PAHs From Benzene by Shock Reaction

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium [138]. They are found in carbonaceous chondrites that have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be synthesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, the simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furthermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmochemistry. 

The shock against benzene produced H2, light alkanes from Ci to C3, light alkenes from C2 to C3, C2H2, and aromatic hydrocaibons with molecular weights ranging from 102 (phenylacetylene) to 306 (quaterphenyl). This reaction produced H2, CH4, polyphenyl compounds such as biphenyl, terphenyl, and quaterphenyl. Other major products were naphthalene, fluorene, trans-stilbene, phe-nanthrene, isomers of phenylnaphthalene and chrysene. The shock produced ethenyl in greater abundance than ethyl derivatives. 

The molar yields (mol of products/initial mol of benzene) of products increased with increasing projectile velocity. The composition of products, however, was independent of the projectile velocity. Many structural isomers were identified in the products. Mutual ratios between the structural isomers for each product did not vaiy greatly with projectile velocity. The results at low temperatures were practically the same as those at room temperature with regard to the composition of products, the dependence of molar yields on projectile velocities, and the ratios between structural isomers. 

Reaction Mechanisms Controlling PAH Formation in Shocked Benzene 

On the other hand, shock waves generate high pressures as well as high temperatures, and, consequently, some factor in addition to heat must be involved in the shock reaction. Drickamer [145], for example, has suggested a close relationship between photochemistry and high-pressure chemistry. He experimentally showed that high-pressure conditions promoted the formation of pentacene dimers with cross-linked structure, the formation of which usually occurred in the photochemical reaction. If the shock reaction is a type of some reactions in excited states such as a photochemical reaction, many valence isomers such as Dewar benzene and benzvalene would be generated from benzene by shock waves, and the interaction between these isomers would produce various compounds such as derivatives of fulvene. Such valence isomers are unstable and would not have been detected in our study. 

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