Another Challenge Large Molecules, the Case of PAHs

Another Challenge Large Molecules, the Case of PAHs 

It is commonly accepted that reactions of CH with unsaturated hydrocarbons proceed without any entrance barrier via the formation of an initial intermediate which decomposes via hydrogen elimination. These reactions are exothermic and show several exit pathways due to the existence of isomers of final products. However there is debate on the exact nature of the reaction mechanism. As for the reactions of CH with acetylene and ethylene, the most likely mechanism seems to be the addition of the CH radical on the carbon multiple bonds to form a 3-carbon-atom cycle. It is believed that this cyclic intermediate isomerises to give linear intermediates which decompose to give the final products. The insertion of the CH radical in the C-H bond of the molecule is not thought to be a favourable entrance chaimel. For the reactions of CH with aromatic compounds a similar mechanism is expected. With respect to anthracene the observed behaviour of the rate coefficient is usually indicative of a reaction which 

An insertion mechanism would lead to a benzyl hke radical that would need to be stabilised. In order to confirm the mechanism of this reaction and the formation of cyclopropa[b]anthracene, ab initio calculations are needed. 

Various explanations have been proposed to account for this behaviour, but it is now generally agreed that it can be understood in terms of a two transition state modeE as first proposed by Klippenstein and co-workers for the reaction OH+C2H4. Briefly, the MEP for radical-molecule reactions can be represented schematically as in Fig. 2.11 at long range, attractive 

Motivated by this proposal, Sabbah et al. performed low temperature measurements of the rate coefficients for these reactions. 0( P) atoms were generated in the cold CRESU flow via 355 nm laser photolysis of NO2, and their concentration followed using chemiluminescence from excited NO2, formed in the association of 0( P) with NO, as a marker for the oxygen atom concentrations, which decayed exponentially as a result of reaction with the added alkene. Results were obtained in most cases down to 23 K or 27 K, except for the 0( P) + ethene reaction, for which only an upper limit to the rate coefficient could be measured at 39 K. 

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