The reaction H + C₄H₂: Absolute rate constant measurement and implication for atmospheric modeling of Titan

Abstract

Diacetylene (C4H2), 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 C4H2 has importance concerning formation and survival of the unsaturated species in the atmosphere of Titan. The reaction kinetics of H + C4H2 are also of significance in incomplete combustion processes forming higher unsaturated hydrocarbons from acetylene and its radicals in the C2H2/O/H system. The absolute rate constant for the reaction H + C4H2 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 [C4H2], 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 + C4H2 are contrasted with those for the corresponding reactions of H with C2H2 and C3H4. 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 + C2H2, appears to make H + C4H2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.

A model of the ionosphere of Titan

“…The rate used by Yung (2.0 x 10 -12 cm 3 s -1) was measured at room temperature. A more recent measurement by Nava et al [1986] found the temperature dependence of this rate coefficient to be given by the relationship k3a = (1.39 + 0.25) x 10-1ø x exp[-(1184 _+ 44)FF] cm 3 s -1 over the temperature range 210 K _< T < 423 K. Extrapolating this relationship to T = 174 K (the atmospheric temperature used by Yung) gives a value for k3a of 1.5 x 10 -13 cm 3 s -1, smaller by a factor of 13 than that used by Yung. Since this is the first step in the critical scheme for removing atomic hydrogen in the stratosphere, a slower rate for this reaction would increase the atomic hydrogen density and consequently reduce hydrocarbon species' densities to produce closer agreement to those proposed by Coustenis et al [1989].…”

“…After analyzing infrared spectra from both the polar and equatorial regions of Titan, Coustenis et al [1991] remarked that current models (specifically that of Yung)were in "acceptable agreement" with observations within at least a factor of 2. Lellouch et al [1990] suggested that incorporation of the rate measured by Nava et al [1986] into Yung's model would produce "much higher C2H2 abundances ..." (p. 310), hence resolving the discrepancy between Yung's model and the UVS measurement of acetylene at 840 km. It seems that Lellouch et al [1990] were referring to the relative rates of reactions (3b) and (3c) above, i.e., a decrease in rate (3b).…”

The reaction of atomic hydrogen with germane: Temperature dependence of the rate constant and implications for germane photochemistry in the atmospheres of Jupiter and Saturn

“…The Results which we obtained at 298 K were presented at the First International Conference on Laboratory Research for Planetary Atmospheres, and a preliminary report is given in the conference proceedings [Nava et al, 1990].…”

“…an additional H atom source in our study of the reaction of H with diacetylene[Nava et al, 1986]. Photolysis occurred at wavelengths above 110 nm (MgF2 cutoff) to produce [H] 0 -< 10 TM cm -3.…”

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

“…Table 1 provides a comparison of the CBS-QB3 data with the QCISD(T) quantum chemical data, extrapolated to the infinite basis-set limit, as reported by Miller and Klippenstein, [44,45] for the activation energy barriers at 0 K, order to bring the rate coefficient in agreement with experimental measurements. [68] The reaction energies are also lower by 1 to 4 kJ mol -1 , except for the addition to ethene. Due to the lower CBS-QB3 barriers, the CBS-QB3 rate coefficients are slightly larger than the QCISD(T)/∞ values, by 10 to 70% for the addition to ethene and ethyne, and up to a factor 3 at 300 K for the addition to 1,3-butadiyne.…”