Molecules In Interstellar Clouds Research Articles

Formation of S-Bearing Complex Organic Molecules in Interstellar Clouds via Ice Reactions with C2H2, HS, and Atomic H.

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past few decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH3CH2SH and CH2CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C2H2 molecules and SH radicals-both thought to be present in interstellar icy mantles-at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH3CH2SH, CH2CHSH, HSCH2CH2SH, H2S2, and tentatively CH3CHS and CH2CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH3CH2SH, and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed.

Potential energy curves, partition functions and line intensities of HF and HCl molecules under the improved Hulburt‐Hirschfelder quantum mechanical anharmonic oscillator model

AbstractAn improved Hulburt‐Hirschfelder (IHH) quantum mechanical anharmonic oscillator model which describes the exact analytical potential energy curves (PECs) of diatomic molecules is presented. The IHH analytical potential function that involves a variable parameter, four empirically determined low‐order molecular vibrational and rotational spectral constants and the dissociation energy was applied to the ground electronic states of HF and HCl, yielding a good agreement between the Rydberg‐Klein‐Rees (RKR) potential energy points and the calculations. Correspondingly, with the determined IHH PECs, the partition functions at temperatures 300–6000 Kelvin (K) and rovibrational line intensities are evaluated, and coincide well with those quoted in the HITRAN2020 database compilation. The methodology could be valuable for the detection of molecules in interstellar clouds using infrared spectroscopy, as well as thermochemical reactions.

Unusual Chemical Processes in Interstellar Chemistry: Past and Present

The chemistry that occurs in interstellar clouds consists of both gas-phase processes and reactions on the surfaces of dust grains, the latter particularly on and in water-dominated ice mantles in cold clouds. Some of these processes, especially at low temperature, are very unusual by terrestrial standards. For example, in the gas-phase, two-body association reactions form a metastable species known as a complex, which is then stabilized by the emission of radiation under low-density conditions, especially at low temperatures. In the solid phase, it has been thought that the major process for surface reactions is diffusive in nature, occurring when two species undergoing random walks collide with each other on a surface that has both potential wells and intermediate barriers. There is experimental evidence for this process, although very few rates at low interstellar temperatures are well measured. Moreover, since dust particles are discrete, modeling has to take account that reactant pairs are on the same grain, a problem that can be treated using stochastic approaches. In addition, it has been shown more recently that surface reactions can occur more rapidly if they undergo any of a number of non-diffusive processes including so-called three-body mechanisms. There is some experimental support for this hypothesis. These and other unusual gaseous and solid-state processes will be discussed from the theoretical and experimental points of view, and their possible role in the synthesis of organic molecules in interstellar clouds explained. In addition, their historical development will be reviewed.

Quantum Chemical Cluster Studies of Cation-Ice Reactions for Astrochemical Applications: Seeking Experimental Confirmation.

ConspectusInterstellar clouds and the outer reaches of protostellar and protoplanetary systems are very cold environments where chemistry is limited to processes that have little or no reaction barrier (in the absence of external energy input). This account reviews what is known about cation-ice reactions, which are not currently incorporated in astrochemical network models. Quantum chemical cluster calculations using density functional theory have shown that barrierless reactions can occur when gas phase cations such as HCO+, OH+, CH3+, and C+ are deposited on an icy grain mantle with energies commensurate with other gas phase species. When cations react with molecules on ice surfaces, the pathways and products often differ significantly from gas phase chemistry due to the involvement of water and other molecules in the ice. The reactions studied to date have found pathways to abundant and important astromolecules such as methanol, formic acid, and carbon dioxide that are very favorable and may be more efficient pathways than gas phase processes. Other products that can be produced include glycolonitrile, its precursors, and related isocyanide compounds. This account describes for the first time ice surface reactions between the carbon cation, C+, and two common astromolecules, methanol (CH3OH) and formic acid (HCOOH), which can yield precursors to glyoxal, hydroxyketene, vinyl alcohol, and acetaldehyde. The quantum chemical methodology used to explore reaction surfaces is also used to predict both vibrational and electronic spectra of reactant and product ices, which offers guidance for possible experimental studies of these reactions. While theoretical calculations indicate that cation-ice reactions are efficient and offer novel pathways to important astrochemical compounds, experimental confirmation would be very welcome. Cations and ice-covered grain mantles are certainly present in cold astrophysical environments. The account concludes with a discussion of how cation-ice reactions could be incorporated into reaction network models of the formation and destruction of molecules in interstellar clouds and protoplanetary systems. Further studies will involve characterizing additional rcactions and more extensive treatment of the most important cation-ice reactions to better ascertain reaction branching outcomes.

Cosmic biology in perspective

A series of astronomical observations obtained over the period 1986 to 2018 supports the idea that life is a cosmic rather than a purely terrestrial or planetary phenomenon. These include (1) the detection of biologically relevant molecules in interstellar clouds and in comets, (2) mid-infrared spectra of interstellar grains and the dust from comets, (3) a diverse set of data from comets including the Rosetta mission showing consistency with biology and (4) the frequency of Earth-like or habitable planets in the Galaxy. We argue that the conjunction of all the available data suggests the operation of cometary biology and interstellar panspermia rather than the much weaker hypothesis of comets being only the source of the chemical building blocks of life. We conclude with specific predictions on the properties expected of extra-terrestrial life if it is discovered on Enceladus, Europa or beyond. A radically different biochemistry elsewhere can be considered as a falsification of the theory of interstellar panspermia.

Composition of Comets and Interstellar Dust

AbstractRecent observational results from ground and space-based observatories in combination with laboratory simulations allow us to monitor the evolution, survival, transport and transformation of cosmic dust from molecular clouds and the diffuse interstellar medium, until their incorporation into Solar System material. The composition of comets encodes information on their origin and can be used as a tracer for processes which were predominant in the protosolar nebula. A comparison of the composition and abundances of interstellar molecules and cometary volatiles reveals a general correspondence, but discrepancies are also apparent. Detailed studies indicate that cometary ices are a mixture of original interstellar material with material that has been moderately to heavily processed in the pre-solar nebula. The inventory and distribution of ice and organic molecules in interstellar clouds and in comets and their common link are briefly described.

Crossed beam investigation of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbon (PAH)-like molecules in extraterrestrial environments

The reactions of ground state carbon atoms, C( 3P j), with benzene, C 6H 6, and phenyl radicals, C 6H 5, with methylacetylene, CH 3CCH, were investigated in crossed beam experiments at collision energies of 21.8 and 140 kJ mol −1 to investigate elementary reactions relevant to the formation and chemistry of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The C( 3P j) reaction proceeds via complex formation and gives a cyclic, seven-membered C 7H 5 doublet radical plus atomic hydrogen. This pathway has neither an entrance nor exit barrier, and is exothermic. Together with the experimental verification of the carbon versus hydrogen exchange under single collision conditions, the findings have an important impact on the chemistry of aromatic molecules in interstellar clouds and outflow of carbon stars. Even in the coldest molecular clouds ( T=10 K), the benzene molecule can be destroyed upon reaction with carbon atoms, whereas they are resistant toward an attack of oxygen and nitrogen atoms. Since the aromatic benzene unit is ubiquitous in extraterrestrial, PAH-like material, our results suggest that PAHs might react with carbon atoms as well. On the other side, the reaction of C 6H 5 radicals with methylacetylene to form phenylmethylacetylene is direct. Since an entrance barrier inhibits the reaction in cold molecular clouds and in the atmospheres of hydrocarbon rich planets like Jupiter and Saturn and satellites such as Titan, this reaction is expected to play a role in PAH synthesis only in high temperature interstellar environments, such as circumstellar outflows of carbon stars.

Chemical physics: Molecular clouds, clusters, and corrals

Vignettes of three frontier areas illustrate the eclectic scope of modern chemical physics. (1) Radioastronomy has revealed a profusion of organic molecules in interstellar clouds, now attributed to sequences of exoergic, bimolecular ion-molecule reactions proceeding far from thermodynamic equilibrium. The organic profusion occurs because ${\mathrm{He}}^{+}$ ions react far more readily with CO than with the much more abundant ${\mathrm{H}}_{2}$ molecules. (2) Molecular clusters, generated by supersonic expansions, have become a favorite medium for study of reactions and spectra. Exemplary episodes are the discovery of carbon-60 and kindred fullerene molecules and the observation of sharp rotational spectra of guest molecules in superfluid helium clusters. (3) New means to control molecular trajectories are being developed. These include spatial orientation or alignment by field-induced hybridization of rotational states, corralling (or trapping) molecules after collisional or mechanical quenching of translational kinetic energy, and laser control of photochemical reaction pathways.

An ISO View on Interstellar and Cometary Ice Chemistry

The ISO-SWS instrument offering a large wavelength coverage and a resolution well adapted to the solid phase has changed our knowledge of the physical-chemical properties of ices in space. The discovery of many new ice features was reported and the comparison with dedicated laboratory experiments allowed the determination of more accurate abundances of major ice components. The presence of CO2 ice has recently been confirmed with the SWS (Short Wavelength Spectrometer) as a dominant ice component of interstellar grain mantles. The bending mode of CO2 ice shows a particular triple-peak structure which provides first evidence for extensive ice segregation in the line-of-sight toward massive protostars. A comparison of interstellar and cometary ices using recent ISO data and ground-based measurements has revealed important similarities but also indicated that comets contain, beside pristine interstellar material, admixtures of processed material. The investigation of molecules in interstellar clouds is essential to reveal the link between dust in the interstellar medium and in the Solar System.

The chemistry of complex molecules in interstellar clouds

This paper is concerned with the chemical evolution of large molecules in interstellar clouds. We consider the chemistry and ionisation balance of large polycyclic aromatic hydrocarbon (PAH) type molecules in diffuse clouds and show that certain PAH molecules can be doubly ionised by the interstellar ultraviolet radiation field. If recombination of the dications so produced with electrons is dissociative rather than radiative, then PAHs are rapidly destroyed. PAHs which can only be singly ionised have much smaller recombination energies and can be long lasting in these regions. This type of property may be very important in selecting the PAH species which can populate the general interstellar medium and account for certain of the diffuse bands observed in optical spectra. Destruction of PAH molecules via formation of dications may be responsible for the weakening of the diffuse bands observed in regions of high UV flux. We also discuss the chemistry of large, saturated molecules in hot molecular cores in star-forming regions. Here it appears that gas phase abundances are determined by surface chemistry on the cold ice mantles of interstellar grains. We present the preliminary results of a study of deuterium chemistry in these hot cores with the aim of probing both the detailed surface processes by which complex molecules are synthesised and the gas phase reactions which destroy them.

Gas-Phase Reactions and Energy Transfer at Very Low Temperatures

Experimental studies of gas-phase chemical reactions and molecular energy transfer at very low temperatures and between electrically neutral species are reviewed. Although work of collisionally induced vibrational and rotational transfer is described, emphasis is placed on very recent results on the rates of free radical reactions obtained by applying the pulsed laser photolysis (PLP)-laser-induced fluorescence (LIF) technique in a CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) apparatus at temperatures as low as 13 K. These measurements demonstrate that quite a wide variety of reactions-including those between two radicals, those between radicals and unsaturated molecules, and even some of those between radicals and saturated molecules-remain rapid at very low temperatures. Theoretical efforts to explain some of these results are described, as is their impact on attempts to model the synthesis of molecules in interstellar clouds.

Cosmic evolution of the biogenic elements and compounds

This paper traces the evolution of the biogenic elements H, C, N, O, P and S from their creation by cosmic nucleosynthesis to their inclusion in living systems on the surface of the Earth. Evidence for the presence of significant prebiotic molecules in interstellar clouds and in primitive meteorites is reviewed. The possible relevance of this discovery to the origin of life on Earth is assessed in the light of evidence suggesting that such molecules could not easily be synthesized in a primitive CO2-dominated terrestrial atmosphere.

The chemistry of PAH and fullerene molecules in interstellar clouds

Recent laboratory data on the ion-neutral chemistry of PAH and fullerene ions and molecules have been incorporated into chemical kinetic models of interstellar clouds. The laboratory data show that the second ionization potentials of many complex molecules are less than the first ionization potential of helium. Thus collisions between He + , generated by cosmic ray ionization, and PAH and fullerene neutrals produce doubly charged cations.

Hydrogenation of fullerene cations in the gas phase: reactions of fullerene cations and dications with atomic and molecular hydrogen

The recent discovery' and synthesisZ of the fullerene series of carbon allotropes has opened up many exciting vistas in chemistry? So far, hydrogenation of fullerenes has been achieved in solution by the Birch reduction, but only with C60.4 C60H36 was the observed product, and ab initio calculations have predicted that C&, should also be table.^ A previous attempt to hydrogenate the monocation of Cm in the gas phase was unsuccessful-C60*+ has been found to be unreactive toward molecular hydrogen6-but the monohydrogenated Cm and C70 cations have been formed by gas-phase proton t r a n ~ f e r . ~ Here we report results of an experimental study which demonstrate that both the monocations and the dications of Cm and C70 can be hydrogenated in the gas phase by reactions with atomic hydrogen. We have studied the reactions of Cm*+, c60z+, C70*+, and C70z+ with H and with Hz and can report that all of these fullerene ions are observed to add atomic hydrogen with remarkably high efficiency. Hydrogenation of fullerene cations in this manner has importance in the chemistry of sooting flames* and in the chemistry which leads to partially hydrogenated fullerane molecules in interstellar clouds and circumstellar envelope~.~J~ Atomic hydrogen (atomic deuterium) was produced by the action of a microwave discharge upon a dilute mixture, -3%, of Hz (D2) in He. The discharge source and the method of calibrating the dissociation efficiency have been described previously,*'z as has the selected ion flow tube (SIFT) apparatus used in these experiments to measure reaction kinetic^.'^ Fullerene ions were produced by electron bombardment (at 50 eV) of c 6 0 and C70 vapor entrained in argon carrier gas. The fullerene sample

Interstellar H3O+

The H3O+ ion is a key species in the oxygen chemistry leading to H2O, OH and O2. Chemical models predict O2 and H2O to be the dominant oxygen-bearing molecules in interstellar clouds. However, neither of them can easily be observed in the bulk of the interstellar medium because of blockage from the Earth's atmosphere. Determination of the abundance and distribution of the precursor H3O+ ion might thus provide an important indirect measure of their abundances.

The gas-phase chemistry of organic molecules in interstellar clouds with dynamical mixing

view Abstract Citations (68) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Gas-Phase Chemistry of Organic Molecules in Interstellar Clouds with Dynamical Mixing Chieze, J. -P. ; Pineau des Forets, G. ; Herbst, Eric Abstract It is shown that, if mixing between diffuse and dense clumps with no gas intermediate in density is considered, it is necessary to mix in large amounts of neutral atomic carbon rather than the singly ionized variety to maintain high abundances of organic molecules at t much greater than 10,000 yr. On the other hand, if a more natural model in which gas condensation occurs before mixing is considered, the very act of compression is a significant source of complex molecules. In addition, high abundances of organic molecules are preserved for a large range of mixing times. Publication: The Astrophysical Journal Pub Date: May 1991 DOI: 10.1086/170030 Bibcode: 1991ApJ...373..110C Keywords: Abundance; Interstellar Chemistry; Interstellar Gas; Molecular Clouds; Organic Compounds; Astronomical Models; Carbon Compounds; Neutral Gases; Astrophysics; INTERSTELLAR: ABUNDANCES; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS | data products SIMBAD (1)

Interstellar Dust in Collected Interplanetary Dust Particles

During the past decade interplanetary dust particles (IDPs) have been collected in the earth's stratosphere. Isotopic studies of these particles have demonstrated that many of them are greatly enriched in deuterium and at least some of them carry this enrichment in smaller subcomponents. Deuterium enrichments of a similar magnitude are seen in simple molecules in interstellar clouds. Deuterium enrichment in IDPs can be taken as evidence for the presence of interstellar material. It is not clear at this time whether the carriers of the isotopic anomalies represent true, unaltered interstellar dust grains, or whether they represent an altered component with a molecular ‘memory’ of original interstellar grains. The spectra of different components in the collected dust provide suggestive matches to similar components evident in the astronomical spectra of dust in comets, dense molecular clouds, and emission nebulae. The known extraterrestrial nature of the particles, the possible presence of interstellar material in them, and their spectral similarity to many astronomical objects all argue that the collected IDPs provide useful analogs for the modelling of interstellar dust.

A study of reactions of C nH +m ions ( n = 4, 5, 6; m = 0–6) with H 2 and CO at 300 K and 80 K

The reactions of ions in the series C n H + m ( n = 4, 5, 6, m = 0–6), derived by electron impact on benzene vapour, have been studied with H 2 and CO in a selected ion flow tube apparatus at 80 K and 300 K (and in some cases also at 210 K and 475 K) and over limited pressure ranges of helium carrier gas; both the rate coefficients and the product ions of the reactions have been determined. In general only the less-hydrogenated ions ( m ⩽ 1) showed significant reactivity with H 2 and CO, a notable exception being C 6H + 5, which additionally was generated in two differently reacting forms. Further, the ions C 4H + 2, C 4H + 3 and C 5H + 2 were observed to react only with CO. The reactions with H 2 proceed mainly by H-atom abstraction producing ions possessing an additional H atom. Thus hydrogenation of the bare carbon chain ions can only proceed to m = 2 by these reactions. The reaction with CO were exclusively association reactions producing ions of the form C n H m CO +. In addition, the bare carbon chain ions were the only ones observed to add two CO molecules. Some of these association reactions are very fast, especially at 80 K, the effective two-body rate coefficient being close to the collisional limiting value and independent of the helium pressure over the available range. Therefore only lower limits to the three-body rate coefficients can be given. The intriguing possibility arises that some of these fast association reactions may be occurring in two-body collisions, the binding energy being released by the emission of photons. This would have major implications for the synthesis of carbon—oxygen chain molecules in interstellar clouds.

Formation and destruction of molecular ions in interstellar clouds

A brief review is given of the role of molecular positive ions in interstellar chemistry. It is indicated how the simplest ions, which are formed by photoionization and cosmic-ray ionization, are converted to polyatomic ions by sequential gas-phase ionic reaction processes such as proton transfer, carbon insertion and radiative association. The importance of H3 and CH3 ions in the initial stages of molecular synthesis and the analogous roles of H2D+ and CH2D+ (which are formed in the reactions of H3 and CHg with HD) in the production of deuterated molecules are stressed. Recent work is also mentioned concerning the possible routes to c-C3H2 the first cyclic molecule to be detected in interstellar clouds. The process of dissociative recombination of positive ions with electrons is also discussed, because it is commonly invoked as the final step in the destruction of polyatomic ions and in the formation of many of the observed neutral molecules in interstellar clouds, even though the products of such reactions are currently a matter for speculation. It is stressed how spectroscopic studies of the structures and the products of reactions of molecular ions can further advance understanding in the field of interstellar chemistry.

Oscillator Strengths for Visible and Ultraviolet Observations of Interstellar Molecules

The accuracy of the information obtained from observations of optical-wavelength, molecular absorption features in interstellar clouds is directly limited by the accuracy of the rotational-line oscillator strengths used in the analysis of data. This paper briefly discusses optical observations of molecules in interstellar clouds; surveys molecular lifetime and oscillator strengths; and reviews laboratory techniques for obtaining these data.