Cyanopolyynes

The formation of the cyanopolyynes (HC2nCN) is closely tied to the synthesis of hydrocarbons. Both hydrocarbon ion-atomic nitrogen reactions 41 viz.,... 

Of the alkenes (Figure 5.5) only ethene has been detected and of the aromatics only benzene has been seen unambiguously surprisingly propene has not been seen despite its well-understood microwave spectrum. Of interest to the origins of life is the onset of polymerisation in HCN to produce cyanopolyynes. These molecules could provide a backbone for the formation of information-propagating molecules required for self-replication. The survival of these species in a planetary atmosphere depends on the planet oxidation would be rapid in the atmosphere of today s Earth but what of the early Earth or somewhere altogether more alkane-based such as Titan ... 

Figure 5.5 Alkanes, alkenes, aromatics and cyanopolyynes 1, methane 2, ethene 3, propene 4, benzene 5, acetylene 6, propyne 7, butadiyne 8, pentadiyne 9, hexatriyne 10-14, cyanopolyynes (the largest family with the longest molecule to date, 14). (Reproduced from Guillemin et at. 2004 by permission of Elsevier)...

Representative trace constituents HNC,c-C3H2) polyacetylenic compounds Cyanopolyynes, polycarbon sulfides, metal cyanides, silicon compounds Methylated organics Alcohols, ethers, esters, carboxylic acids, amino acids ( )... 

After these more general comments, we would like to discuss within the context of recent laboratory data some of the progress which has l n made specifically in the area of complex molecules such as cyanopolyynes. The interstellar carbon chemistry in dense molecular clouds (n 10 cm ) is used as an example. 

This presents a short summary of the reactions which are supposed to be important gas-phase reactions leading to the complex hydrocarbon molecules, and some of their derivatives. In interstellar space the cyanopolyynes constitute an important class, since only molecules with a permanent dipole moment can be observed in radioastronomical measurements. Thus hydrocarbons which have no dipole moment elude radioastronomical detection. 

Before discussing the possibility of gas-phase cyanopolyyne chemistry, it seems necessary to summarize the present status of carbon chain molamle detections in interstellar space. Table 7 presents an overview of where these molecules are found and their respective abundances. These tables are an abbreviated update from Table 1 taken from Winnewisser and Walmsley (1979). It is seen that these molecules are found essentially in every type of molecular cloud from the cold dark cloud to the warm circumstellar environment, underlining the trend which has been observed over the past few years namely that complex organic molecules are not limited to a few sources only (in particular to the galactic center sources) but that they are spread over sources with rather different physical conditions. A qualifying statement may be in order here. 

Hayatsu and Anders (this volume) point out from a comparison of the relative abundance pattern of cyanopolyynes H(C=C) N (n = 1, 2, 3, 4) with that of alcohols C H2 , 0H (Fig. 13 of their contribution) that the probability of the cyano... 

Cyanoalkynes and Cyanopolyynes From Crossed Beam Experiments to Astrochemistry.285... 

Cataldo, F. Polyynes and cyanopolyynes synthesis from the submerged electric arc about the role played by the electrodes and solvents in polyynes formation. Tetrahedron 2004, i59(19), 4265-4274. 

Cyanoacetylene and higher cyanopolyynes (H-(C=C) -C=N) are alleged to be possible intermediates towards the formation of simple amino acids and this motivates detailed studies focused on confirming the speculated formation routes [11,25]. The idea behind it is that the study of the chemical evolution of Titan s atmosphere will help us understand how biologically... 

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. 

As already discussed in Section 14.1, the CN-I-C2H2 reaction is alleged to be the most important step in the formation of cyanopolyynes in different environments, such as planetary atmospheres and the interstellar medium. Its potential importance has been confirmed by kinetic studies which have found this reaction to be very fast in a large range of temperatures [2 5]. The possible reaction channels include ... 

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. 

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