Interstellar intermediates

Gas-phase ion chemistry is a broad field which has many applications and which encompasses various branches of chemistry and physics. An application that draws together many of these branches is the synthesis of molecules in interstellar clouds (Herbst). This was part of the motivation for studies on the neutralization of ions by electrons (Johnsen and Mitchell) and on isomerization in ion-neutral associations (Adams and Fisher). The results of investigations of particular aspects of ion dynamics are presented in these association studies, in studies of the intermediates of binary ion-molecule Sn2 reactions (Hase, Wang, and Peslherbe), and in those of excited states of ions and their associated neutrals (Richard, Lu, Walker, and Weisshaar). Solvation in ion-molecule reactions is discussed (Castleman) and extended to include multiply charged ions by the application of electrospray techniques (Klassen, Ho, Blades, and Kebarle). These studies also provide a wealth of information on reaction thermodynamics which is critical in determining reaction spontaneity and availability of reaction channels. More focused studies relating to the ionization process and its nature are presented in the final chapter (Harland and Vallance). 

As far as acyclic allene hydrocarbon intermediates are concerned, these begin with a C3H2 species, 254, about which there has been considerable discussion since it has also been detected in radioastronomy studies to belong to the (growing) number of organic compounds present in interstellar and circumstellar space [105]. 

Lim and Lampe119 have studied the ion-molecule reactions in mixtures of SiR and CO. During the course of these studies the authors observed that the intermediate CSiH+ ion readily protonates CO and proposed that this reaction (equation 22) might be a possible source for die neutral SiC molecule in interstellar space. 

Experiments with deuterated species show extensive H/D scrambling, which implies longer-lived intermediates in the reaction. Calculations at the B3LYP/6-311+G(2d,p) level suggest that the reaction pathway involves bond formation between the cycloheptatrienylidene dication and acetylene, followed by isomerization and hydrogen loss (Scheme 9). The authors of this study note the possibility of similar chemistry generating polycyclic aromatic compounds in interstellar space. 

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. 

Because most radicals have an odd number of electrons on an atom, the octet rule cannot be satisfied at that atom. It is no surprise, then, that most radicals are unstable species and are quite reactive. They are most often encountered, like carbocations, as transient intermediates in reactions. However, alkyl radicals tend to have longer lifetimes than carbocations because they are less electron deficient, and therefore more stable. In fact, the lifetime of a radical can be appreciable in an environment where nothing is available with which to react. For example, hydrogen atoms are the principal type of matter in interstellar space. And die methyl radical has a lifetime of about 10 min when frozen in a methanol matrix at 77 K. 

The intermediate stages between interstellar matter and the formation of protostars are seen in very dense and very cool clouds of interstellar matter. 

While small carbon molecules have been found in locations as diverse as interstellar molecular clouds, carbon stars, hydrocarbon flames and laser-ablated carbon, bulk quantities are much less easily produced and have therefore been little studied. However, as with many organic intermediates, stabilization of these reactive molecules by coordination to metal centers can be achieved and is the subject of this review. The preparation of systems containing metal centers linked by carbon-atom chains is both a synthetic challenge (in spite of the first such being obtained over 45 years ago) and of considerable current relevance. 

The aim of this article is to give a short outline of current theories of molecule formation and destruction in interstellar clouds, together with a short summary of the observational material which has been accumulated up to early 1981. Although this article will address itself predominantly to simple molecules a section on complex molecules has been added. We will, therefore, discuss some general aspects of cosmochemistry and then turn to molecule formation in diffuse clouds followed by a discussion of the chemistry of dense interstellar clouds. A section has been added to summarize recent observational results and theoretical proposals in understanding the formation of intermediate and complex molecules, an area of considerable current activity. Finally the article closes with a short summary of the molecular species found in planetary atmospheres and a short discussion of what the relation might be to the interstellar molecules. 

Electron transfer from methane is significant because it seems likely that the radical cation and secondary intermediates derived from it (CH3 , CH3+) played a significant role in the chemical evolution preceding the origins of life. Methane is a probable constituent of early planetary atmospheres and its radical cation has potential significance as an interstellar species. 

In this chapter, we will review the characteristics of thermonuclear processing in the three environments we have identified (i) intermediate-mass stars (ii) massive stars and type II supemovae and (iii) type la supemovae. This will be followed by a brief discussion of galactic chemical evolution, which illustrates how the contributions from each of these environments are first introduced into the interstellar media of galaxies. Reviews of nucleosynthesis processes include those by Arnett (1995), Trimble (1975), Truran (1984), Wallerstein et al. (1997), and Woosley et al. (2002). An overview of galactic chemical evolution is presented by Tinsley (1980). 

Figure 25. Pattern of chemical abundances in the ambient interstellar medium of cB58 deduced by Pettini et al. (2002b). The vertical height of the boxes shows the typical uncertainty in the abundance determinations. O, Mg, Si, P, and S are thought to be synthesised by massive stars which explode as Type II supernovae, while Mn, Fe and Ni are predominantly produced by Type la SN. Their release into the ISM, as well as that of N from intermediate mass stars, lags behind that of the Type II SN products by several 100 Myr.

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