Synthesis of polycyclic aromatic hydrocarbons from benzene by impact shock: Its reaction mechanism and cosmochemical significance

Abstract

The synthesis of polycyclic aromatic hydrocarbons (PAHs) from benzene by shock waves was studied in order to search for a novel possibility of PAH formation under cosmochemical conditions. Shock waves generated by projectile impacts were transmitted into pure benzene, and then the shocked samples were analyzed by FID gas chromatography and gas chromatography-mass spectrometry. The projectile velocity ranged from 100 to 1200 m/s. The physical conditions of shocked benzene in liquid form were estimated by the Hugoniot data. The shock waves caused reactions between benzene molecules to produce PAHs with high-molecular weights ranging from 128 (naphthalene) to 306 (quaterphenyl). Major products were naphthalene, biphenyl, fluorene, trans-stilbene, phenanthrene, and chrysene. Striking aspects emerge from the experiments: (1) the molar yields of products were enhanced exponentially with increasing projectile velocity, (2) the composition of products remained constant independent of the projectile velocity, and (3) the mutual ratios between structural isomers and the ratios of various products to chrysene showed definite values independent of the projectile velocity. These results were identical for experiments at two different temperatures, 77 K (benzene in solid form) and 290K (benzene in liquid form). I propose in this study that thermochemical reactions of ground states play a major role in the shock synthesis, although reactions of excited states cannot be ruled out. Examination of the yield relationships among structural isomers in products suggests that concerted cycloaddition reactions controlled by Woodward-Hoffmann rules explain the formation of some products better than do radical addition reactions. Most species of PAHs reported to be present in carbonaceous chondrites and interplanetary dust particles were synthesized during the present experiment. Furthermore, abundance ratios between some structural isomers in shock-induced PAHs are approximately the same as those in carbonaceous chondrites such as the Murchison meteorite. Shock synthesis must have operated during shock events in cosmochemical environments, and the shock-induced PAHs may be present in the interstellar medium, in atmospheres of Jovian planets, and in carbonaceous chondrites.