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

Geochimica et Cosmochimica Acta, Vol. 59, No. 3, pp. 579-591, 1995 Copyright © 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .00

Pergamon

0016-7037(95)00326-2

Synthesis of polycyclic aromatic hydrocarbons from benzene by impact shock: Its reaction mechanism and cosmochemicai significance KOICHI MIMURA Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-01, Japan ( Received April 19, 1994; accepted in revised form August 25, 1994)

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 shockinduced 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

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 C-beating 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 ( F I T ) 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 describe in detail the experiments under various conditions of shock energy and

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, USA. Lipschutz and Anders (1961) and Lipschutz (1964) suggested that diamonds detected in ureilites and the Canyon Diablo meteorite were formed by shock during breakup of their parent bodies. McKay et al. (1988) reported the importance of strong impact-induced shock 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 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 the 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 (CI-L + NH3 + H20) 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 579

580

K. Mimura

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. la). It contained 6 mL 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, lb). When an aluminum projectile of 4.6 g from a vertical powder gun struck 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 and the products lost. In the room temperature experiments, the pressure and the ratio of V (the specific volume behind the shock front) against V, (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 25°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 25 m × 0.25 mm fused silica capillary column coated with a 0.3 #m layer of SE-52 ( 5% phenyl, 95% methyl polysiloxane). The column temperature was programmed from 100 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 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 the following section.

(a)

T=290K

(b)

T=77K

Projectile Styrofoambox Benzene

R

Container

tcm

FIG. 1, Cross section view of experimental apparatuses used for room temperature experiment (a), and for low-temperature experiment (b),

T a b l e 1. The e s t i a a t e d benzene. Projectile

velocity(=/s)

physical

c o n d i t i o n s of shocked

Pressure

Relative

(xl0' pascal)

volume(V/Vo)*

100 373 450

0 . 3 8 ± 0.02 1.55±0.09 1.92±0.11

574

2.84±0.14

841 708

2.89±0.15 3.24±0.17

0 . 9 8 2 ± 0.001 0.937±0.004 0,926±0,005 0.908±0,005 0.899±0,006 0.891±0,008

712 755

3 . 2 7 ± 0.17 3.51±0.18

0 . 8 9 0 ± 0.006 0.884±0.007

757

3.52± 0.18

0.884±0.007

783

3.67±0.19

0.881±0.007

814

3.85±0.19

0.877±0.007

896 899 922

4.33 ± 0.21 4 . 3 5 ± 0.22 4.49 ± O. 22

932

4.55±0.22

992 1000

4.92±0.23 4 . 9 7 ± 0.24

0 . 8 6 7 ± 0.007 0 . 8 6 7 ± 0.007 O. 864± 0.007 0 , 8 8 3 ± 0.007 0.858±0.008 0 . 8 5 6 ± 0.008

1020

5.10±0.24

0 . 8 5 3 ± 0.008

1050

5.29±0.25

0.850±0,008

1080 1140

5 . 4 8 ± 0.26 5.87±0.27

0 . 8 4 7 ± 0.008 0.840±0.008

* ; The a b b r e v i a t i o n s

of V and Vo s t a n d f o r the s p e c i f i c

volume behind the shock front and the specific voluue ahead of the shock front, r e s p e c t i v e l y . RESULTS R o o m T e m p e r a t u r e E x p e r i m e n t s ( 2 9 0 K, B e n z e n e 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 and contained soot-like materials. On the representative gas chromatographic record for the products (Fig. 2a), thirty-one peaks were identified by retention times a n d / o r fragmentation patterns. The shock on benzene yielded two-, three-, and 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 he 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 m o l / m o l ) . The ratios of various

Formation of PAH by shock waves in space

581

(a) 468

26 27

19

290K

T =

(projectile velocity=992m/s)

14

15 '0 21 1

'<, 25

28

29

L i

i

i

=

lo

15

20

25

3O

i

i

i

J

35

40

45

50

55 ( min I

(b) 689

26 27

T

19

77K

=

(projectile velocity= lO00mls)

14

11

I° I,..

J

5

10

15

20

25

30

35

40

i

45

50

55 ( min )

FIG. 2. Gas chromatogram of products from shocked benzene at room temperature (a), and at low temperature (b). Peak numbers correspond to those in Table 2. products vs. chrysene are constant throughout these experiments, and the representative ratios (biphenyl/chrysene, mterphenyl/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.

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).

Low-Temperature Experiment (77K, Benzene in Solid Form)

Ground States or Excited States?

Many species of PAHs were synthesized also from solid benzene. The results of low-temperature experiments were

CHEMISTRY

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 con-

582

K. Mimura

Table 2, Yields of products (n tool/tool ) at various projectile velocities in room and low temperature experiments.

Room temperature (290K)

Projectile velocity (m/s)

Peak No.*

Molecular

100

373

450

574

574

641

706

712

755

757

783

814

896

899

Compound

weight 1 2 3 4 5 6 7 8 9 10 11 12, 13 14 15 16 17 18

128 142 142 154 168 168 152 168 168 180 166 180 180 178 178 204

20,21

192

22 19 23 24 25 26 27 28 29,30,31

230 Z04 202 202 230 230 228 306

Napthalene 2-Methylnaphthalene 1-Methylnaphthalene Biphenyl 2-1dethylbiphenyl Diphenylmethane Acenaphtylene 3-Methylbiphenyl 4-Methylbiphenyl 1,1 -Diphenylethane Fluorene Ethenylbiphenyl trans~Stilbene F'nenanthrene Anthracene 1-Phenylnaphthalene Methylp/nenanthrene or Me~yla~thracene o-Terphenyl 2-Phenylnaphthalene Fluoranthene Pyrene m-Terphenyl p-Terphenyl Chrysene Quaterphenyl

<0.1 6.88 96.50 56.10 77.70 31,90 84.90 135,00 36.20 35.70 145,00 173.00 108.00 39.90 <0.1 2.05 12.70 9,79 17.40 8.55 12.70 23.90 13.20 10.00 38.40 23.30 14.00 14.30 < 0.1 1.43 10.30 8.41 15.50 7.07 10.90 23.20 10.70 9.37 34.60 19.30 12.30 12.70 <0.1 176.00 408.00 334.00 478.00 382.00 462.00 432.00 654.00 315.00 1040.00 605.00 457.00 590.00 < 0.1 1.28 5.94 5.39 6.02 6.70 7.42 5.17 23.50 11.10 43.60 24.20 6.72 22.40
n,d.

n.d.

n. cl.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

<0.1 <0.1 <0.1 < 0.1 < 0.1 <0.1 < 0.1 n.d,

2.54 1.64 0.59 0.53 7,58 3,82 0,50 n.d.

4.80 5.34 1.66 1.31 14.30 8.16 1.28 n.d.

4.43 5.26 1.62 1.36 11.80 7.39 1.05 n,d.

8.03 9.41 3.24 2.60 22.60 10.20 2.29 n.d.

4.51 5.46 2.10 1.78 14.60 7.51 1.38 n.d.

5.49 6.21 2.05 1.64 15.40 9.51 1.43 n.d.

7.39 7.39 3.50 2.89 13.10 9.12 1.70 n,d.

9,89 8.40 2.33 1.96 27.00 14.50 2.11 n.d.

5.Z5 6,49 2.51 2.02 11.40 8.74 1.50 n.d.

20.50 21.80 8.12 6.76 46.40 36.10 5.95 n.d.

11.40 14.20 5.29 4.66 24.50 19.50 3.09 n,d.

4.99 5.53 2.10 1.84 14.10 8.86 1.23 n.d.

11.50 18.00 5.73 5.51 32.50 19,40 6.28 n.d.

* ; Peak numbers corespond to those in Fig. Z. n, d. ; not determined

sider 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 correlates 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, 290 K) and for solid benzene (at low temperature, 77 K) suggest that shock synthesis proceeds thermochemicalty in high-temperature conditions irrespective of the temperature difference of 20OK. 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 ( i 967). He conducted an experiment showing that high-pres-

sure conditions promoted the formation of pentacene dimers with a 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 excited 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 > l-MeNap (Figs. 6c, 7c), 2-PhNap > 1-PbNap (Figs. 6c, 7c ), phenanthrene > anthracene (Fig. 5a,b), and fluoranthene > pyrene (Fig. 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 re-

Formation of PAH by shock waves in space

583

Table 2. ConlJnued.

Low temperature(77K)

Room temperature (290K)

Projeclflle velocity (m/s)

Peak NO,*

Molecular

922

932

992

1000

1020

1050

1080

1140

600

800

1000

1200

Compound

we~m 1 2 3 4

5 6 7 8 9 10 11 12, 13 14 15 16 17 18 20,21 22 19 23 24 25 26 27 28 29, 30, 31

128 142 142 154 168 168 152 168 168 180 166 180 180 178 178 204 192 230 204 202 202 230 230 228 306

Napthlllene 2-Methylnapbthatene l-Methylnaphthalene Biphenyl

Z-Me~y~ Diphenylme~ne Acemmmylene 3-MeU-~t~he~ 4 - - ~ y l 1,1-Olfdlnyfethane Fluurene Ethenylbiphenyl trans-Stllbene ~tNene Anthracene 1-Phenyinal~thalene Methylpil~anthrene Or Methylanmracene o-Terphenyt 2-Phenyteaphff~aler~e Fkxxanthene Pyrene m-Terphenyl p-Terphenyl Chrysene Q u a ~

111.00 136.00 68.20 74.10 133.00 49.40 58.50 113,00 22.70 23.80 35.10 26.20 30.40 20.70 20.20 35.90 21.50 1 5 . 8 0 22.10 17~70 23,80 15.30 13.80 34.20 573.00 652.00 1390.00 805.00 1530.00 1480.00 1070.00 1320.00 6.68 24.50 48.80 32.30 45.50 54.20 31.60 41.50 9.85 24.80 4 S . 2 0 32.00 43.00 52.90 28.00 46.80 5.16 12.10 7.79 1 2 . 0 0 1 1 . 9 0 17.00 9.91 17.50 13.00 39A0 59.30 40.30 74.80 77.20 39.00 73.40 7.10 20.20 34.90 23.40 3 7 . 6 0 42.30 22.10 40.10 13.20 9.60 19.80 1 7 . 7 0 1 3 . 8 0 27.50 21.50 23.60 15.60 22.10 25.70 20.70 24.80 45.70 21.00 33.50 n.d. n.d. n.d. n.d. rl.d. n.d. n.d. n.d. 25.30 26.00 32.80 33.80 31.70 5 8 . 6 0 47.40 54.70 16.60 24.30 31.80 25.60 28.60 36.20 23.90 33.40 2.18 3.10 4.64 3.30 3.57 4.66 3.22 4.25 5.26 7.46 10.40 8.26 1 0 . 7 0 12.00 7.46 18.40 n.d.

n.d.

n.d.

n.d.

n.d.

9.96 9.12 3.77 3.02 19.80 14.60 2.54 n.d.

13.20 13.10 5.19 4.88 32.60 22.60 3.91 n.d.

30.60 17.50 6.73 6.38 79.60 53.60 8.88 n.d.

16.80 14.30 5.68 5.13 38.30 27+40 4.61 n.d.

25.60 18.80 5.31 4.04 74.50 45.60 5.48 n.d.

n.d. 28.30 20.30 8.12 7.24 66.20 46.90 8.16 n.d.

n.d. 14.60 14.20 4.40 3.84 42.10 25.10 3.51 n.d.

n.d. 27.30 20.90 6.96 5.88 62.60 44.20 6.80 n.d.

10.10 20.10 295.00 211.00 5.89 1 0 . 9 0 61.10 86.70 3.14 6.28 53.80 76.90 322.00 750.00 2370.00 2670.00 5.45 2 4 . S 0 90.00 120.00 5.82 19.30 84.10 105.00 0.84 2.94 1 5 . 7 0 22.30 9.50 27.10 147.00 148.00 5.87 1 8 . 0 0 92.30 102.00 3.47 8.20 33.70 32.50 7.04 9.08 50.70 80.60 n.d. n.d. n.d. n.d. 9.68 1 8 . 1 0 62.20 82.90 8.30 1 3 . 3 0 48.10 69.70 1.14 1.83 6.76 12.30 4.97 6.82 22.50 29.30 n.d.

n.d.

n.d.

n.d.

7,64 1 3 , 5 0 46,10 71.30 5.25 1 2 . 2 0 32.20 57.40 2.10 3.41 1 0 . 2 0 15.00 2.05 2.66 9.87 14.10 15.20 34.00 115.00 180.00 8.48 1 9 . 7 0 66.20 112.00 1.85 4.40 1 1 . 1 0 16.70 n.d. n.d. n.d. ll.d.

* ; Peak numbers c o ~ t x ) ~ to those in Fig. 2. n.d.; not determined

action mechanism triggered by the shock wave, and on the degree of the steric hindrance for the structural isomers, but are independent of the energy given by the projectile. In the following discussion, two production mechanisms of the compounds having the structural isomers are considered on the basis of the yield relationships. ( 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 formed 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 > 3MeBip 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 ra-Ter in Ter isomers would be minor products. Another pathway through which MeBip is produced from phenyl rad-

icals 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 orthoposition (2-position) and is enhanced as the attacking molecules become bigger. In this study, the molar yields of 2MeBip 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 paraisomers > 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, re-

584

K. Mimura 1

I

(a) 0

E

I

I

103

0

E t-" -o

101 ..... [3.... 171ET"

0

[]

[]

10-1

I

I

I

I

400 soo 8oo lOOO Projectile velocity (m/s) 1

I

I

1200

enyl 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 l-MeNap > 2MeNap and l-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 Ph-Nap molecule is formed by the [4 + 2] cycloaddition of a toluene or biphenyl molecule with a benzene molecule, respectively, followed by retro Diels-Aider reaction and dehydrogenation (Fig. 8 ), then the yield relations based on statistical consideration would be 2-MeNap > l-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.

I

(b)

103

I

m

O

E

I

QI

0

0

° m

O

E

>.,

I

0 0

o

(a) o

%

10 2

"o ° ~

I

10 3

--"E"

J

A J/X

0

._.N.

--[D

101

101 0 c--

O

&

¢.-

10 0 10 "1

I 600

I

1

t

800

1000

1200

o

[]

tO 10-1

Projectile velocity (m/s)

I 400

FIG. 3. Molar yields (n tool of products/initial mol of benzene as reactant) of representative products at room temperature (a), and at low temperature (b). Symbols of circle, triangle and square represent biphenyl, trans-stilbene and pyrene, respectively. The regression lines were drawn by the least square method.

I

1

1

600

800

1000

1200

Projectile velocity (m/s) 103

I

1

1

I

(b)

-o

spectively, followed by isomerization and by dehydrogenation (Fig. 8), the yield relations between structural isomers based on statistical consideration would be 3-MeBip > 2MeBip > 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 > pTer > 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 eth-

0

0

0

102

O

N

E O c-

101

A

A

¢-.

E',

10 °

n

1 0 "t

I

[]

D

D

I

I

I

tO

600 800 1000 1200 Projectile velocity (m/s) FIG. 4. Ratios of representative products against chrysene at room temperature (a), and at low temperature (b). Symbols of circle, triangle and square represent biphenyl/chrysene, m-terphenyl/chrysene and fluoranthene/chrysene, respectively.

Formation of PAH by shock waves in space

585

Table 3. Mutual ratios or structural Isomers at various projectile velocities In room and low temperature experlmems.

Room temperature (290K)

ProjecUlevelocity (m/s)

373

450

574

574

641

706

712

755

757

783

814

896

899

1.21 2.32 1.21 0.43 8.31 1.65 1.18 3.25 1.67 0.31

1.16 2.30 1.26 0.44 8.00 1.83 1.25 2.80 1.73 0.36

1.03 2.26 1.28 0.44 7.14 1.51 1.21 1.77 1.23 0.56

1.23 1.28 0.65 0.78 8.83 1.50 1.19 2.73 1.46 0.37

1.07 1.78 1.01 0.56 8.47 1.84 1.24 2.16 1.66 0.46

1.11 1.52 0.80 0.66 7.72 1.86 1,20 2.26 1.76 0.44

1.21 1.43 0.75 0.70 7.68 1.83 1.14 2.15 1.70 0.47

1.14 2.18 1.19 0.46 7.96 1.66 1.14 2.82 1.78 0.35

1.12 1.47 0.86 0.68 7.80 1.92 1.04 2.83 1.69 0.35

Ratio 2-MeNap/1-MeNap 3JdeBip/2-MeBip 4-MeBip/2-MeBip 2-MeSip/3-MeBip Phenanthrene/Anthracene 2-PhNap/1-PhNap Fluoranthene/Pyrene rrPTer/o-Ter p-Ter/o-Ter o-Ter/m-Ter

1.44 1 . 2 4 1 . 1 6 1 . 1 2 2.32 2 . 0 7 2.17 1 . 2 8 1.44 1 . 0 7 1 . 1 2 0 . 6 3 0.43 0.48 0.46 0.78 7.11 12.00 7 . 4 6 9 . 4 8 1.55 1 . 6 3 1.71 1 . 4 3 1.11 1.27 1 . 1 9 1 . 2 5 2.98 2.97 2 . 6 7 2 . 8 2 1.50 1 . 7 0 1 . 6 7 1 . 2 6 0.34 0 . 3 4 0.38 0 . 3 6

Room temperature (290K)

Projectile velocity (m/s)

922

932

992

1.05 1.95 1.06 0.51 7.60 1.74 1.25 1.99 1.47 O.SO

1.51 1.61 0.83 0.62 7.86 1.76 1.06 2.47 1.71 0.41

1.59 1.22 0.72 0.82 6.85 1.68 1.05 2.60 1.75 0.38

Low temperature (77K)

1000 1020 1050 1080 1140

600

800

1000 1200

Ratio 2-MeNap/1-MeNap 3-MeBip/2-MeBip 4-MeBip/Z-MeBip 2-MeBIp/3-MeBip Pheflanthrene/Anthracene 2-PhNap/1-PhNap Fluoranthene/l~rene m-Ter/o-Ter p-Ter/o-Ter o-Ter/m-Ter

1.48 1.25 0.73 0.80 7.76 1.73 1.11 2.28 1.63 0.44

1.28 1.64 0.82 0.61 8.00 1.75 1.31 2.91 1.78 0.34

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 pen-

1.35 1.42 0.78 0.70 7.76 1.69 1.12 2.34 1.66 0.43

1.46 1.23 0.70 0.81 7.42 1.90 1.15 2.87 1.71 0.35

1.05 1.77 0.97 0.57 7.85 1.14 1.18 2.29 1.62 0.44

1.87 1 . 7 4 1 . 1 4 1.74 1.11 1 . 6 3 1.08 0 . 7 4 1 . 0 3 0.81 0 . 6 1 0.90 7.30 7 . 2 4 7 . 1 3 1.06 1 . 7 9 1 . 4 3 1.03 1 . 2 8 1 . 0 4 1.98 2 . 5 2 2.49 1.11 1 . 4 6 1 . 4 3 0.40 0.40 0.40

1.13 1.23 0.85 0.57 5.68 1.96 1.06 2.53 1.57 O.SO

tagon 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 is 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; NeUis 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

586

K. Mimura 10 2

I

I

!

1

(a) ._(2 101

100

10

"1

0

o

zx zx

I

8 o ~cbo ~ ~ c o o

~ zx & z ~

I

~x~zx

I

P A H s in C a r b o n a c e o u s

I

400 6oo Boo lOOO Projectile velocity (m/s) 102

I

I

I

0

0

1200

I

(b)

.o 101 4..,,

0

-t

A

10 °

10-1

I

600

O

A

A

I

I

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800

1000

1200

Projectilevelocity(m/s) FIG. 5. Mutual ratios of structural isomers in shocked products at room temperature (a), and at low temperature (b). Symbols of circle and triangle represent phenanthrene/anthraceneand fluoranthene/pyrene, respectively. 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 does 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 C O S M O C H E M I C A L

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 a low temperature (77 K), 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.

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.,

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 cbondrites, such as naphthalene, bipbenyl, 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 2MeNap/l-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 Cbearing 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.

Formation of PAH by shock waves in space 101

I

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(a)

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101

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°

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10 0

10-1

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I

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~AA

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600

800

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(b)

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_

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400

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800

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Projectile velocity (m/s)

I~G. 6. Mutual ratios of structural isomers in shocked products at room temperature. (a) Symbols of circle and triangle represent 3-MeBip/2-MeBip and 4-MeBip/2-MeBip, respectively. (b) Symbols of circle and triangle represent m-Ted o-Ter and p-Terlo-Ter, respectively. (c) Symbols of circle and triangle represent 2-MeNap/1-MeNap and 2-PhNap/lPhNap, respectively. (d) Symbols of circle and triangle represent 2-MeBip/3-MeBip and o-Terlm-Ter, respectively.

PAils 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 at. ( 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, NI-I3, CH4, C2I-I6, and C2H2 (Pollack and Yung, 1980). Furthermore, the Voyager 1 IRIS experiment indicated the presence of benzene on Jupiter (Kim et at., 1985). As to the genesis of these hydrocarbons in Jovian planets, three possibilities have been generally pointed out; F T r 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 the results of Warnes (1970) 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 (Fig. 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 waves, which are produced by the impact of comets and meteorites on the early Jovian planets and

588

K. Mimura 101

i

i

1

101

i

I

(a)

.9

0 10 °

&

._o

0 A

I

I

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600

800

1000

1200

I

1

0 10 °

A

10 °

zx

10 1

I 600

0

0

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1

800

1000

1200

-1

Projectile velocity (m/s)

I

101

I

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(d) 0

0

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A

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A

.9 =m "~ .-!

.--i

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A 6

(b)

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Projectile velocity (m/s) 101

I

0

0 A

-t

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I

(c)

t

I

I

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600

800

1 000

1200

10 °

o

O o

10-1

A

A

A

I

I

I

800

1000

1200

I

600

Projectile velocity (m/s)

Projectile velocity (m/s)

FIG. 7. Mutual ratios of structural isomers in shocked products at low temperature. (a) Symbols of circle and triangle represent 3-MeBip/2-MeBip and 4-MeBip/2-MeBip, respectively. (b) Symbols of circle and triangle represent m-Ter/ o-Ter and p-Terlo-Ter, respectively. (c) Symbols of circle and triangle represent 2-MeNap/l-MeNap and 2-PhNap/lPhNap, respectively. (d) Symbols of circle and triangle represent 2-MeBip/3-MeBip and o-Terlm-Ter, respectively.

Titan, have affected the precursor PAHs and benzene in their atmospheres and regoliths, and have formed heavier PAHs. The shock-induced PAHs are expected to be discovered in the atmosphere and tholins of Jovian planets and Titan by more careful investigation in the future.

X

x..

~

I

x(;~,;'~,p)

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 C-rich giants are thought to dominate the production. Most carbonaceous materials (e.g., acetylene) in the outflow from C-rich giants are converted into PAHs by the gas-phase pyrolysis of hydrocarbons through some chem-

(~l~ ')

,-~°' X

H~

. .

~

(IMo%.;) X

X =

-{3

~

HC---CH

--CH 3 ,

2 Pl~lap

+

H2

FIG. 8. Possible concerted cycloaddition reactions for forming Ter, MeBip, PhNap, and MeNap.

Formation of PAH by shock waves in space

589

Table 4. Comparison of PARs in shocked benzene and in m e t e o r i t e s . Peak

Compound

Detected in

No.$

leteorites(Ref.)t#

1 2 3

Naphthalene 2-Methylnaphthalene 1-Methylnaphthalene

yes(l.2.4,5) yes(1.2,4.5) yes(].2,4,5)

4

Biphenyl

yes(i,2,4.5)

5

2-Methylbiphenyl

6

Diphenylnethane

7 8

Acenaphtylene 3-Methylbiphenyl

yes(2,4,5) yes(1,2) yes(4) yes(2.4,5)

9 l0 11 12,13 14

4-Methylbiphenyl 1,1-Oipbenylethylene

19 23 24

Fluorene Ethenylbipbenyl trans-Stilbene Phenanthrene Anthracene 1-Phenylnaphthalene Methylphenanthrene or Methylanthracene o-Terphenyl 2-Phenylnaphthalene Fluoranthene

25 26 27 28 29.30,31

Pyrene m-Terphenyl p-Terphenyl Chrysene Quaterphenyl

15

19 17 18 20,21 22

yes(2,4.5) no yes(I,4,5)

so no yes(l,3,4,5) yes(l.3,4.5) no yes(l,3,4,5)

no no yes(l.3.4.5) ¥es(1,3,4,5) no no yes(3) no

t ; Peak nunbers correspond to those in Fig. 2. * * ; Ref. : (1) Pering and Ponnamperuaa (1971), (2) $ t u d i e r et al. (3) Basile et al. (1984), (4) Naraoka ~t al. (1988) (5) Shimoyama et al. (]989)

ical 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 C-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 protoplanetary 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.

(1972)

CONCLUSION The experimental results and the Woodward-Hoffmann rules suggest that the shock synthesis of PAHs from benzene involves the concerted cycloaddition reaction. The comparison between PAHs in carbonaceous chondrites and those of shock-induced PAHs, and the result of the experiments in a low temperature environment simulating interstellar space, suggest that shock synthesis plays an important role in chemical reactions of astrophysical processes. Many PAHs detected in carbonaceous chondrites, such as naphthalene, biphenyl, phenanthrene, and chrysene, were synthesized by shock in this study. It is generally accepted that carbonaceous chondrites contain complicated organic materials, such as alkyl-substituted PAHs (Zenobi et al., 1989), aromatic polymers with functional groups such as COOH, OH, and CO (Hayatsu et al., 1977), and amino acids; these

Table 5. Ratios of 2-MeNap/1-MeNap, phenanthrene/antheracene and fluoranthene/pyrene from the carbonaceous chondrites and shocked benzene.

Ratio 2-MeNap/1-MeNap Phenanthrene/knthracene Fluoranthene/Pyrene

Murcbison(l)* Murchison(II)* Yaato-791198** Yaato--74962*** Shocked benzene**** 1. O0 51.2 1.12

l. 60 48.0 l. O0

1.50 13. l 1.09

1.03 8.80 1.14

* ; Pering and PonnaperuBa(1971), ** ; Naraoka et al. (1988). *** ; Shl~yama et al. (1989) **** ; The average values in shocked benzene.

1.26 7.85 1.16

590

K. Mimura

compounds were not synthesized 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 shock from a mixture (low molecularweight carboxylic acids, alcohols, aldehydes, and aliphatic and aromatic hydrocarbons), the cosmochemical significance of shock synthesis would be enhanced; such a mixture may be present in interstellar mediums and may have been present in meteorite parent bodies. This expectation may be investigated in the future. Mimura ( 1 9 9 3 ) and Sugisaki and Mimura ( 1 9 9 4 ) reported recently that unaltered mantle-derived rocks such as mantle xenoliths and tectonized peridotites commonly include highmolecular weight hydrocarbons, and they pointed out the possibility that the mantle hydrocarbons originated from abiotic synthesis of primordial hydrocarbons in carbonaceous chondrites. If this is true, some abiotic synthesis may have been triggered by shock. Chyba and Sagan ( 1992 ) emphasized the role of shocks in the genesis of primordial organics. It seems, 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 provide more representative information of cosmochemical problems including the genesis of abiotic organic materials in the Earth. Acknowledgments--1 thank the members of Department of Earth and Planetary Sciences at Nagoya University. I especially wish to express my gratitude to Ryuichi Sugisaki in our Department, who continually gave me helpful instructions and criticism. I would like to thank Nobuhiko Handa at Nagoya University for providing analytical facilities and useful suggestions, and Manabu Kato in our Department for providing the vertical powder gun and constructive suggestions. 1 also thank Mamoru Ohashi at University of Electro-Communications for critically reading my manuscript and giving useful comments in terms of organic chemistry, and Syoji Eguchi at Nagoya University for instructive discussions on organic chemistry. 1 am indebted to Gunzo Takamatsu, Tadashi Masuda, Syuzo Ishikawa, Kazuji Suzuki, Tatsuharu Torii, Chiyomi Miwa, and Masami Hamajo for their technical assistance. M, D. Schulte and three anonymous reviewers provided constructive reviews. Editorial handling: C. Koeberl

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