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On the Formation of Higher Carbon Oxides in Extreme Environments

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On the Formation of Higher Carbon Oxides in Extreme Environments Chemical Physics Letters 465 (2008) 1–9 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett FRONTIERS ARTICLE On the formation of higher carbon oxides in extreme environments Ralf I. Kaiser a,*, Alexander M. Mebel b a Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall (Bil 301A), Honolulu, HI 96822, USA b Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA article info abstract Article history: Due to the importance of higher carbon oxides of the general formula COx (x > 2) in atmospheric chem- Received 15 May 2008 istry, isotopic enrichment processes, low-temperature ices in the interstellar medium and in the outer In final form 22 July 2008 solar system, as well as potential implications to high-energy materials, an overview on higher carbon Available online 29 July 2008 oxides COx (x = 3–6) is presented. This article reviews recent developments on these transient species. Future challenges and directions of this research field are highlighted. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction tion has no activation energy, proceeds with almost unit efficiency, and most likely involves a reaction intermediate. However, neither In recent years, the interest in carbon oxides of higher complex- reaction products (kinetics studies) nor the nature of the interme- 1 + ityP than carbon monoxide (CO; X R ) and carbon dioxide (CO2; diate (kinetics and dynamicsP studies) were determined. On the 1 þ 1 + 3 À X g ) of the generic formula COx (x > 2) has been fueled by com- other hand, a CO(X R )+O2(X g ) exit channel was found to have plex reaction mechanisms of carbon oxides with atomic oxygen in an activation energy between 15 and 28 kJ molÀ1 in the range of the atmospheres of Mars [1–4] and Venus [5]. Here, carbon dioxide 300–2500 K [14]. Atreya et al. pointed out the necessity to incorpo- represents the major atmospheric constituent [6]. Laboratory stud- rate heterogeneous reactions on aerosols or carbon dioxide ice par- ies suggested that the photodissociation of carbon dioxide by solar ticles in the Martian atmosphere [15–17]. However, these 0 photons (k < 2050 ÅA) generates a carbon monoxide molecule plus processes have not been investigated in the laboratory so far. Also, 3 atomic oxygen. Near the threshold, only ground-state O( P) atoms the explicit structure of the atmospherically relevant CO3 interme- are produced; shorter wavelengths supply also electronically ex- diate has not been unraveled to date [18,19]. cited oxygen atoms O(1D) [1]. Electronically excited oxygen atoms Besides the atmospheric chemistry of Venus, Earth, and Mars are thought to be mainly quenched involving a carbon trioxide and the 18O enrichment processes, solid carbon dioxide presents molecule (CO3) of a hitherto unknown structure. Once ground- an important constituent of interstellar ices as present on sub- state oxygen atoms O(3P) and carbon monoxide have formed, it micrometer sized grain particles in cold interstellar clouds holding is difficult to recycle carbon dioxide, since the reversed reaction temperatures as low as 10 K. Here, ice mixtures comprising water presents a spin-forbidden process. (H2O) (100), carbon monoxide (CO) (7–27), methanol (CH3OH) Besides the quenching of electronically excited oxygen atoms, (<3.4), ammonia (NH3) (<6), methane (CH4) (<2), and carbon diox- the carbon trioxide (CO3) molecule has been implicated as an ide (CO2) (15) were identified unambiguously via infrared spec- important transient species in the 18O isotope enrichment of car- troscopy towards the dense cloud TMC-1 employing the field star bon dioxide in the atmospheres of Earth and Mars [7,8]. Computa- Elias 16 as a black body source; the numbers in parentheses indi- tions and laboratory experiments indicate that the 18O enrichment cate the relative abundances compared to water ice. These clouds in ozone might be transferable to carbon dioxide [9,10], possibly interact with ultraviolet photons (<13.2 eV) and energetic parti- via a CO3 intermediate. Due to the potential role of carbon trioxide cles, predominantly from galactic cosmic rays. Henceforth, pristine in atmospheric chemistry, various kinetics and dynamics studies ice mantles are processed chemically by the cosmic ray-induced have been carried out to access the CO3 surface and to determine ultraviolet radiation present even in the deep interior of dense the temperature-dependent rate constants of the reaction of elec- clouds [20]. This can lead to the formation of new molecules in tronically excited oxygen atoms, O(1D), with carbon dioxide. At the solid state via non-equilibrium (non-thermal) chemistry even room temperature, rate constants of a few 10À10 cm3 sÀ1 have been at temperatures as low as 10 K. Likewise, carbon dioxide ices have derived [11–13]. This order of magnitude suggests that the reac- been detected in the outer solar system on Triton and Ganymede (Neptune’s and Jupiter’s largest moons, respectively). Here, charged particles from the planetary magnetospheres can poten- * Corresponding author. Fax: +1 808 956 5908. tially release oxygen atoms which may react with carbon dioxide E-mail address: [email protected] (R.I. Kaiser). to form carbon trioxide and higher carbon oxides. 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.076 2 R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9 Finally, it should be recalled that intensive research has been position of carbon trioxide molecules leads to carbon dioxide mol- carried out to develop new high-energy materials (HEMs) to meet ecules and atomic oxygen. Shortly after this study, Weissberger needs for future defense and space science applications like novel et al. carried out photolysis experiments of solid carbon dioxide explosives and rocket propellants [21]. Particular attention has matrices and of mixtures of ozone and carbon dioxide dispersed been devoted to cyclic carbon oxides of the generic formula COx in solid argon. These experiments strongly indicated that an elec- (x = 3–6). These higher-order oxides of the main group IV element tronically excited oxygen atom (O(1D)) can react with carbon diox- carbon are highly energetic and therefore, candidates for high-en- ide to actually synthesize the C2v symmetric carbon trioxide isomer ergy density materials; Shkrob suggested these oxides also as mod- [26]. An early review on carbon trioxide concluded that further el compounds to study high velocities of detonation (VOD) and to studies are necessary to elucidate the structure and the electronic possibly control the energy release from high-energy density configuration of this transient species [27]. material [22]. The subsequent years elucidated the electronic structures of the Due to the importance of higher carbon oxides in atmospheric carbon trioxide molecule(s). Applying Hueckel molecular orbital chemistry, isotopic enrichment processes, low-temperature ices theory, correlation diagrams, and Walsh rules, Gimarc and Chou in the interstellar medium and in the outer solar system, as well predicted that the molecule favors a planar, C2v symmetric struc- as potential implications to high-energy materials, this Frontiers ture [28]. Calculations predicted that this molecule is metastable Article presents a historical overview on higher carbon oxides COx toward dissociation into carbon monoxide (CO) and electronically 1 (x = 3–6) and reviews recent developments on how to synthesize excited molecular oxygen (O2; a Dg); an alternative decomposition these transient species experimentally and computationally. The pathway could involve linear carbon dioxide (CO2) and electroni- final chapter highlights future challenges and directions of this re- cally excited oxygen atoms (O(1D)). The latter mechanism was also search field. verified in photolysis experiments of ozone in carbon dioxide [29]. Olsen and coworkers [30] applied the extended Hueckel and inter- mediate neglect of differential overlap (INDO) methods to investi- 2. Historical overview gate various molecular geometries of the carbon trioxide molecule in its pyramidal (C3v) and symmetric trigonal C2v shaped struc- 2.1. Carbon trioxide (CO3) isomers tures. Both methods predicted the C2v shaped structure holding a 1 X A1 electronic ground-state to be favorable in energy. The authors 1 Experimental and theoretical studies of the properties and for- also suggested that the closed shell A1 ground-state correlates 1 0 mation routes of carbon trioxide isomers started more than four with the E wave function in D3h symmetry; consequently, the decades ago. In 1966, Ung and Schiff [23] conducted gas phase adoption of the C2v symmetry of carbon trioxide can be attributed experiments and photolyzed carbon dioxide under bulk conditions. to the Jahn–Teller effect. Subsequent calculations [31,32] corre- Based on the kinetics, the mass balance of the detected carbon lated with the previous results favoring a C2v symmetric structure monoxide (CO) and molecular oxygen (O2) products, and the reac- of the molecule. The short carbon–oxygen bond distance of the tions of methane (CH4), molecular hydrogen (H2), and dinitrogen- exocyclic unit of only 126.5 pm indicated a carbonyl group; like- oxide (N2O), the authors postulated the presence of a hitherto wise, the extended carbon–oxygen bonds of 147.3 pm as found in elusive reactive species, carbon trioxide (CO3), in the gas phase. the ring suggest the existence of carbon–oxygen single bonds. Fi- However, an explicit detection of this unstable molecule failed; nally, the oxygen–carbon–oxygen angle was derived to be about further, the authors did not recommend any structure of the car- 66°. This geometry was also verified in subsequent matrix studies bon trioxide molecule. by Jacox and Milligan [25]. Finally, high pressure kinetics studies of In the same year, Moll et al. [24] identified a C2v symmetric car- the gas phase reaction of electronically excited oxygen atoms 1 bon trioxide molecule (CO3) in carbon dioxide and carbon dioxide- (O( D)) have been conducted [33].
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