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14

Preferred Annellated Structures of Polycyclic Aromatic Compounds in Coal-Derived Materials

Masaharu Nishioka and Milton L . Lee 1

Department of Chemistry, Brigham Young University, Provo, UT 84602

The structures and relative abundances of polycyclic aromatic compounds (both hydrocarbons and heterocycles) in a solvent-refined coal liquid and in a coal tar were compiled and compared. These structures and relative abundances were determined by detailed analyses performed in our laboratory during the past 7 years. The purpose of this comparison was to determine (1) if preferred aromatic structural features exist in the complex mixture of compounds present in a single coal-derived material and (2) to what extent these preferred structures are evident in different coal-derived materials produced from different feedstocks and under different conditions. Although different feedstocks and process conditions were associated with each of the two coal-derived products studied, remarkably similar structural trends could be seen. If one disregards the structures of the compounds produced by mild autocatalytic hydrogenation in the solvent-refined coal-liquefaction process, the structures of the remaining polycyclic aromatic compounds in both samples are similar. These results suggest that the major compounds identified are either representative of similar aromatic moieties in the original coal feedstock or are a result of processing conditions involving complex reactions that lead to similar stable final products. Because many similar aromatic moieties are found in coal in comparison with coal-derived materials, many of the same complex reactions may occur during diagenesis. Such reactions may include cyclo-coupling dehydrogen-ation.

1To whom correspondence should be sent.

0065-2393/88/0217-0235$06.00/0 © 1988 American Chemical Society

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In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

236 POLYNUCLEAR AROMATIC COMPOUNDS

OOAL-DERIVED MATERIALS such as coal liquids and coal tars are highly aromatic, and these materials contain polycyclic aromatic compounds (PACs) as major components. Although average descriptive parameters such as molecular weight range, aromaticity, and abundances of functional groups are usually obtained to characterize such materials, detailed chemical analysis is also important for properly assessing health risks due to exposure to such materials and for understanding fundamental chemical reactions involved in upgrading technologies such as coal gasification and liquefaction (1, 2). In addition, detailed identification of constituents in coal-derived products could provide important information relevant to coal structure.

Because coal-derived liquids and tars are usually complex mixtures of organic chemicals, the separation and identification of individual PACs in these samples have been best accomplished by using open-tubular-column (capillary-column) gas chromatography (GC) (3, 4). Selective detectors for nitrogen and sulfur, as well as mass spectrometry (MS), can be easily combined with G C . Especially when authentic standard reference compounds and selective stationary phases (5-10) are used, capillary-column G C is, by far, the easiest and most reliable technique available.

During the past 7 years, we have developed and applied new methodologies using capillary-column G C for the separation and identification of PACs in coal-derived liquids. The primary samples that were used throughout these studies included a solvent-refined coal (SRC II) heavy distillate and a coal tar. Details of the isolation and identification of polynuclear aromatic hydrocarbons (PAHs) (11), sulfur heterocycles (12, 13), nitrogen het-erocycles (14,15), amino- substituted PACs (16-18), and hydroxy-substituted PACs (14, 19, 20) in an SRC II heavy distillate have already been published (11, 13, 15, 18, 21).

In the study reported here, the abundances and structural characteristics of the numerous PACs identified in the SRC II heavy distillate and the coal tar were compiled and compared. The purpose of this comparison was to determine (1) if preferred aromatic structural features exist in the complex mixture of compounds present in a single coal-derived material and (2) to what extent these preferred structures are evident in different coal-derived materials produced from different feedstocks and under different conditions. Explanations for differences in the compositions and relative abundances of PACs in these materials are proposed.

Experimental Details

The solvent-refined coal heavy distillate (SRC II H D , bp 260-450 °C) that was analyzed and used in this study was collected during the processing of a West Virginia coal from the Pittsburgh seam. Data from the analysis of a solvent-refined coal vacuum residue (SRC II VR) from the same process was also used. A coal tar was obtained from the National Bureau of Standards. This tar was a medium crude coke-oven tar

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14. NISHIOKA & L E E Annellated Structures in Coal-Derived Materials 237

from an unknown coal feedstock. The samples were approximately 2 years old and were stored in a freezer at -10 °C. Elemental analyses of the SRC II H D and the coal tar are given in the following table (values are weight percents):

Element SRC II HD Coal Tar

C 89.5 89.4 H 7.0 4.9 Ν 1.6 1.2 S trace 0.8

Most of the compounds reported in this chapter were positively identified by comparison with standard reference compounds by using capillary GC and G C - M S . Standard compounds were obtained commercially or synthesized in one of our laboratories. The detailed identification procedures and results are reported elsewhere (11-21). The abbreviations for various PACs used in this chapter are given in the following list according to the format defined by Bartle et al. (21).

Abbreviations of Compound Classes Discussed in This Chapter

PAC Polycyclic aromatic compound PAH Polycyclic aromatic hydrocarbon PAS H Polycyclic aromatic sulfur heterocycle PAO H Polycyclic aromatic oxygen heterocycle N-PAC Nitrogen-containing polycyclic aromatic compound (both

heterocyclic and nonheterocyclic compounds) 2°-PANH Secondary-nitrogen polycyclic aromatic nitrogen heterocycles

(nitrogen in a five-membered ring) APAH Amino-substituted polycyclic aromatic hydrocarbons 3°-PANH Tertiary-nitrogen polycyclic aromatic nitrogen heterocycles

(nitrogen in a six-membered ring) HPAH Hydroxy-substituted polycyclic aromatic hydrocarbons APASH Amino-substituted polycyclic aromatic sulfur heterocycles PANS H Polycyclic aromatic nitrogen sulfur heterocycles H PAS H Hydroxy-substituted polycyclic aromatic sulfur heterocycles HPANH Hydroxy-substituted polycyclic aromatic nitrogen heterocycles

Results and Discussion

Quantitative comparisons of the PACs in the SRC II H D and in the coal tar are given in Table I. The PACs containing two heteroatoms, such as amino-substituted polycyclic aromatic sulfur heterocycles (APASHs), polycyclic aromatic nitrogen-sulfur heterocycles (PANSHs), hydroxy-substituted polycyclic aromatic sulfur heterocycles (HPASHs), and hydroxy-substituted polycyclic aromatic nitrogen heterocycles (HPANHs), were present at 1-10 parts per million (ppm) in these samples. In comparison, the corresponding PACs containing a single heteroatom, such as the polycyclic aromatic sulfur heterocycles (PASHs), nitrogen-containing polycyclic aromatic compounds

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Table I. Quantitative Comparisons of Selected PACs in an SRC II Coal Liquid and in a Coal Tar

Compound

Concentration11

Structure SRC II Coal Coal Tar liquid1

Phenanthrene

Anthracene

Pyrene

Benzo[c]phenanthrene

Triphenylene

Benz[a]anthracene

Chrysene

Naphthacene

Benzo[a]pyrene

Benzo[#]pyrene

Fluorene

PAHs

GOT

69

co9

occo

95,000 53,000

,500

2,000

3,600

Cyclopenta-Containing PAHs

4H-Cyclopenta[tfe/]phenanthrene

12,000

61,000 31,000

1,700

2,000

8,000

8,600

9,700

6,100

13,000 13,000

6,300

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14. NISHIOKA & L E E Annellated Structures in Coal-Derived Materiah 239

Table I.—Continued

Concentration0

Compound Structure SRC II Coal Coal Tar liquidh

Fluoranthene

Benzo[a]fluorene

Benzo[Z?]fluorene

Benzo[c]fluorene

Benzo[g/ii]fluoranthene

Benz[a]aceanthrylene

Benz[e]aeephenanthrylene

Benzo[/]fluoranthene

Benzo[fc]fluoranthene

Dibenzothiophene

Naphtho[2,3-fc]thiophene

Phenanthro[4,5-bcd] thiophene

Benzo[l?]naphtho[2,1-d] thiophene

Continued on next page.

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Concentration0

Compound Structure SRC II Coal Coal Tar liquidb

Benzo[&]naphtho[2,3-d]thiophene

Benzo[Z? ]naphtho[ 1,2-d] thiophene

Phenanthro[2,3-fc]thiophene ClQC^

Phenanthro[3,2-fo]thiophene O^X^j — d

Benzo[2,3]phenanthro[4,5-Z?cii]thiophene Ç^-^Ç^ 25

Chryseno[4,5-bcd] thiophene

Triphenyleno[4,5-focd] thiophene

Carbazole

4H-Benzo[efe/]earbazole

llff-Benzo[a]carbazole

5H-Benzo[Z?]carbazole

7H-Benzo[c]carbazole

N-PACs

55

7,400

790

220

700

15

10

51

53

85

4,400

710

1,200

700

860

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Compound

Concentration0

Structure SRC II Coal Coal Tar liquid1

1- Azadibenzothiophene




1- Methylphenanthrene




4-Methyldibenzothiophene


2- M e thyldibenzothiophene


1- M ethlyearbazole

op Op QP"

Methylated PACs

\ h 3

Oft

0.05

4,200

C H s 4

1.7

1,100

30,000 2,500

18,000 1,800

140

150

0.5 1

1,700 190


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Compound

Concentration0

Structure SRC Π Coal Coal Tar liquidb

2- M e thly carbazole

3- M ethlycarbazole

4- M ethlycarbazole

2- Aminobiphenyl

3- Aminobiphenyl

4- Aminobiphenyl

Aminophenylnaphthalenes

1- Aminodibenzo thiophene

2- Aminodibenzothiophene



1-Hydroxynaphthalene

°*3

1,700

3,500

1,000

Amino PACs

<X3

Hydroxy PACs

o5

70

50

( X H 1 5 0

0.32

0.11

Q^P—» 2.5

0.33

220

160

120

80

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Concentration0

Compound Structure SRC II Coal Coal Tar liquidb

2- Hy droxynaphthalene

2-Hydroxybiphenyl

3-Hydroxybiphenyl

4-Hydroxybiphenyl

Naphthylphenols

1-Hydroxydibenzo thiophene

2-Hydroxydibenzothiophene



3-(3- Hydroxypheny l)thiophene

7-(4-Hydroxyphenyl)benzothiophene

1-Hydroxyisoquinoline

Hydroxyphenylpyridines


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Compound

Concentration"

Structure SRC II Coal Coal Tar liquidb

1- Hydroxy carbazole

2- Hy droxycarbazole

3- Hydroxycarbazole

4- Hydroxycarbazole

0.63

€ ^ P ~ ° " 0.51

OH

0.89

0.76

"Approximate concentrations are given in micrograms per gram of original material. ^Concentrations of PASHs were determined for the SRC II VR, whereas concentrations of all others were determined for the SRC II H D . cCompound was not detected at a concentration > 100 μg/g. ^Compound was not detected at a concentration > 1 μg/g. eCompound was not detected at a concentration > 0.01 μg/g. Concentration of compound was not quantified because of coelution with other compounds or because several isomers were listed together. gCompound was not detected at a concentration > 0.1 μg/g.

(N-PACs) , and hydroxy-substituted polycyclic aromatic hydrocarbons (HPAHs) were present at 0.1-5%. In the following sections, compounds found in the two sample types are compared, and their origins are discussed. Because many details are not known concerning (1) the sources and compositions of the coal feedstocks and (2) the conditions and complexities of chemical reactions during processing, only general observations and explanations can be made. A number of exceptions to the general trends are expected in light of the complexities involved; nevertheless, noteworthy trends do exist and can be explained in a reasonable fashion.

Mild Autocatalytic Hydrogénation. The most significant difference between the compounds identified in the coal liquid and in the coal tar is the pronounced presence of hydrogenated compounds, tetrahydro-phenanthrene, tetrahydrodibenzothiophene, and tetrahydrobenzoquinoline in the coal liquid (14). Recently, a large number of hydroaromatic compounds in the SRC II H D were identified (23, 24). Although the SRC II process is not considered to be a hydrogénation coal-liquefaction process, mild auto-

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catalytic hydrogénation does take place. The 9- and 10- positions of anthracene are markedly reactive toward hydrogénation in comparison with other PAHs such as benzene, naphthalene, and phenanthrene. Similar results were reported in the correlation between the reaction constants for bromination, and reactivity values of selected PAHs (25). The abundance of anthracene relative to phenanthrene was low in the SRC II H D in comparison to that in the coal tar. This result seems to be due to the high reactivity of anthracene towards hydrogénation. Similar arguments can be applied to acridine; the abundance of acridine relative to the benzoquinolines and benzoisoquino-lines in the SRC II H D was quite low (21).

Cyclopenta-containing compounds were relatively less abundant in the SRC II H D than in the coal tar, whereas biphenyl, phenylnaphthalene, and their alkylated isomers were found in significant concentrations in the SRC II H D (II). The cyclopenta-containing compounds such as fluorene and benzofluorene are thought to be hydrogenated as shown in Schemes I and II. The SRC II coal liquid lacked naphtho[2,3-b]thiophene and its benzo analogues. These compounds, with fusion on only one side of the thiophene ring, can be hydrodesulfurized more easily than thiophenic compounds with aromatic rings fused on both sides, as shown in Scheme III (26). Although the SRC II process is generally considered to be a noncatalytic thermal process in comparison with a thermal catalytic process, for example, the Exxon Donor Solvent process (27), it is assumed to be somewhat autocatalytic as discussed earlier, but only mild hydrogénation occurs. The hydrogénation reaction is important for understanding the structural characteristics of the SRC II materials.

The effects of autocatalytic hydrogénation are demonstrated by the relative abundances of PACs containing functional groups, for example, H P A H s and amino-substituted polycyclic aromatic hydrocarbons (APAHs) in the SRC II coal liquid and coal tar. The suggested reactions for H P A H s and HPASHs in the SRC II H D are shown in Schemes IV-VI I . The condensed H P A H s such as the hydroxyphenanthrenes, hydroxypyrenes. and hydroxychrysenes were predominant in the coal tar, whereas the hydrogenated compounds such as the hydroxybiphenyls, naphthylphenols, hydroxyphenylthiophenes,

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Scheme IV

Scheme V

Scheme VI

Scheme VII

and hydroxyphenyl, benzothiophenes were major components in the SRC II H D . Similarly, the hydroxyphenylpyridines were one of the major compound types in the SRC II H D (Scheme VIII). The H P A H fraction of the SRC II H D (8.8%) was a larger percentage of the original sample than the H P A H fraction of the coal tar (2.8%). The different abundances of two- and three-ring APAHs and APASHs in the two samples can also be explained by hydrogénation (Schemes IX-XII) .

Scheme VIII

Scheme IX

Scheme X

Scheme XI

Scheme XII

Recently, quantitation of PACs in samples produced under different SRC conditions was reported (28). The concentrations of carbazole, which is thought to be the compound from which the aminobiphenyls were formed is as follows: 4500 ppm (SRC I process solvent), 2200 ppm (SRC II fuel-oil blend, which corresponds to the total process solvent fraction), and 31 ppm (high-hydrogen SRC II fuel-oil blend). The process bottoms were not re-

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cycled in the SRC I process. These similar trends were also observed for fluorene and dibenzofuran. Although the compositions of the feedstocks were not known, the variation in concentration suggests the possibility of hydrogénation of earbazole, fluorene, and dibenzofuran. The most abundant ami-nobiphenyl isomer in the SRC II H D was 2-aminobiphenyl (15). This result also supports the suggested hydrogénation reaction. The 2-aminobiphenyl isomer may be a useful indicator of the concentration of A P A H s . Similarly, 2-hydroxybiphenyl and 2-methylbiphenyl could be indicators of hydrogénation. 2-Hydroxybiphenyl, which was the most abundant hydroxybiphenyl isomer, was most highly concentrated in an SRC II middle distillate (180-392 °C) as shown by White and L i (29).

The foregoing discussion indicates that the aminophenylpyridine and aminophenylquinoline isomers would be produced by hydrogénation of heterocycles containing two nitrogen heteroatoms (Schemes XIII and XIV). These compounds were concentrated in the third fraction during adsorption chromatography on silicic acid. The major components were tentatively identified by using G C - M S as aminophenylpyridines, aminophenylquinolines, and their alkylated products (30).

Comparative Trends in Preferred PAC Structures. If the structures of the compounds produced by autocatalytic hydrogénation in the SRC II process as just described are disregarded, the structures of the remaining compounds in the coal liquid and coal tar are remarkably similar. Major alternate PAHs are phenanthrene, pyrene, chrysene, benz[a]anthra-cene, benzo[a]pyrene, benzo[e]pyrene, benzo[g/ii]perylene, dibenzo-[cfef,mno]chrysene, and coronene. Two major factors contributing to the existence of these major PAHs are (1) the structural composition of the coal feedstock, which is composed of crosslinked macromolecules (31-33) of biological origin, and (2) the stabilities of the aromatic components produced during processing, for which an important factor is the resonance energies for the conjugated systems.

The phenanthrene and chrysene structures could be derived from cyclic terpenoids such as abietic acid, sterols, and hopanes (34). Although tri-phenylene, which has the highest resonance energy among the four-ring catacondensed PAHs (35), was not present in the SRC II H D at a significant level, this compound was found in the coal tar which is a higher temperature

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product than the SRC II H D . The order of Huckel ττ-electron energies for the benzofluoranthene isomers is benz[a]aceanthrylene < benzole] fluoranthene < benzo [/]fluoranthene < benz[e]acephenanthrylene (35). The abundances of benz[a]aceanthrylene and benz[e]acephenanthrylene in the coal tar were the lowest and highest, respectively.

Although the abundances of the benzofluoranthene isomers are consistent with their resonance energies, other factors must be considered when comparing other isomer pairs. For example, in comparing benzo[a]pyrene with benzo[e]pyrene, benzo [a]pyrene is the less resonance-stable isomer but is the one that can be formed most easily from the biologically derived chrysene or benz[a]anthracene. In this case, benzo[a]pyrene is the most abundant isomer of the two in the coal tar. No universal rules can explain all observations; only general trends can be indicated.

Structural correlations between the PASHs and PAHs found in petroleum, coal tar, and pitch have been noted by Karcher et al. (36) and Burchill et al. (37). They found that the sulfur-containing materials contained thiophene derivatives analogous to the PAHs; the materials differed only in the replacement of one aromatic ring in the PAHs by a thiophene ring. We (13) recently reported that the structures and relative abundances of the major PAS H containing three- to- six rings in the SRC II V R and in the coal tar were analogous to those of the major PAHs in the same samples. By replacing one of the aromatic rings in the most abundant PAHs with a thiophene ring, the most abundant PAS H could be generally derived. Similarly, this correlation holds for cyclopenta-containing PAHs and secondary-nitrogen polycyclic aromatic nitrogen heterocycles (2°C-PANH), as shown in Chart I. The most abundant cata-condensed four-ring PAHs in both samples studied here was chrysene. Benzo[a]fluorene benzo[fe]naphtho[2,l-d]thiophene, phenanthro[3,2-fo]thiophene, and llii-benzo[a]earbazole, which have analogous structures to chrysene, were the most abundant among their respective isomers. Isomers such as benzo[£>]fluorene, which also corresponds to a n o t h e r a b u n d a n t c a t a - c o n d e n s e d f o u r - r i n g P A H , n a m e l y , benz[a]anthracene, were also major compounds. Benzo [a]pyrene was more abundant than benzo[e]pyrene in the coal tar. The PASHs corresponding to benzo[a]pyrene were benzo[2,3]phenanthro[4,5-fec(i]thiophene and chry-seno[4,5-fccd]thiophene, whereas the only P A S H corresponding to benzo[e]pyrene was triphenyleno[4,5-fec(i]thiophene. The structures and relative abundances of the major PACs containing a single heteroatom are analogous to those of the major PAHs, as shown in Chart I.

Likewise, the major PACs containing two heteroatoms in both samples could be structurally derived from the major PACs containing a single heteroatom. Dibenzothiophene, phenanthro[4,5-fecd]thiophene, and the ben-zonaphthothiophenes were the major three- and four-ring PASHs in both samples; the benzoquinolines, azapyrenes, and benzophenanthridines were the major three- and four-ring N-PACs. Similarly, azadibenzothiophenes

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Chart I. Structural similarities of the major PACs identified in the SRC II coal liquids and in the coal tar.

and azabenzonaphthothiophenes were found in both samples. Azabenzo-t h i o p h e n e s , a m i n o d i b e n z o t h i o p h e n e s , and a z a p h e n a n t h r o [ 4 , 5 -focd]thiophenes were identified in one or the other of the samples at low concentrations. Similar compounds were found for the hydroxy-sulfur-con-taining PACs and hydroxy-nitrogen-containing PACs; that is, the structures of the major PACs containing a single heteroatom reflect those of major parent PAHs, and the structures of the major PACs containing two heter-oatoms reflect those of the major PACs containing a single heteroatom.

Several correlations were also found for positions of substitution on the aromatic molecules, β methyl-substituted isomers, such as 2- and 3-meth-ylphenanthrene, were more abundant than α-substituted isomers, such as 1- and 4-methylphenanthrene. This trend was observed for the methylna-phthalenes, methylphenanthrenes, and methylchrysenes (II). However, the relative abundances of positions of substitution were not consistent for the methyldibenzothiophenes and methylcarbazoles when compared with the methylphenanthrenes; different factors are involved in their production. The methyl-substituted PACs are probably highly related to the alkyl linkages in the original coal structure. For the hydroxy- and amino-substituted PACs, hydrogénation in the SRC II process is an important factor.

Cyclo-Coupling Dehydrogenation during Liquefaction and Coalification. The correlations of compound structures described in the foregoing discussion suggest the possibility of similar reactions occurring

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during diagenesis or coal upgrading. One important reaction is believed to be coupling dehydrogenation of aromatic moieties by heating. Model condensation reactions expected for the coupling of two aromatic moieties are shown in Schemes X V - X X I L The reactions in Schemes X V - X V I I were extensively studied under heating with no catalyst by Badger (38). The PAHs, benz[a]anthracene and benzo[a]fluoranthene, can be produced according to

Scheme XV

Scheme XVI

Scheme XVII

Scheme XVIII

Scheme XIX

Scheme XX

Scheme XXI

Scheme XXII

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Schemes X I X and X X . Secondary cyclo-coupling dehydrogenations are apparently important for the production of highly pericondensed compounds such as dibenzo[def,rano]ehrysene, coronene, and triphenyleno[4,5-fecd]thiophene (39, 40). For heteroatom-containing compounds, Schemes X X I and XXII are possible examples. Recently, many polycyclic aromatic oxygen heterocycles (PAOHs) were produced by self-coupling of hydroxy compounds by McMil len et al. (41). These types of coupling or condensation reactions may be possible for molecules such as aromatic triterpanes (Scheme XXIII) by bond breakage and ring formation of aromatic moieties linked together with alkyl or heteroatom bridges (Scheme XIV), and cyclization of aromatic moieties in a macromolecular network (Scheme XXV). Further investigation into such reactions in coal may prove to be invaluable in future studies of coal structure.

Scheme XXIII

Scheme XXIV

Scheme XXV

Acknowledgment

This work was supported by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract No. D E - F G 0 2 -86ER60445.

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RECEIVED for review September 29, 1986. ACCEPTED April 4, 1987.

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