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[12.02] Nitrile Chemistry in Titan's Atmosphere: Low Temperature Rate Constants for H + Cyanoacetylene Reaction

R.J. Cody, J.K. Parker, W.A. Payne, L.J. Stief (NASA/Goddard Space Flight Center)

Cyanoacetylene (HC₃N) is one of the few nitrile compounds observed in the atmosphere of Titan by Voyager and from the ground. The three body reaction H + HC₃N + M arrow products (1) constitutes one of the important loss processes for cyanoacetylene in the photochemical models of the Titan atmosphere; e.g. the 1984 model of Yung, Allen and Pinto and the 1995 model of Toublanc et al. The models calculate a mixing ratio for HC₃N that differs considerably from that derived from the observations. The rate constant for Reaction 1 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, 250 K, and 200 K with helium as the bath gas at a pressure of 1 Torr; additionally, measurements of k₁ were made at T = 298 K with P = 0.5 and 2.0 Torr. 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 temperature and pressure, first order rate constants (k_obs) were measured for a range of [H] ~ 1 - 13 x 10¹³ molecule cm^-3. The bimolecular rate constants (k₁) were derived from the slopes of the plots of k_obs 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. This indicates that the rate constant has reached its high-pressure limit by 1 Torr for T \leq 298 K. The results are k₁(298K) = 2.1x10^--13, k₁(250K) = 1.5x10^--13 and k₁(200K) = 0.93x10^--13, all in units of cm³ molecule^-1 s^-1. These rate constants can be fit by the Arrhenius expression k₁ = 1.1x10^-12exp(-500/T) 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 and have a much weaker temperature dependence. Theoretical calculations of the rate constant were performed to understand the temperature dependence of Reaction 1. These results could render Reaction 1 a more important loss process in Titan's atmosphere than previously estimated.

The Planetary Atmospheres Program of NASA Headquarters supported this research.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.