Interstellar chemistry

Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles. 

Petrie S, Bohme DK (2003) Mass Spectrometric Approaches to Interstellar Chemistry. 225 35-73... 

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,... 

Association reactions, in particular, seem to present a severe problem for structural determination. In these reactions, an ion and a neutral species form a complex which is stabilized either by collision with a third body or, at especially low pressures, by the emission of radiation. The radiative mechanism, prominent in interstellar chemistry, is discussed below. Although some studies of radiative association have been performed in the laboratory,30,31 90 most association reactions studied are three-body in nature. It is customarily assumed that the product of three-body association is the same as that of radiative association, although this assumption need not be universally valid. 

Guillemin J.-C., Bouyahyi M. and Riague E. H. (2004). Prebiotic, planetary and interstellar chemistry starting from compounds detected in the interstellar medium. Advances in Space Science 33 81. 

We are aware that our review is by no means complete, the topic of diene and polyene radical cations having ramifications into such diverse fields as biology or interstellar chemistry. In the following we shall first discuss the photoelectron spectra of dienes... 

Busemann, H., Young, A. F., Alexander, . M. O D. el al. (2006) Interstellar chemistry recorded in organic matter from primitive meteorites. Science, 312, 727-730. 

Gravitational instability can occur in a cloud characterised by high density and low temperature (Jean s criterion). More precisely-, the mass of the cloud must be of the order of 20 M0 (where M0 is the solar mass) if the density (expressed as the number of dihydrogen molecules per cubic centimeter) is around 103 and the temperature around 10 K. Such a density is precisely what is observed in so-called dense clouds. These dense clouds are also regions of space where a lot of complex molecules are detected by spectroscopy, and where dust particles are observed. Readers interested in interstellar chemistry will find an excellent review of the subject in the recent book by Duley and Williams 13). 

As we will see, some anomalies in the isotopic composition of carbon, hydrogen and oxygen can be explained on the basis of this assumption, and we will start the discussion with the deuterium-rich matter in carbonaceous chondrites. This deuterium-rich matter is essentially present as complex macromolecules 70 73 96 97). The carbon in these samples is essentially normal 76,98). For some polymer-type fractions, the deuterium content is up to 32 times higher than the galactic value (D/H 2 x 10s in the number of atoms per cubic centimeter). High deuterium enrichments are known in interstellar molecules and the mechanism of this enrichment is fully understood. For an excellent review dealing with interstellar chemistry, see the paper by Winnewisser 99) and the previously mentioned book by Duley and Williams 13). 

Duley, W. W., Williams, D. A. Interstellar Chemistry, Academic Press, London 1984... 

The specific mechanisms that produce small interstellar molecules via ion-molecule reactions have been discussed by a large number of investigators since the original paper by Herbst and Klemperer (1973) and a large number of detailed model calculations have been undertaken. For a general discussion of processes, the reader is referred to earlier reviews by us (Winnewisser 1981 Herbst and Klemperer 1976). For the reader who is unfamiliar with gas phase interstellar chemistry, we will briefly consider some of the processes that form and destroy two significant molecules — H20 and the radical OH. 

Model calculations that include at least some of the reactions we have discussed for the syntheses of complex molecules have been performed in the last several years. Both steady-state and chemical time dependent models have been published. Unfortunately, as models include more and more complex species, they become more and more sensitive in their predictions to small changes. As an example, consider two models that in their predictions of the abundances of one-carbon-atom hydrocarbons differ by a factor of 3. This factor is not considered to be a major one in the field of interstellar chemistry. However, since the two-carbon-atom hydrocarbons are formed by reactions between one-carbon atom species, the model will differ in their predictions for the abundances of the larger hydrocarbons by a factor of 9. As one can easily discern, the situation becomes worse as the size of the hydrocarbons increase. Given this extreme sensitivity, modelers should attempt to make sure that at each stage of molecular complexity, they consider all depletion mechanisms and do not overestimate the abundances of the complex molecules that are intermediates in the formation of still more complex species. Unless this is done, models can become in our view overly optimistic about the growth of complexity in the interstellar medium. 

The real breakthrough of interstellar chemistry belongs to the post-war period and depended on the development of radioastronomy and microwave technologies that opened up quite new perspectives in celestial chemistry. However, since this modern phase only started in 1963 (with the detection of the radio signature of OH),... 

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