Formation of Interstellar Molecules

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

The first step in interstellar chemistry is the production of diatomic molecules, notably molecular hydrogen. Observations of atomic hydrogen in dense clouds show that this species cannot be detected except in a diffuse halo surrounding the cloud, so that an efficient conversion of H into H2 is necessary. In the gas phase this might be accomplished by the radiative association reaction, 

Once a significant amount of molecular hydrogen is produced, a rich gas-phase chemistry ensues.24 Ion-molecule processes are initiated in the interiors of dense clouds mainly via cosmic ray ionization, the most important reaction being, 

The hydrogen molecular ion is rapidly (within a day at a standard gas density n of 104 cm-3) converted to H3 via the well-studied reaction  

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions  

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Most molecules observed to date in interstellar space can be dissociated by UV radiation of wave lengths longer than 912 A. In fact, their average lifetimes in interstellar space are <100 yr, unless they are protected by a dust layer (Section IV. E). This, and the fact that surface reactions on dust grains play an important role in the formation of interstellar molecules (see Section IV. 

The scope of this section is limited to a discussion of the formation and dissociation processes of molecules in cool clouds of interstellar gas with densities n > 10 cm- 3 and with kinetic gas temperatures 7 > 5 °K together with some laboratory work related to the formation of interstellar molecules. Chemical processes suggested to be operative in solar nebulae are briefly mentioned. 

Table 8. Schematic representation of the formation of interstellar molecules (after Winnewisser, 1972)... 

In a more recent paper by Watson and Salpeter (1972a) the formation of interstellar molecules on the surface of grains is also discussed. 

The fractionation of deuterium in interstellar molecules continues to excite considerable interest. Cosmologists identify the cosmic D/H ratio as a parameter critical to the assessment of cosmological models. Astrophysicists can use the isotopic ratio of species found in interstellar clouds as a probe of the conditions in those clouds. Isotopic abundances can help ion chemists to map synthetic pathways for forming interstellar molecules. Rnally to chemical kineticists, interested in the formation of interstellar molecules at temperatures approaching absolute zero, isotope effects offer a unique challenge — what is a minor perturbation at 300 K must exercise a profound influence at 10 K. Thus the equilibrium constant for the reaction... 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

Several different types of this dust are distinguished by astronomers. On average, interstellar dust resides in widely separated diffuse clouds. But there are also dense regions of gas and dust into which little ultraviolet radiation can penetrate, thereby providing an environment for the formation of complex molecules these are referred to as molecular clouds. Clouds of particles expelled by cooler stars into the regions around them are called circumstellar... 

In 1985. similar experiments were conducted at Rice University. In a 1988 paper. Curl and Smalley (Rice University) outlined their experiments with carbon cluster beams, essentially using the clusler-generaung apparatus previously described by the Exxon researchers. Initially, this experimentation was motivated by an interest that had been shown by die astrophysicist, Krotu (University of Sussex), who had been modeling the formation of carbon molecules in circumstellar shells. As a consequence, the Rice University team concentrated its studies on the smaller (2- to 30-atom) carbon clusters. As pointed out in the Curl-Smalley paper, the objective was to determine if some or all of the species had the same form as the long linear carbon chains known to be abundant in interstellar space."... 

The following sections deal with II the physical conditions in interstellar space, III observations of interstellar molecules and their interpretation, including relevant laboratory measurements, and Section IV experimental and theoretical investigations of the processes of formation and destruction relevant to interstellar molecules. This review covers primarily the interstellar matter proper and refers only briefly to observations of molecules in circum-stellar shells, or molecule formation in protostellar nebulae. Table 1 sets out certain quantities which are used in astronomy and are necessary to an understanding of this paper. 

If these values are typical, even a young cloud should contain appreciable amounts of carbon cycled through solar nebulae. Abundances of interstellar molecules relative to CO are at least 2 orders of magnitude lower than yields in FTT syntheses (Gammon, 1978). It appears that only a moderate degree of star formation and CO processing would suffice to account for the interstellar molecules. 

In this cosmochemistry series Topics in Current Chemistry , Winnewisser, Mezger and Breuer 1974 have given a general review of interstellar molecules, with some consideration of molecule formation mechanisms. Recently Winnewisser, Churchwell and Walmsley 1979 have given a detailed account of the Astrophysics of Interstellar Molecules with a chapter specially devoted to mola ule formation mechanisms. This article is based on these earlier reviews with emphasis on some of the more recent developments. 

Detailed studies show that the rate coefficients for 3-body association increase with decreasing temperature (Smith and Adams, 1978) probably due to the longer lifetime of the excited complex (AB ). Arnold (1979) suggest i that radiative association of Hj to molecular ions may proceed with high rate constants at the low temperatures of dense molecular clouds, (iii) Laboratory observation of a large number of chemical reactions which lead to the formation of complex molecules. Often though, it is not easy to assess the precise relevance of these new data to the interstellar conditions. 

Kaiser, R.I. Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. Suits, A.G. A combined experimental and theoretical study on the formation of interstellar C3H isomers. Science 1996, 274, 1508-1511. Kaiser, R.L Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. Neutral-neutral reactions in the interstellar medium. IL Isotope effects in the formation of linear and cyclic C3H and C3D radicals in interstellar environments. Astrophys. J. 1999, 510, 784—788. Kaiser, R.L Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. The formation of HCS and HCSH molecules and their role in the collision of comet Shoemaker-Levy 9 with Jupiter. Science 1998, 279, 1181-1184. Kaiser R.L Stranges D. Lee Y.T. Suits A.G. Neutral-neutral reactions in the interstellar medium. 1. Formation of carbon hydride radicals via reaction of carbon atoms with unsaturated hydrocarbons. Astrophys. J. 1997, 477, 982-989. 

Figure 10.5 illustrates the chemical structure of a plane parallel PDR by giving the relative abundances of C+, C and CO as a function of the penetration depth into the model cloud [11]. We assumed a kinetic equilibrium and determined the relative abundances from a chemical network consisting of 38 different species formed and destroyed in 434 reactions. The PDR is illuminated from the left by the mean interstellar radiation field and extends from the predominantly atomic surface region to the point where almost all carbon is bound into CO. One of the difficulties in calculating the chemical and thermal structure of a PDR arises from the effect of self-shielding. Molecules already formed absorb UV photons which are able to dissociate the respective molecule. In other words they cast a shadow into the cloud which enhances the further formation of the respective molecule. This effects becomes especially important for the formation of key molecules like O2, H2 and CO. Our current research addresses the question under which physical conditions an instability due to shadowing effects could occur. Another difficulty concerns the effects of small-scale fluctuations of the UV radiation field on the chemical network. 

The chemistry of dense, dark molecular clouds prior to planet formation is the topic of this paper. Dr. Ziurys has discussed the inventory and measurement of gas phase interstellar molecules associated with dense molecular clouds in the chapter, "Identifying Molecules in Space" 8). Here, the focus is on the chemistry in and on the ices and the interaction of these ices with species in the gas. Since these ices represent the largest repository of interstellar molecules in dense clouds, they tie up a large fraction of the chemical inventory in molecular... 

Both these ice and PAH spectral features are now being used as new probes of the interstellar medium of our Galaxy, the Milky Way and other galaxies throughout the Universe. The ices probe chemical processes in dark clouds and planet forming regions and the PAH features reveal the ionization balance as well as the energetic and chemical history of the medium. This paper has focused on the role of ices in the formation of complex molecules that are part of the raw materials from which stars, planets, satellites, and comets form. 

Cumulenes with ten or more cumulated carbon atoms have been detected in the interstellar space and some theories propose that such molecules played very important role in the formation of organic molecules during chemical evolution. 

Passing from atoms to radicals, the possibility of the formation of a complex has to be taken into account. This formation considerably increases the value of Tc in Eq. (19.1), thus increasing Pj. In the case of complex radicals, this may even lead to considerable (up to 10 —10 cm s) rate constants of recombination accompanied by radiative transitions between vibrational levels of the same electronic state. Such processes, though not yet observed, are believed to play an important part in the formation of complex molecules in interstellar gas Luds[184]. 

Interstellar molecules are formed in the clouds themselves from atoms and dust particles ejected from older stars, either explosively, as in the case of supernovae, or less violently, as in the case of low-mass stars. The stellar ejecta eventually lead to the formation of interstellar clouds through the force of gravity, which also causes the formation of denser smaller structures... 

The detection of molecules via their rotational spectra allows astrophysicists to probe interstellar clouds to provide information on their environment, star formation, interstellar chemistry, mechanisms for synthesis and destruction of interstellar molecules, isotopic distributions, etc. 

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