Interstellar clouds, chemical

With so many molecules now being observed in interstellar clouds, chemical reaction models which can explain how these molecules are produced and destroyed are becoming increasingly more valuable. The most modern chemical reaction networks that have been proposed involve following the concentration of several hundred atomic and molecular species as a function of time, and reliable temperature-dependent rate coefficients for several thousand reactions are a vital requirement in such simulations. The role of ion-molecule reactions has been shown to be of particular Importance in these networks as these reactions can have very large rate coefficients at the low temperatures of interstellar clouds [2]. Furthermore, a more limited number of neutral species, particularly radicals and open-shell atoms, can have large rate coefficients at low temperatures [3]. Since only a relatively small number of reactions have been studied in the laboratory at the temperatures relevant to Interstellar chemistry, theory plays an Important role in producing many of the required rate coefficients. 

Smith D (1992) The ion chemistry of interstellar clouds. Chemical Reviews 92 1473-1485. 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Although most of the ion-molecule reactions used in large chemical models of interstellar clouds have not been studied in the laboratory, a few classes of reactions... 

Figure 2. The new species added to our chemical models of interstellar clouds. The species range in complexity from 10-64 carbon atoms and comprise the following groups of molecules linear carbon chains, monocyclic rings, tricyclic rings, and fullerenes. The synthetic pathways are also indicated. See ref. 83. Reproduced from the International Journal of Mass Spectrometry and Ion Processes, vol. 149/150, R.P.A. Bettens, Eric Herbst "The interstellar gas phase production of highly complex hydrocarbons construction of a model", pp 321-343 (1995) with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV, Amsterdam, The Netherlands.

The rates of a chemical reaction generally increase with temperature, an example of which is the hydrolysis of sucrose, which is 4.13 times faster at body temperature (35°C) than at room temperature (25°C). This represents a surprisingly large change in the rate of chemical reactions due to the simple rise to body temperature. It is then clear that cooling the same reactions down will slow the reactions and there is a big difference between body temperature and 10 K in an interstellar cloud. 

Our knowledge of the chemical composition of interstellar space is growing rapidly due to recent advances in observation techniques using detection at ultraviolet, infrared, and radio wavelengths. Studies of interstellar clouds, which are composed... 

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

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. 

Summary The matrix-spectroscopic identification and photochemical interconversion of the isomeric silylenes 3-5, and silacyclopropyne (6) are of interest in many ways. For one, their isolation serves to illustrate the potential of matrix isolation spectroscopy. In addition, the structural assignments for these species are based on the comparison of the experimentally observed and calculated IR spectra and therefore emphasize the importance of simultaneously applying quantum chemical calculations and spectroscopic measurements. Moreover, practically no examples exist for this class of silylene rearrangements. Lastly, the C2H2Si isomers eventually play a decisive role in the chemistry of interstellar clouds. 

The chemical composition of diffuse interstellar clouds is simple and essentially limited to diatomic molecules which have been summarized in Table 2, together with the wavelength region where detection was made. It has to be noted, however, that there is a marginal detection of H2O at about the 2a level by Snow (1980). If confirmed a new H O mechanism has to be thought of. 

However, since we are dealing in this review primarily with problems related to star formation and galactic evolution, we shall ignore the work that has been done on dust formation and nucleation of classical (chemical) systems. These have been extensively reviewed by Abraham,Burton, and Draine and Salpeter for problems of astrophysical interest. We shall only refer to this literature for analogies which may be of some aid in establishing new directions for work on megascopic systems like interstellar clouds. 

So far, unsuccessful searches for Hj in BN, GL 2591, LkH 101, NGC 2024/IRS, W33IR, NGC 2264 and AFGL 2591 have been reported. Black et alP observed spectral lines of CO simultaneously with their search for Hj. The abundance of CO thus obtained together with the upper limit of the Hj column density set a limit on the rate of the cosmic ray ionization C through eqn. (7). van Dishoeck and Black have proposed on chemical grounds that the abundance of Hj may be equally high in diffuse interstellar clouds. ... 

In applications of stellar abundances to questions of cosmochemistry, an assumption is invoked. Put in its strongest form, this states that the star s present surface composition is identical to that of its natal interstellar cloud. When applicable and in conjunction with corresponding assumptions about interstellar clouds, this assumption allows the chemical evolution of the Galaxy to be traced using stars of different ages or more realistically stars of different metallicity. 

Duley, W.W. Chemical evolution of carbonaceous material in interstellar clouds. Astrophys. J. 2000, 528, 841-848. 

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