecular rate constants (kbi) were derived from the slopes of the plots of kfirst versus [H].
Within our experimental uncertainty, we did not see a pressure dependence of the bimolecular rate constant for the limited pressure range of 0.5 - 2 Torr at T = 298K. The results at T=298K are kbi(0.5 Torr)=2.0x10-13, kbi(1.0 Torr)=2.2x10-13 and kbi(2.0 Torr)=1.8x10-13, all in units of cm3 molecule-1 s-1. These measured rate constants are about a factor of 50 faster than those estimated by analogy with the H + C2H2 reaction. These results could render Reaction 1 a more important loss process than previously estimated. Measurements of the rate constants for reaction 1 are continuing at T = 250K and 200K.
The Planetary Atmospheres Program of NASA Headquarters is providing the funding for this research.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.