Synthesis of Polycyclic Aromatic Hydrocarbons from Benzene by Impact Shock: Its Reaction Mechanism and Cosmochemical Significance

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 weight 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, (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, 77K (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 can not 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. INTRODUCTION The shock wave has been widely recognized as a common and important phenomenon giving rise to various materials in the Universe. Chao et al. (1962) and Chao (1967) identified high pressure minerals in the sandstone around Meteor Crater, Arizona. Lipschutz and Anders (1961) and Lipschutz (1964) suggested that diamonds detected in ureilites and the Canyon Diablo meteorite were formed by shock during break-up of their parent bodies. McKay et al. (1988) claimed the importance of strong shock induced by impacts at late stages of the accretion of interplanetary particles in converting NH3 to N2 in Titan's atmosphere. Shock-compression produces unusual and distinctive states of materials under the high pressure and elevated temperature, which are not realized in other processes. These states may be in static compressive conditions, and they can be easily revealed by the Rankine-Hugoniot relations. Changes in composition and properties of materials have been studied under these extreme conditions (e.g., Duvall and Fowles, 1963). The influence of the shock wave on organic materials has been reported by several authors. Bar-Nun et al. (1970) and Bar-Nun and Shaviv (1975) demonstrated that shock heating of gas mixtures (CH4+NH3+H2O) in the laboratory yielded amino acids in a high yield. Sugisaki et al. (1994) reported shock synthesis of light hydrocarbons from CO and H2. In relation to these circumstances, I directed attention to the shock reaction of organic materials, particularly to shock syntheses of polycyclic aromatic hydrocarbons (PAHs). PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium (Allamandola et al., 1989). They are found in carbonaceous chondrites (Basile et al., 1984, Hahn et al., 1988, Zenobi et al., 1989), and in interplanetary dust particles (Clemett et al., 1993). Shock and Schulte (1990) suggested that amino acids could be synthesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. Thus PAHs are crucial materials involved in a variety of cosmochemical phenomena. Several methods for the genesis of PAHs in extraterrestrial environments have been proposed. Many reports (e.g., Anders et al., 1973) have claimed that PAHs were synthesized in the early solar nebular by a Fischer-Tropsch-type (FTT) process. On the other hand, a hypothesis by Harris and Weiner (1985) and Frenklach et al. (1989) emphasized that PAHs may have been formed by pyrolysis of hydrocarbons such as acetylene in the solar nebular. In order to test another possibility of PAH formation under cosmochemical conditions, Mimura et al. (1994) preliminary reported shock synthesis of PAH from benzene, the most simple aromatic hydrocarbon detected in the atmosphere of Jupiter (Kim et al., 1985) and in carbonaceous chondrites (e.g., Belsky and Kaplan, 1970). In this paper, I described in detail the experiments under various conditions of shock energy and temperature. To simulate a possible synthesis occurring in interstellar space, the experiment was carried out at low temperature (77 K) as well as room temperature (290 K). Furthermore, I examined the reaction mechanism of shock synthesis on basis of the quantum chemistry and discussed the implication for cosmochemistry. EXPERIMENTS A stainless steel container capped at one end was filled with reactant and subsequently capped at the other end. The container consists of two parts, a cylindrical vessel and a lid that were welded to each other (Fig. 1a). It contained 6ml of pure benzene distilled from a commercial reagent of the highest quality. In the low temperature experiment, the container was cooled with liquid nitrogen (Fig. 1b). When an aluminum projectile of 4.6g from a vertical powder gunstruck the lid of the container, shock waves were transmitted into the benzene. The projectile velocities ranged from 100 to 1200 m/s in these experiments. Above 1200 m/s, the container was destroyed to lose the products from the container. In the room temperature experiments, the pressure and the ratio of V (the specific volume behind the shock front) against Vo (the specific volume ahead of the shock front) in shocked benzene were calculated (Table 1) using the Hugoniot data for benzene in liquid form (Dick, 1970). At low temperatures, however, these values could not be estimated because of the lack of the Hugoniot data for solid benzene. Shock temperatures at impact cannot be estimated, owing to unavailability of the value for the Gruneisen gamma. A mixture of the shocked benzene recovered from the container and internal standard compounds was carefully concentrated with a rotary evaporator. The shocked benzene was concentrated at 2525°C on a water bath, but was not completely evaporated. The concentrated solution was analyzed by gas chromatography (Ohkura GC 202) with a flame-ionization detector and by gas chromatograph-quadrupole mass spectrometry (Shimadzu GC-MS QP2000). The column used for GC and GC-MS was a 25m × 0.25mm fused silica capillary column coated with a 0.3μm layer of SE-52 (5% phenyl, 95% methyl polysiloxane). The column temperature was programmed from 100°C to 280°C at 4°C/min. To avoid laboratory contamination, the stainless container and glassware for these procedures were all baked at 450°C before use. An unshocked procedural blank was carried through each experimental step. The blank showed no laboratory or instrumental contamination. This analysis was made on the compounds with boiling points from 200°C to 500°C. Shocked benzene, however, probably contains the compounds with boiling points <200°C and >500°C. Their presence is strongly suggested by the unpleasant smell and the presence of "soot-like" matter in the concentrate as described in "RESULTS". RESULTS Room temperature experiments (290K, benzene in liquid form Many kinds of aromatic hydrocarbons with high-molecular weight were synthesized from pure benzene during the experiment. The dark yellow concentrated solution smelled unpleasant smell and contained soot-like materials. On the representative gas chromatographic record for the products (Fig. 2a), 31 peaks were identified by retention times and/or fragmentation patterns. The shock on benzene yielded two-, threeand four-ring PAHs, and the molecular weights of these products ranged from 128 (naphthalene) to 306 (quaterphenyl). The molar yields (n mol of products / initial mol of benzene) of the identified products are shown in Table 2. The list of the products (Table 2) shows that this reaction especially favors the synthesis of polyphenyl compounds such as biphenyl, terphenyl and quaterphenyl. Other major products were naphthalene, fluorene, trans-stilbene, phenanthrene, isomers of phenylnaphthalene and chrysene. The shock produced more abundant ethenyl than ethyl derivatives. Because of a relatively lower boiling temperature and tendency to sublime, naphthalene and biphenyl were partly lost during the analysis, and hence the determined yields for these compounds are likely to be lower than the actual ones. Isomers of ethenylbiphenyl, methylphenanthrene, methylanthracene and quaterphenyl were not determined because authentic standard compounds were not available. The molar yields of products increase exponentially with increasing projectile velocity (Fig. 3a). The composition of products, however, is independent of the projectile velocity. At 100 m/s of projectile velocity, the molar yields are below the detection limit (0.1 n mol/mol). The ratios of various products versus chrysene are constant throughout these experiments, and the representative ratios (biphenyl / chrysene, m-terphenyl / chrysene and fluoranthene / chrysene) are shown in Fig. 4a. Many structural isomers were identified in the products. Mutual ratios between the structural isomers for each product did not vary greatly with projectile velocity (Table 3). The representative ratios (phenanthrene / anthracene and fluoranthene / pyrene) are shown in Fig. 5a. Low temperature experiment (77K, benzene in solid form) Many species of PAHs were synthesized also from solid benzene. The results of low temperature experiments were practically the same as those of room temperature experiments with regard to the composition of products (Fig. 2b), the molar yields (Table 2), the dependence of yields on projectile velocities (Fig. 3b), the relative molar yields (Fig. 4b), and the ratios between structural isomers (Fig. 5b, Table 3). CHEMISTRY Ground states or excited states? The experimental results show that these instantaneous high energy conditions generated by shock waves promote the formation of PAHs from benzene. It is of interest to consider what kind of reactions are involved in the shock synthesis and the reaction mechanisms. Figure 3 indicates that the molar yields increase exponentially with increasing the shock temperature at impact, because the shock temperature linearly correlated with the projectile velocity within the velocity range in this study. This suggests that shock synthesized amounts depend on the shock temperature. Shock synthesis may correspond to "pyrolysis" of benzene at high temperatures caused by shock wave, i.e., shock synthesis is a thermochemical reaction of the ground states. Lewis (1980) and Greinke and Lewis (1984) showed that heat brought up the polymerization of aromatic hydrocarbons. Stein (1978), in a study of concentrations of PAHs in idealized equilibrium systems, suggested that PAHs such as biphenyl, naphthalene, phenanthrene, and pyrene are formed through the most thermodynamically stable pathway. However, he did not mention other major PAHs found in this study such as methylbiphenyl, trans stilbene and fluorene. The identical results of experiments for liquid benzene (at room temperature, 290K) and for solid benzene (at low temperature, 77K) suggest that shock synthesis proceeds thermochemically in high temperature conditions irrespective of the temperature difference of 200K. On the other hand, shock waves generate high pressure conditions in addition to high temperatures, and consequently some factor as well as heat must be involved in shock synthesis. For example, a close relation between photochemistry and high pressure chemistry has been claimed by Drickamer (1967). He conducted an experiment showing 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 shock synthesis is some reaction of 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 the present study. Although the reaction of the ground states in shock synthesis cannot be ruled out, I propose here that a thermochemical reaction of the ground states dominates shock synthesis. This hypothesis is discussed below. Reaction mechanisms of shock syntheses Many kinds of structural isomers are detected in the shocked benzene (Table 2). The yield relations between these structural isomers are 3-MeBip > 4-MeBip = 2-MeBip (Figs. 6a, 7a), m-Ter > p-Ter > o-Ter (Figs. 6b, 7b), 2-MeNap > 1-MeNap (Figs. 6c, 7c), 2-PhNap > 1-PhNap (Figs. 6c, 7c), phenanthrene > anthracene (Figs. 5a, b) and fluoranthene > pyrene (Figs. 5a, b). The abbreviations, MeBip, Ter, MeNap, and PhNap stand for methylbiphenyl, terphenyl, methylnaphthalene and phenylnaphthalene, respectively. As described earlier, mutual molar yields between the structural isomers remain constant independent of projectile velocities. These results suggest that yield relations depend on the reaction mechanism triggered by shock wave, and on the degree of the steric hindrance for the structural isomers, but are independent of the energy given by projectile. In the following discussion, two production mechanisms of the compounds having the structural isomers are considered on the basis of the yield relationships. There are (1) the shock synthesis is a radical addition reaction, and (2) it is a concerted cycloaddition reaction controlled by the Woodward-Hoffmann rules (Hoffmann and Woodward, 1965, Woodward and Hoffmann, 1970). Toluene is assumed to be ormed by the radical reaction. Methylbiphenyl and terphenyl (1) Formation by a radical addition reaction. If we assume that a MeBip or Ter molecule is formed by the combination of a biphenyl molecule and a methyl or phenyl radical, respectively, and if we use the free valence and the localization energy as the reactivity index, the yield relations between structural isomers would be 2-MeBip > 4-MeBip > 3-MeBip and o-Ter > p-Ter > m-Ter, because these indices show that the ortho-position (2-position) of biphenyl is the most reactive. If we use the frontier electron density as the index, however, the yield relations would be 4-MeBip > 2-MeBip > 3-MeBip and p-Ter > o-Ter > m-Ter, because this index shows that the para position (4-position) of biphenyl is the most reactive. Both cases show that 3-MeBip in MeBip isomers and m-Ter in Ter isomers would be minor products. Another pathway through which MeBip is produced from phenyl radicals is also conceivable. If we assume that a MeBip molecule is formed by the addition of a toluene molecule with a phenyl radical, the yield relations between these isomers estimated from the three reactivity indices remain unchanged. The experimental yield relations between structural isomers, however, do not agree with these theoretical expectations. These arguments ignore the steric hindrance effect on isomer ratios. In general, the effect is maximum at the ortho position (2-position) and is enhanced as the attacking molecules become bigger. In this study, the molar yields of 2-MeBip and o-Ter are rather low in comparison with other isomers, and the ratio of 2-MeBip / 3-MeBip (av. 0.61) is higher than that of o-Ter / m-Ter (av. 0.40) (Figs. 6d, 7d and Table 3). Although the steric hindrance effect adequately accounts for the relation of para-isomers > ortho-isomers, other features of yield estimations are incompatible with the experimental result that 3-MeBip in MeBip isomers and m-Ter in Ter isomers are the most abundant among the isomers. Thus, the dominant reaction mechanism of polyphenyl compounds cannot be regarded as the simple radical reaction described above. (2) Formation by a concerted cycloaddition reaction. If the reaction mechanism is a concerted cycloaddition reaction, a biphenyl molecule would be produced by a thermal [4+2] cycloaddition (Diels-Alder reaction) of two benzene molecules, followed by isomerization and dehydrogenation. Some typical examples are shown in Fig. 8. If a MeBip or Ter molecule is formed by the [4+2] cycloaddition of a toluene or biphenyl molecule with a benzene molecule, respectively, followed by isomerization and by dehydrogenation (Fig. 8), the yield relations between structural isomers based on statistical consideration would be 3-MeBip > 2-MeBip > 4-MeBip and m-Ter > o-Ter > p-Ter. In this way, the predominant formation of 3-MeBip in MeBip and m-Ter in Ter is easily explained. Furthermore, the yields of 2-MeBip and o-Ter would be lower than expected because of the steric hindrance effect. Therefore the predicted yield relations would be 3-MeBip > 4-MeBip = 2-MeBip and m-Ter > p-Ter > o-Ter, in agreement with the shock synthesized products. Methylnaphthalene and phenylnaphthalene (1) Formation by a radical addition reaction. The presence of methyl and ethenyl groups in the products indicates that shock waves destroyed the structure of benzene and formed some lower-molecular weight radicals (e.g., methyl and ethenyl radical). If we assume that a MeNap or PhNap molecule is formed by attack of a naphthalene molecule by a methyl or a phenyl radical, respectively, the yield relations in isomers estimated from the reactivity indices would be 1-MeNap > 2-MeNap and 1-PhNap > 2-PhNap. These relative amounts are inconsistent with those of the shock products. Therefore, it is unreasonable to invoke a radical reaction for the synthesis of MeBip and PhNap. (2) Formation by a concerted cycloaddition reaction. If a MeNap or PhNap molecule is formed by the [4+2] cycloaddition of a toluene or biphenyl molecule with a benzene molecule, respectively followed by retro Diels-Alder reaction and dehydrogenation (Fig. 8), then the yield relations based on statistical consideration would be 2-MeNap > 1-MeNap and 2-PhNap > 1-PhNap, in agreement with the experimental results. The preferred mechanism of synthesis of MeNap and PhNap is the concerted cycloaddition as in the case of MeBip and Ter. Phenanthrene and anthracene (1) Formation by a radical addition reaction. When phenanthrene and anthracene are assumed to be formed by the reaction of naphthalene with a 1,3-butadienylene biradical, the estimated yield relations would be phenanthrene > anthracene; furthermore, a phenanthrene molecule could be formed by the addition of a biphenyl molecule with an acetylene molecule, but an anthracene molecule would not be formed through the same pathway. These expectations are in agreement with the experimental results. (2) Formation by a concerted cycloaddition reaction. If a phenanthrene molecule is formed by the [4+2] cycloaddition of biphenyl with two benzene molecules followed by retro Diels-Alder reaction and by dehydrogenation, or it is formed by the [4+2] cycloaddition of naphthalene with benzene followed by the retro Diels-Alder reaction and dehydrogenation, and further if an anthracene molecule is formed by the [4+2] cycloaddition of naphthalene with benzene followed by retro Diels-Alder reaction and by dehydrogenation, then the yield relation would be phenanthrene >> anthracene, because biphenyl is produced more abundantly than is naphthalene during shock synthesis. This statistical consideration fits the experimental results. Fluoranthene and pyrene The reaction mechanism of fluoranthene is probably different from that of pyrene, because fluoranthene has a pentagon in its carbon skeleton whereas pyrene has not. Therefore the dominant mechanism in the synthesis of fluoranthene and pyrene cannot be determined from the yield relation between them. It should be noted that in the experiments fluoranthene and pyrene are synthesized in the same amounts. Comments on the analysis of reaction mechanism From inspection of the reaction described above, we would conclude that the production mechanism of the compounds having structural isomers generally and rationally explained by the concerted cycloaddition reaction controlled by the Woodward-Hoffmann rules at this time. Some reports have argued that shock waves promote the synthesis of complicated compounds from simple ones (e.g., Warnes, 1970 and Nellis et al., 1984), but they did not examine the shock-synthesis mechanism on the basis of the concerted cycloaddition reaction controlled by the Woodward-Hoffmann rules. Thus, the formation of organic materials in nature has generally been accepted to rely upon radical reactions (e.g., Allamandola et al., 1989) and upon ion/molecule reactions (e.g., Bohme, 1992). I suggest that the concerted cycloaddition reaction controlled by the Woodward-Hoffmann rules is the mechanism by which the chemical reactions in shock synthesis occur in nature. The statistical argument for the concerted cycloaddition reaction controlled by the Woodward-Hoffmann rules, which is summarized in the reaction scheme of Fig. 8, seems to be oversimplified. The actual picture may not be as simple as depicted in Fig. 8. The scheme in Fig. 8 dose not take into account of the steroelectronic effects, which could influence the relative amounts of the three intermediates, and the rates of the nine decomposition reactions. More strict discussion will be available based on more reliable experiments in the future. IMPLICATION FOR COSMOCHEMICAL PROCESSES PAHs are recognized as cosmochemically important molecules, because they are detected in abundance in interstellar mediums (e.g., Allamandola et al., 1989), carbonaceous chondrites (e.g., Zenobi et al., 1989), and interplanetary dust particles (e.g., Clemett et al., 1993). Although FTT processes (e.g., Studier et al., 1972), the pyrolysis of hydrocarbons such as the polymerization of acetylene (e.g., Allamandola et al., 1989) and ion/molecule reactions (e.g., Bohme, 1992) have been accepted to be responsible for the PAH genesis, the shock process discussed in the present study can be nominated as a strong candidate for PAH formation. Most species of PAHs detected in meteorites and interplanetary dust particles were synthesized during the present experiment at low temperature (77K), which suggests that synthesis occurs in interstellar space; some molecules detected in interstellar environments such as pyrene and chrysene (Allamandola et al., 1989) were produced also by the present study (Table 2). With regard to these extraterrestrial PAHs, the cosmochemical significance of the shock reaction is discussed below. PAHs in carbonaceous chondrites Carbonaceous chondrites generally include many kinds of organics, which may have been abiotically synthesized and may record the early thermal history of the solar system. Predominant organic materials detected in carbonaceous chondrites are aromatic polymers; two-, three-, and four-ring PAHs such as naphthalene, phenanthrene, pyrene, and chrysene (e.g., Pering and Ponnamperuma, 1971). Moreover, carbonaceous chondrites include volatile aromatic hydrocarbons such as benzene (e.g., Studier et al., 1972). There is a difference between the shock synthesized PAHs and those found in carbonaceous chondrites. The former are dominated by polyphenyl compounds, whereas the latter are predominantly condensed ring compounds. However, many PAHs reported to be present in carbonaceous chondrites could be produced by the shock synthesis from benzene (Table 4). Major species of PAHs in carbonaceous chondrites such as naphthalene, biphenyl and phenanthrene were formed abundantly in this study (Table 2). Furthermore, the mutual ratios of structural isomers in the Murchison meteorite (Pering and Ponnamperuma, 1971), the Yamato-791198 meteorite (Naraoka et al., 1988) and the Yamato-74662 meteorite (Shimoyama et al., 1989) resemble those of the present shock products; in particular, the coincidence in the ratios of 2-MeNap / 1-MeNap and fluoranthene / pyrene is striking (Table 5). This implies a genetic connection between the shock products and the organic materials in carbonaceous chondrites. It has been believed that PAHs in carbonaceous chondrites are secondary materials formed by the mild aqueous and thermal alteration of the primitive materials (Shimoyama et al., 1989, Shock and Schulte, 1990). In these discussions, however, the importance of shock waves has not been noticed. Before carbonaceous chondrites arrive on the earth, the carbon-bearing materials in them may undergo shock events at least in the following three stages; the formation of parent bodies by accretion of interstellar medium particles, the break-up of the parent bodies by their mutual collisions, and the fall of meteorites on the Earth traversing the atmosphere. Through these shock events, primitive carbonaceous materials which had been present in interstellar medium particles would become more complex compounds and they would be detected in meteorites. Shock synthesis as well as the aqueous and thermal alteration may have promoted the secondary production of heavier and more complicated PAHs such as the insoluble polymers of multiple benzene rings detected in meteorites. PAHs in the atmospheres of Jovian planets and Titan Jovian planets and Titan are different essentially from our planet in compositions of their interiors and atmospheres; in particular, Jovian planets and Titan contain complex organic solids named tholins (Hanel et al., 1981). Sagan et al. (1993) detected PAHs in organic materials that were synthesized from simulated atmospheres of Jupiter and Titan, and they predicted the presence of PAHs in their atmospheres, although at present no PAHs are identified on Jovian planets and Titan. Jovian planets possess reduced atmospheres composed of H2, He, NH3, CH4, C2H6 and C2 H2 (Pollack and Yung, 1980). Furthermore, the Voyager 1 IRIS experiment indicated the presence of benzene on Jupiter (Kim et al., 1985). As to the genesis of these hydrocarbons in Jovian planets, three possibilities have been generally pointed out; FTT reactions (Prinn and Fegley, 1989), thermodynamic equilibrium (Prinn and Fegley, 1981) and photochemical reactions (Atreya et al., 1978). In contrast, Mckay et al. (1988) noted that Titan's atmosphere is composed mainly of N2, unlike other Jovian planets, and they suggested that shock waves during high velocity impacts at late stages of the accretion triggered the conversion of NH3 into N2. Sugisaki et al. (1994) argued the important contribution of shock waves to the genesis of planetary atmospheres. Warnes (1970) reported the production of high molecular weight substances from anthracene by shock waves. The present study as well as Warnes' results demonstrates that the shock wave is an effective accelerator in high polymer synthesis. In particular, the present study suggests that shock synthesis might proceed even at low temperatures in the vicinity of the Jovian planets. The yield of PAHs synthesized by shock waves exponentially increases with increasing projectile velocities (Figs. 3a,b). The shock energy occurring in nature must be great in comparison with that of the laboratory experiments. Several lines of evidence described above suggest that strong shock wave, which produced by the impact of comets and meteorites on the early Jovian planets and Titan, has affected the precursor PAHs and benzene in their atmospheres and regoliths, and have formed heavier PAHs. The shock-induced PAHs are expected to be present in atmospheres and tholins of Jovian planets and Titan by more careful investigation in the future. Interstellar medium Spectrum analyses indicate that PAHs, such as pyrene, chrysene, and coronene, are abundantly present in the interstellar medium. Carbon-rich red giants and supernovae are regarded as stellar contributors of carbonaceous materials to the interstellar medium. The relative importance of the two sources in terms of carbonaceous material formation is not well known, although carbon-rich giants are thought to dominate the production. Most carbonaceous materials (e.g., acetylene) in the outflow from carbon-rich giants are converted into PAHs by the gas-phase pyrolysis of hydrocarbons through some chemical pathways (Frenklach et al.,1989). PAHs must be present in grains including silicates, ice and carbonaceous material (Mathis, 1988), and/or in SiC grains that occur within molecular envelopes of carbon-rich red-giant stars (Frenklach et al., 1989). Benzene cannot be detected in interstellar spaces owing to the lack of an electric dipole moment. It is, however, possible that benzene is present in the interstellar medium as an intermediate during the formation of PAHs by the pyrolysis of hydrocarbons. When the grains including PAHs and solid benzene of the interstellar medium condensed and constituted proto planetary nebulae of dense clouds, these PAHs must have experienced continual shock waves produced by adiabatic compression of the nebulae. Shock waves in cosmochemical environments generate vast amount of shock energy, which is involved in synthesis of heavier and abundant PAHs from precursor PAHs and benzene, although the pyrolysis of hydrocarbons may simultaneously occur. CONCLUDING REMARKS The experimental results and the Woodward-Hoffmann rules suggest that the shocksynthesis of PAHs from benzene involves the concerted cycloaddition reaction. Thecomparison between PAHs in carbonaceous chondrites and those of shock-induced PAHs,and the result of the experiments in a low temperature environment simulating interstellarspace suggest that shock synthesis plays an important role in chemical reactions ofastrophysical processes.Many PAHs detected in carbonaceous chondrites such as naphthalene, biphenyl,phenanthrene and chrysene were synthesized by shock in this study. It is generally acceptedthat carbonaceous chondrites contain complicated organic materials such asalkyl-substituted PAHs (Zenobi et al., 1989), aromatic polymers with functional groups suchas COOH, OH and CO (Hayatsu et al., 1977) and amino acids; these compounds were notsynthesized in this study owing to the lack of the essential elements (nitrogen and oxygen)in the reactant. If other organic compounds detected in meteorites are synthesized by shockfrom a mixture ( low molecular weight carboxylic acids, alcohols, aldehydes, aliphatic andaromatic hydrocarbons), the cosmochemical significance of shock synthesis would beenhanced; such a mixture may be present in interstellar mediums and may have beenpresent in meteorite parent bodies. This expectation may be investigated in the future.Mimura (1993) and Sugisaki and Mimura (1994) reported recently that unalteredmantle-derived rocks such as mantle xenoliths and tectonized peridotites commonly includehigh-molecular weight hydrocarbons, and they pointed out the possibility that the mantlehydrocarbons originated from abiotic synthesis of primordial hydrocarbons in carbonaceouschondrites. If this is true, some abiotic synthesis may have been triggered by shock. Chybaand Sagan (1992) emphasized the role of shocks in the genesis of primordial organics. Itseems, therefore, that shock waves are promising practical means of abiotic synthesis of primordial organic materials on the early Earth. Further study in this field may providemore representative information of cosmochemical problems including the genesis of abioticorganic materials in the Earth.Acknowledgements---. I thank the members of Department of Earth and Planetary Sciencesat Nagoya University. I especially wish to express my gratitude to Ryuichi Sugisaki in ourDepartment, who continually gave me helpful instructions and criticism.I would like to thank Nobuhiko Handa at Nagoya University for providing analyticalfacilities and useful suggestions, and Manabu Kato in our Department for providing thevertical powder gun and constructive suggestions. 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