Hydrocarbon interstellar

The reactions of ground state atomic carbon, C(3Pj), with unsaturated hydrocarbons are another important class of reactions characterized by multiple pathways. These reactions, besides being of fundamental interest, are of great relevance in the chemistry of the interstellar medium and also in combustion.12,93-95... 

Recent studies with a crossed-beam apparatus not only show that the products shown above are the correct ones, but that both the linear and cyclic isomers, each of which is a detected interstellar molecule, are formed.47 Crossed-beam studies also show that other reactions between C atoms and unsaturated hydrocarbons proceed to form similar products 48... 

It is possible that electrons are not the major carrier of negative charge in dense interstellar clouds. It has been suggested that if a large fractional abundance of polycyclic aromatic hydrocarbons (PAHs) exists throughout dense interstellar clouds (see Section I), then electron sticking reactions of the type,... 

Since two mechanisms are possible for the competition between association and reaction, detailed ab initio calculations of the potential surface are even more necessary in theoretical determinations of the rates of association channels. More experimental work is also needed it is possible that as a larger number of competitive systems is studied, our understanding of the competition will increase. Critical systems for interstellar modeling include the association/reactive channels for C+ and bare carbon clusters, as well as for hydrocarbon ions and H2. 

A second major class of ion-molecule reactions that is relatively poorly studied consists of systems involving very unsaturated hydrocarbon neutrals, especially radicals. The unsaturated nature of the organic chemistry in interstellar clouds leads to sizeable abundances of very unsaturated hydrocarbons such as the polyacetylenes HC H, the carbenes H2C , the radicals C H, and the clusters Cn. Although some work has been done on the chemistry of such species, much of the relevant ion-molecule chemistry involving ions such as C+, CH3, and even C2H2 must be guessed at from generalizations based on a small number of studied systems. 

A much more detailed and time-dependent study of complex hydrocarbon and carbon cluster formation has been prepared by Bettens and Herbst,83 84 who considered the detailed growth of unsaturated hydrocarbons and clusters via ion-molecule and neutral-neutral processes under the conditions of both dense and diffuse interstellar clouds. In order to include molecules up to 64 carbon atoms in size, these authors increased the size of their gas-phase model to include approximately 10,000reactions. The products of many of the unstudied reactions have been estimated via simplified statistical (RRKM) calculations coupled with ab initio and semiempirical energy calculations. The simplified RRKM approach posits a transition state between complex and products even when no obvious potential barrier... 

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.

Later we found that diperester 5 is a much better precursor for the preparation of 2.26 But the easiest way of generating 2 is the thermal HC1 elimination from 3-chlorocyclopropene (6).27,28 This reaction is astonishing and is probably due to the resonance stabilization of 2 in the singlet ground state, as the thermal elimination of HC1 (in the absence of a base) does not work with ordinary halides. It should be added that 2 has been detected in interstellar clouds and it is claimed that 2 may be one of the most abundant hydrocarbons in interstellar space. 

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]. 

Formaldehyde has been detected recently in the interstellar medium by microwave spectroscopy (593), It is a combustion product of hydrocarbons. The photolysis of H2CO by sunlight in the troposphere may produce H02 radicals by reactions such as... 

The C60 molecule is difficult to destroy by ultraviolet radiation or by collisions with other particles. While other molecules have serious difficulties to survive in the interstellar medium, the robustness of C60 and of the other fullerenes allows their long survival. The bonds between carbon atoms make them at least as robust against dissociation in the interstellar medium as polycyclic aromatic hydrocarbons (PAHs) can be. 

The imperfect match between the IR spectra of benzenoid hydrocarbons with the interstellar emission bands has been taken to indicate that rather than pure benzenoid compounds being responsible, benzenoid-like (or PAH-like) species are present in the interstellar medium. In many regions of the interstellar medium PAHs are expected to be ionized [12] and those containing less than about 20 carbon atoms are expected to be dehydrogenated [10, 11, 12]. 

In the interstellar medium less stable PAHs will be weeded out, leaving a mixture dominated by the most stable structures [17, 19]. These more stable PAHs are those which are symmetric and generally in the most condensed forms. An example of a mixture comprised largely of the most stable molecular forms is the soot formed in the high temperature combustion of hydrocarbons. A comparison of the Raman spectrum in the 1800 to 1000 cm-1 region of auto soot with the interstellar IR emission spectrum from Orion is shown in Fig. 7. The Raman spectrum of soot... 

Summing up, the 1610 and 1310 cm-1 bands are indicative of aromatic hydrocarbons and have been assigned to the aromatic C-C stretch. The variability of the peak position and profile of the 1310 cm-1 feature implies further that symmetric PAHs dominate the interstellar mixture. These are, of course, the most stable members of the PAH family. 

It remains true that the subtle signature that corresponds to the 813C has never been completely exploited due to the lack, until recently, of very precise 813C value measurements for homologous derivatives (cf. Fig. 21). Work is in progress in our laboratory to demonstrate clearly that, at least for hydrocarbons, FTT is really the best candidate. The endogenous polymeric matter (which coexists with interstellar... 

Modified thermal (Bates 1983) or phase space (Herbst 1985c) calculations of radiative association rates indicate, as expected, an inverse temperature dependence and a direct dependence on the complexity of the reaction partners. Thus, if theory is to be believed, the importance of radiative association is enhanced by complex molecules reacting in cold clouds. Let us consider two important examples in the synthesis of interstellar methane (Huntress and Mitchell 1979). Although methane can only be observed with difficulty via radioastronomical methods (by centrifugal distortion induced rotational transitions) because it does not possess a permanent dipole moment, its synthesis is an important one because methane is a precursor to more complex hydrocarbons which can be and have been detected. This synthesis can proceed via the following series of normal and radiative association reactions, most of which have been studied in the laboratory ... 

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