Cyanopolyynes, production

To verify if reactions 14.1 and 14.2 are responsible for the formation of cyanoacetylene/cyanopolyynes in the low temperature environments of Titan and ISM, a confirmation from laboratory experiments is required. Provided that the elementary reactions of interest are thermodynamically feasible, it is necessary to reach the knowledge of at least two other factors the relevant rate constants and the yield of the possible reaction products. Particularly this last piece of information will allow us to draw the sequence of elementary steps which account for the global reaction. 

Probably, the most important conclusion of the present study is that the triple acetylenic bond is preserved in the reaction products and this in spite of the fact that the initial attack by the unpaired electron of the CN radical destroys it. The three possible reaction intermediates have, in fact, lost the triple bond, but when, because of the high internal energy with which they are formed, they dissociate to products, the triple bond is retrieved in the new molecular products. This observation can be generalized to the case of the reactions of CN with polyynes and is quite important for the speculated routes of formation of cyanopolyynes in various environments. 

Interestingly, the presence of the methyl group of methylacetylene strongly increases the complexity of the reaction if compared to that of the unsubstituted acetylene. In particular, amongst the possible products only cyanomethylacetylene and cyanoacetylene (routes 14.9a and 14.9c) preserve the triple acetylenic bond in the nitrile products, while the formation of cyanoallene (routes 14.9b) or allene (route 14.9d and 14.9e) can also occur. This observation implies two consequences relevant to observations and ast-rochemical models on one side, it should be possible to observe cyanoallene in the same extraterrestrial environments where cyanomethylacetylene was observed, if reaction 14.9a is the real formation route of cyanomethylacetylene on the other side, the models should take into account that the reactions of CN radical with short methyl-substituted polyynes do not necessarily form cyanopolyynes. 

Neutral molecule plus radical reactions in the outer CSE are probably the major production channel of longer C-chain molecules such as C2 H, and the cyanopolyynes (HCan+iN). For example, the reaction channel C2nH2 + C2H — C2n+2H2 + H and subsequent photodissociation C4H2 + H C4H +H can build up C4H and CeH from the parent molecule C2H2 62). 

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