Reactions of ethynyl radicals as a source of C4 and C5 hydrocarbons in Titan's atmosphere

doi:10.1016/S0032-0633(02)00014-4

Planetary and Space Science

Volume 50, Issues 7–8, June–July 2002, Pages 685-692

https://doi.org/10.1016/S0032-0633(02)00014-4 Get rights and content

Abstract

Crossed molecular beam experiments augmented by electronic structure computations of neutral–neutral reactions of the ethynyl radical (C₂H, X²Σ⁺) with the unsaturated hydrocarbons acetylene (C₂H₂), methylacetylene (CH₃CCH), and allene (H₂CCCH₂) are reviewed briefly. All reactions are characterized by a C₂H versus H atom exchange and in the case of the C₂H/C₂H₂ system by an additional molecular hydrogen (H₂) elimination pathway. The attack of the ethynyl radical onto the π-electron density of the unsaturated hydrocarbons has no entrance barrier and initializes each reaction. Consecutive hydrogen atom migrations may precede the exit channels. Diacetylene (HCCCCH), the butadiynyl radical (HCCCC), methyldiacetylene (CH₃CCCCH), ethynylallene (H₂CCH(C₂H)), and penta-4-diyne (HCC(CH₂)C₂H) were identified as products of which only diacetylene has yet been observed in Titan's atmosphere. Our results, however, strongly suggest the presence of all these species on Titan, and the Cassini–Huygens mission is likely to detect these upon arrival in the Saturnian system in 2004.

Introduction

The active and evolving organic chemistry on Saturn's moon Titan, the second largest satellite in the solar system, serves in many aspects as a model for the prebiotic chemistry on Earth (Clarke and Ferris, 1997; Raulin et al., 1999). Analogous to proto Earth, Titan's atmosphere is dense $(1.6 atm)$ and consists mainly of molecular nitrogen (N₂). “Greenhouse gases” (methane and molecular hydrogen on Titan, water and carbon dioxide on Earth), as well as anti-greenhouse species (hazes and clouds on Titan and aerosols and clouds on Earth) are present. Furthermore, both planetary systems exhibit similar vertical temperature profiles with tropospheric and stratospheric layers, but the temperatures deviate significantly: $94 K$ on Titan's surface versus 273 to $<373 K$ on proto Earth. The ambitious NASA-ESA “Cassini–Huygens” mission (Huygens-Science, 1997) is currently on its way to Saturn and is scheduled to arrive in 2004. While the Cassini spacecraft will become an orbiter around Saturn, the Huygens probe will enter Titan's atmosphere to carry out a detailed analysis of its chemical diversity.

The present knowledge of the chemistry on Titan is grounded on observations from Earth-based telescopes, spacecraft instruments, bulk laboratory experiments, as well as on photochemical models (Hanel et al., 1981; Kunde et al., 1981; Lara et al., 1996; Toublanc et al., 1995; Yung et al., 1984). The latter use reaction pathway parameters such as rate coefficients, branching ratios, photolysis frequencies, and absorption spectra (IR, UV) for which laboratory data at low temperatures often are not available or have large uncertainties. All species observed thus far provide a fundament with further chemical and exobiological implications in Titan's atmosphere: diacetylene (1, Fig. 1) is the largest linear polyatomic molecule identified in the outer solar system. The hydrocarbons methane (2), acetylene (3), ethylene (4), ethane (5), methylacetylene (6), propane (7), and the cyano-hydrocarbons hydrogen cyanide (8), cyanogen (9), acetonitrile (10), cyanoacetylene (11), and presumably solid state dicyanoacetylene (12) also were detected (Bandy et al., 1992). The proposed pathways to larger, more complex species are suggested to proceed via polyynes and cyanopolyynes (Yung et al., 1984). Photochemical models predict further the presence of triacetylene (13), tetraacetylene (14), and cyanodiacetylene (15) (Bandy et al., 1992; Zwier and Allen, 1996).

To simulate the chemical processing of Titan's atmosphere, the overwhelming diversity of all processes has to be broken down into binary reactions of a reactant A with a second collision partner BC which have to be studied individually. Hence, precise insight into a single reactive encounter is the most fundamental piece to assemble the complete puzzle accurately. Therefore, all experiments must be performed under single collision conditions. This means that the initial reaction complex ([ABC]*, Eq. (1)) formed by a binary reaction is not stabilized by further collisions (exclusion of three body reactions): $A + BC →[ABC]^{∗} → AB + C .$ Further, highly unstable and reactive reactants must be prepared under well-defined conditions (internal states, velocity, velocity spread) and reaction products with often unknown spectroscopic properties have to be probed. Hence, the majority of interesting unsaturated nitriles cannot be scrutinized by optical detection schemes, such as laser induced fluorescence (LIF) or resonance-enhanced multi-photon ionization (REMPI). Therefore, a universal, mass spectrometric detector operated in the time-of-flight (TOF) mode is crucial. Finally, different structural isomers of a reaction product might be formed and need to be distinguished. The latter can be accomplished by studying the distinct chemical dynamics of a reaction.

Section snippets

A combined experimental and theoretical approach

Our goal was to set up a research program to investigate the formation of nitriles, polyynes, and allene derivatives in planetary environments such as Titan, systematically. The prime directives were the identification of the (different) reaction products and the reaction intermediates, furthermore the elucidation of the reaction mechanisms. We realized this aspiring project by a combination of crossed molecular beam experiments (Fig. 2) linked to a TOF mass spectrometer and high level ab-initio

Study of reactions of the ethynyl radical (C₂H) with unsaturated hydrocarbons

Recent laboratory experiments demonstrate the ability of C₂H (X²Σ⁺) to react with unsaturated hydrocarbons at temperatures as low as $15 K$ . Chastaing et al. (1998) reported reaction rate constants for the reaction of ethynyl radicals with acetylene (3) and ethylene (4) (2.2– $2.3×10^{−10} cm^{3} s^{−1}$ at $15 K$ ). Hoobler and Leone (1999) studied the reactions of C₂H with methylacetylene (6) and allene (16) in the 155– $298 K$ temperature range ( $k_{CH3CCH} =1.9±0.3×10^{−10} cm^{3} s^{−1}$ , $k_{CH2CCH2} =1.7±0.3×10^{−10} cm^{3} s^{−1}$ at $298 K$ ). In

Experimental and theoretical approach

The experiments were carried out under single collision conditions coupling a novel ethynyl, C₂D(X²Σ⁺), source with a universal crossed molecular beams machine (Kaiser et al., 1999; Lee, 1987). Note that the experiments were performed with C₂D, not C₂H, since a mass-to-charge signal of 45 can be caused by $^{12} C^{13} C$ or $^{12} C^{12} C^{1} H$ but when using C₂D a signal from $^{13} C^{13} C$ at 46 is unlikely. The differences in the reaction energies compared to C₂H are only 1– $2 kJ mol^{−1}$ due to the different

The reaction of C₂H (23) with acetylene (3)

The experiments were performed at a well-defined collision energy of $26.1 kJ mol^{−1}$ . Experiment and theory agree that the ethynyl radical (C₂H, $^{2} Σ^{+}$ , 23), attacks the π-electron density of the acetylene molecule (3) without a barrier to form intermediate 24 (Fig. 3). This initial reaction intermediate can undergo cis/trans isomerization to 25 which involves only a small barrier or generate species 26 by a 1,2-hydrogen shift which has a high barrier. Intermediate 25 can decompose via C–H bond

The reaction of C₂H (23) with methylacetylene (6)

Like the C₂H/C₂H₂ system, the reaction of the ethynyl radical with methylacetylene is dominated by the exchange of C₂H for an H atom. The chemical reaction dynamics involve three different channels. The barrier-less addition of the ethynyl radical to the acetylenic π bond leads to two different intermediates: 30 as the result of C₂H binding to the terminal acetylenic carbon atom of 6 or, less favorably, 35 from attack on the central carbon of 6. The former process clearly dominates. Cis/trans

The reaction of C₂H (23) with allene (16)

The reaction of the $C_{2} H (^{2} Σ^{+})$ radical with the second C₃H₄ isomer, allene, is dominated by long-range dispersion forces and proceeds barrier-less by an addition of ethynyl to a terminal carbon of allene. This process leads to the formation of the doublet radical intermediate 38 (Fig. 5) which is bound in a deep potential energy well of $232.4 kJ mol^{−1}$ with respect to the reactants. This complex decomposes via H atom elimination into two distinct C₅H₄ isomers: ethynylallene (37) and 1,4-pentadiyne (

Summary and conclusion

Our combined crossed molecular beam experiments and electronic structure calculations reveal the chemical dynamics of elementary reactions of $C_{2} H (^{2} Σ^{+})$ radicals with acetylene (3), methylacetylene (6), and allene (16) in hydrocarbon-rich planetary and satellite atmospheres. All the potential energy surfaces involve similar characteristics. Firstly, all reactions are initiated by an addition of $C_{2} H (^{2} Σ^{+})$ to the C–C double or triple bond of the hydrocarbon without an entrance barrier. Secondly, all

Acknowledgements

RIK is indebted to the Deutsche Forschungsgemeinschaft (DFG) for a Habilitation fellowship (IIC1-Ka1081/3-1). FS gratefully appreciates a HSP III Doktorandenstipendium from the DAAD (D/99/22963). This work also was supported by the University of Georgia, by the US Department of Energy and, in Germany, by the DFG.

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Reactions of ethynyl radicals as a source of C4 and C5 hydrocarbons in Titan's atmosphere

Abstract

Introduction

Section snippets

A combined experimental and theoretical approach

Study of reactions of the ethynyl radical (C2H) with unsaturated hydrocarbons

Experimental and theoretical approach

The reaction of C2H (23) with acetylene (3)

The reaction of C2H (23) with methylacetylene (6)

The reaction of C2H (23) with allene (16)

Summary and conclusion

Acknowledgements

Planet. Space Sci.

Planet. Space Sci.

Adv. Space. Res.

Icarus

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J. Chem. Phys.

Crossed beam reaction of cyano radicals with hydrocarbon molecules IIIchemical dynamics of vinylcyanide (C2H3CN; X1A) formation from reaction of CN(X2Σ+) with ethylene, C2H4(X1Ag)

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Density-functional thermochemistry. III. The role of exact exchange

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Origins Life Evol. Biosphere

How robust is present-day DFT?

Int. J. Quant. Chem.

Density functional theory (DFT), Hartree–Fock (HF), and the self-consistent field

Infrared observations of the saturnian system from Voyager 1

Science

Ab initio Molecular Orbital Theory

Low-temperature rate coefficients for reactions of the ethinyl radical (C2H) with C3H4 isomers methylacetylene and allene

J. Phys. Chem. A

Reactions of ethynyl radicals as a source of C₄ and C₅ hydrocarbons in Titan's atmosphere

Study of reactions of the ethynyl radical (C₂H) with unsaturated hydrocarbons

The reaction of C₂H (23) with acetylene (3)

The reaction of C₂H (23) with methylacetylene (6)

The reaction of C₂H (23) with allene (16)

Crossed beam reaction of cyano radicals with hydrocarbon molecules. I. Chemical dynamics of cyanobenzene (C₆H₅CN; X¹A₁) and perdeutero cyanobenzene (C₆D₅CN; X¹A₁) formation from reaction of CN (X²Σ⁺) with benzene C₆H₆ (X¹A_1g) and d₆-benzene C₆D₆ (X¹A_1g)

Crossed beam reaction of cyano radicals with hydrocarbon molecules IIIchemical dynamics of vinylcyanide (C₂H₃CN; X¹A) formation from reaction of CN(X²Σ⁺) with ethylene, C₂H₄(X¹A_g)

Formation of nitriles in the interstellar medium via reactions of cyano radicals, CN(X²Σ⁺), with unsaturated hydrocarbons

Direct detection of C₄H₂ photochemical productspossible routes to complex hydrocarbons in planetary atmospheres

Neutral-neutral reactions at the temperature of interstellar clouds; rate coefficients for the reactions of C₂H radicals with O₂, C₂H₂, C₂H₄ and C₃H₆ down to $15 K$

Low-temperature rate coefficients for reactions of the ethinyl radical (C₂H) with C₃H₄ isomers methylacetylene and allene