Proposed fullerene precursor corannulene identified in flames both in

J. Phys. Chem. 1993,97, 13539-13543

13539

Proposed Fullerene Precursor Corannulene Identified in Flames Both in the Presence and Absence of Fullerene Production Arthur L. Lafleur,. Jack B. Howard, Joseph A. Man, and Tapesh Yadav Center for Environmental Health Sciences and Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received: July 27, 1993; In Final Form: October 4, 1993"

Corannulene, CzoHlo, a bowl-shaped polycyclic aromatic hydrocarbon whose curvature and carbon framework are similar to those of fullerenes, has been widely hypothesized to be a fullerene precursor but has never been identified in flames, either in the presence or absence of fullerene production. In research on the mechanism and prevalence of fullerene formation in flames, we have identified corannulene in fullerene-producing flames and also in conventional flames that do not produce fullerenes. The identification was achieved using a combination of analytical methods including high performance liquid chromatography with UV-vis spectrometric detection, and gas chromatography coupled with infrared and mass spectrometric detection. The presence of corannulene in fullerene-forming flames is consistent with the hypothesis that corannulene is a fullerene precursor. The presence of corannulene in fullerene-deficient flames does not necessarily contradict this hypothesis and instead may suggest that at least some stages of fullerene formation chemistry are more generally operative in flames than had previously been known.

Introduction Corannulene, CzoHlo, is a polycyclic aromatic hydrocarbon (PAH) that consistsof five six-membered rings joined along their edges in a ring, resulting in a five-membered ring in the center of the molecule. As illustrated in Figure 1, the five-membered ring introduces curvature into the molecule, giving it the shape of a bowl. Although corannulene was first synthesized in 1966 using classical organic synthetic methods,l the difficulty in producing practical quantities of the material prevented widespread study. Recently, Scott and co-workers reported that flash vacuum pyrolysis of a suitable precursor yields corannulene in a single step, thus providing a method for generating useful quantities of corannulene.2 With the recent dramatic upsurge in fullerene research, corannulene has taken on new importance as a possible fullerene precursor. Fullerenes are composed of carbon atoms arranged in approximately spherical, ellipsoidal or tubular cages reminiscent of the geodesic domes designed by Buckminster Fuller, after whom the molecules are named.3 The most spherical fullerene, which resembles a soccer ball and contains sixty carbon atoms (C60), is called buckminsterfullerene. The fullerene with seventycarbon atoms ((270)is approximately ellipsoidal, reminiscent of a rugby ball. Each known fullerenemoleculecontainsexactly 12CSrings, which provides the curvature, fused together with c6 rings, whose number is characteristic of a particular fullerene structure. Fullerenes can also be visualized as closed-cage PAHs having no edges and thus no H-atoms. Corannulene, a CZOHIO PAH, is related to fullerenes by virtue of its spheroidal curvature and its Cs and Cg ring complement. Indeed, 12 corannulene units can be seen in the structure of any fullerene identified to date, since all fullerene cs rings are completely surrounded by c6 rings, as in corannulene. Fullerenes were first detected in 1985 in carbon vapor produced by laser evaporation of g r a ~ h i t e The . ~ closed shell strucure, which has no edge atoms vulnerable to reaction, was proposed to explain the relatively high stability of certain carbon clusters at high temperatures and in the presence of an oxidizing gas. The

* To whom correspondence should be directed at Core Laboratory in Analytical Chemistry, Room 2OC-032, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. *Abstract published in Advance ACS Absfracrs, November 15, 1993. 0022-3654/93/2097- 13539$04.00/0

Figure 1. Two views of the molecular structure of corannulene, C~OHIO.

proposed structure remained unconfirmed until 1990, when samples large enough for unequivocal spectroscopicidentification were produced by vaporization of graphite rods with resistive heating under an inert atm~sphere.~ This method was quickly adopted by and fullerenesamplessoon became available, quickly accelerating the pace of research. The possibility that fullerenesmight be formed in sooting flames was suggested early in fullerene and recent reviews of these studies are given by Kroto et al.lZ-I4 Early clues to the formation of fullerenes in flames were provided by molecularbeam mass spectrometry studies of Homann and c o - w ~ r k e r s ~ ~ - ~ ~ who reported the presence in low-pressure premixed benzene/ oxygen and acetylene/oxygen flames of all-carbon ions having mass/charge ratios similar to those reported for fullerenes from graphite vaporization. Recently, we reported our observations 0 1993 American Chemical Society

13540 The Journal of Physical Chemistry, Vol. 97, No. 51. 1993

Lafleur et al.

that both Cm and C70 fullerenes can be produced in macroscopic amounts from benzene/oxygen/argon flames operated over a range of conditions.18J9 The yield of Cm+c70 and the C70/caO ratio were found to depend on temperature, pressure, carbon/ oxygen ratio, and residence time in the flame.’8-20 In addition to C a and c70, the flame-derived fullerene soot also yielded the oxides CmO and GOO,the larger fullerenes c 7 6 , (284, CW,and C94, and many hydrogen-containing fullerenes including CmH2, CaH4, CIOHZ,and CaCH4 [Le. (Hz)C~O(CHZ) or (H)Cm(CH3)].21s22More recently, Diels-Alder adducts between Cm and cyclopentadienewere also identified in our flame ~ a m p l e s . ~ ~ . ~ ~ Fullerene formation in flames is a molecular weight growth process analogous to the formation of PAHs and soot, but with important differences.2’ The PAH and soot growth reactions include the addition of small molecules (e.g. C2H2) to larger aromatics at radical sites or dangling bonds and radical addition or recombination of two large species, such as heavy PAHs with themselves in soot-particle inception and with soot particles in surfacegrowth.26 The incorporationof the five- and six-membered rings (C, and C6 rings) required for fullerene structures may occur during molecular weight growth reactions or subsequently through ring rearrangement, or through both pathways.27 Intramolecular shifting of rings has been observed in PAHZ8and hypothesized to occur in fullerene~.~~ One of the first proposed routes for fullerene formation in graphitevaporizationinvolved ablation of single sheetsof graphite followed by ring rearrangement to close the cage. The location Figare 2. Nomenclature and structure of selected polycyclic aromatic of rings was assumed to obey the constraint30that adjoining CS compounds. rings are to be avoided. Sequential addition of smaller (CZ,C3) from a drilled copper burner plate through which the feed mixture units to a curved fullerene precursor was then proposed,I2with is delivered. The flame is surrounded by an annular nonsooting growth along the edges, similar to PAH growth, proceeding to flame which provides a thermal shield, giving a nearly oneclosed-cagestructures. Complicating this picture, recent isotope dimensional core within which temperature and species concenscramblingexperimentswith the carbon arcvaporizationmethod3’ trations vary only with distance,or residencetime, from the burner indicate that the initial solid carbon is randomly rearranged at surface. The burner was previously used in mechanistic studies the atomic level before fullerenes are formed. of soot nucleation and and the flames studied are of The corannulenestructure has often been consideredin relation a type for which considerable data on temperature and chemical to fullerene~~~-’~ or suggested as a plausible fullerene precursor. composition are Additional details about Kroto et a1.339935suggested that imperfectly formed fullerenes in fullerene production with these flames have been reported.18-20925 the form of corannulene-likeseed molecules could serve as nuclei Flames were produced under different sets of conditions over for soot formation. Frenklach and Ebert36,37argued against the following ranges: burner chamber pressure, 1.60-1 3.35 Wa fullerenes as soot nuclei but proposed benzo[ghi]fluoranthene (12-100 Torr); atomic C/O ratio, 0.717-1.08; mol% Ar, He, Nz, (CI~HIO) formation as the point of divergence from planar PAH. or mixtures thereof, 0-50; gas velocity at the burner (298 K), (Structures of benzo[ghi]fluoranthene(BghiF) and of other PAHs 14.6-75.4 cm/s. Maximum flame temperature was approxito be mentioned later are illustrated in Figure 2.) Detailed mately 2000 K. Each flame was maintained for 53-170 min, pathways for c 6 0 and e 7 0 formation through reaction pathways during which condensable compounds and soot were deposited involving corannulene as an intermediate have been formulaton the inside surface of the burner chamber, were collected after ed.20.27,38 Homann et a1.I6 suggested soot particles may serve as each run, and were later consolidated for use in identifying PAHs supports for fullerenegrowth with precursorssuch as corannulene associated with fullerene production. The bulk (75%) of the attaching to the soot surface, growing by acetylene addition, and collected material was obtained from flames operating under the detaching upon cage closure. following conditions: C/O, 0.96; helium diluent, 25%; chamber Thus, in flames, formation of fullerene precursors, first pressure, 40 Torr; gas velocity, 40 cm/s. The collected material identifiable by the onset of curvature, is a subset of the PAH was placed in beakers, extracted with toluene using an ultrasonic growth reactions with the curvature most likely imparted by Cs bath at room temperature, and filtered. The solutions were ring fusion to adjacent (26 rings, such as in benzo[ghi]fluoranthene concentrated by evaporation under a stream of nitrogen. and corannulene. In this work we report recent results from an Fderene-Deficient Fhmes. The jet-stirred/plug-flow comongoing project whose goals are the elucidation of mechanisms bustor was designed to provide well-definedcombustion conditions, of fullerene formation in flames and the identification of and its use is part of an ongoing program aimed at understanding intermediates along the formation pathway. These results include and controlling the combustion chemistry responsible for PAH the unequivocal identification of corannulene in a sample of soot formation. Fuel-rich combustion in a jet-stirred reactor (JSR) and condensed material collected from both fullerene-forming provides baseline input to a closecoupled plug-flow reactor (PFR) and fullerene-deficient flames. where continuing molecular weight growth reactions occur. Chemical analysis of over 70 ethylene combustion samples Experimental Section produced over a range of conditions has indicated no trace of FuUerene-Genemting Flames. Premixed laminar flames of fullerene production. Detailed descriptions of the JSR/PFR benzene and oxygen with argon diluent were stabilizedon a watercombustor are available elsewhere.424 In this study, the fuel cooled burner in a low-pressure chamber quipped with windows was ethylene. Operating conditionswere as follows: C/O, sample and feedthroughs for visual observation, optical diagnostics, and no. 1,0.733; sample no. 2,0.790; residence time, 9.0 me; reactor sample probes and exhausted into a vacuum pump. The flame temperature, 1630 K. Samples were obtained from the plug is stabilized with a flat 70-mm diameter front uniformly displaced flow region of the combustor using an aspirated probe connected

Corannulene Identified in Flames to a high-yield combustion sampler consisting of two refrigerated (0 "C) traps, each containing 1 L of dichloromethane. Samples were concentrated in a Kudema-Danish evaporativeconcentrator to the dissolution limit. Samples were filtered through a 0.2 pm fluorocarbon filter to remove particulates. Hi&-Performance Ldquid Chromatography. The preparative HPLC system used to isolate PAHs from toluene extracts of material collected from the fullerene-forming flames consisted of a Beckman 126Ppumpingsystem and Model 166PUVdetector. Two columns were used sequentially: A 22 mm i.d. by 30 cm poly(diviny1benzene) column (Jordi-Gel-500; Jordi Associates, Bellingham, MA) and a 22 mm i.d. by 50 cm octadecylsilylbonded silica column (Zorbax C-18, 10 pm; packed by Alltech, Deerfield, IL). A fullerene fraction was first isolated from the extracts using the Jordi-Gel column with a dichloromethane to benzene mobile phase gradient. A purified PAH fraction was obtained from the resulting eluate using the Zorbax C- 18 column with an acetonitrile to dichloromethane gradient. The HPLC instrument used for the analysis of PAH fractions and corannulene subfractions consisted of a Hewlett-Packard Model 1050 quaternary gradient pumping system with 1040M (190-600 nm) diode-array detector. For the analysis of the PAH fractions we used a 4.6 mm i.d. Vydac 201TP54 polymeric Cls column (Separations Group, Hesperia, CA) having a length of 250 mm. The flow rate was 1.5 mL/min, and the injection volume was 20 pL. Our mobile phase elution program consisted of two linear gradients: (1) 0-40 min, 40% acetonitrile/60% water to lOO%acetonitrile; (2) 4&80min, acetonitriletodichloromethane. Dichloromethane elution was maintained for another 30 min to remove larger PAHs and residual fullerenes; then the program was reversed to equilibrate the column for subsequent analyses. UV-vis spectra (200400 nm) were acquired immediately and stored for later processing. HPLC analysis of fuel-rich ethylene combustion samples was done with the same column and instrumentation. The conditions were identical except for the following differences: (1) the flow rate was 1.0 mL/min, and (2) in the mobile phase program, the elution time for the acetonitrile segment was doubled to 80 min. HPLC analyses of corannulene-enrichedsubfractions were done using a Vydac minibore 218TP52 polymeric CIScolumn 25 cm in length with an i.d. of 2.1 mm. Injection volume was 5 pL, and the flow rate was 0.4 mL/min. The mobile phase program consisted of the following segments: 0-3 min, 40% acetonitrile/ 60% water; 3-43 min, linear ramp to 100% acetonitrile, 43-53 min, linear ramp from acetonitrile to 100% dichloromethane. CC/MS. Mass spectra were obtained on a Hewlett-Packard GC/MSD consisting of an HP 5890 gas chromatograph coupled to a 597 1A mass-selective detector. Separations were done with a 0.2 mm i.d., 12-m methyl-siliconefused silica column (HewlettPackard, HP-1) having a film thickness of 0.33 pm. The carrier gas was helium with a head pressure of 11.5 psi, the injection volume was 1.0 pL, and the injector temperature was 250 OC. The column temperature program consisted of a 1.5-min hold at 70 OC followed by a linear ramp to 310 OC at 10 OC/min and a final hold at 310 OC for 6 min. CCJFIIR. Infrared spectra were obtained using a HewlettPackard GC/IRD. The instrument consisted of an HP 5890 GC interfaced to a Model 5965A infrared detector. A Model 59965 IRD ChemStation was used for data processing. The GC column was a 25 m, 0.25 mm i.d. Quadrex 007/MPS 5 [methyl-(%phenyl)-silicone] fused silica column with a film thickness of 0.25 Mm. The temperature program consisted of a linear ramp from 40 to 280 OC at 10 OC/min. The injection volume was 1.O PL. Results and Discussion

Corannulenehas been ofspecial interest in our efforts to identify key reaction intermediates in fullerene formation since this molecule has been hypothesized to be a precursor to fullerenes

The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13541

--3 ?i

140 120

I

+ CPPtBghiF

100 80 60

40 20

0

elution volume [mL]

Figure 3. HPLC chromatogram of a polycyclic aromatic hydrocarbon fraction of toluene extracts of condensed material from fullerene-forming flames. Abbreviations: Bghii, benzo[ghi]perylene: Cor,coronene: others. see Figure 2.

as described above. Work presented here focuses on analyses of extracts of condensed compounds and soot sampled from benzene/ oxygen/inert gas flames in which we originally discovered fullerenes to be produced in substantial quantities.ls Although we have found that BghiF, one of the suggested fullerene precursors,26is prevalent in fullerene-forming flames, we have also found that BghiF is prevalent in many other types of common flames that do not produce f~llerenes.~5-4~ Thus, this PAH might not be sufficiently far along in the reaction pathway leading toward fullerenes to be indicative of fullerene formation. Corannulene, on the other hand, not only is one step (Le. one additional ring fusion) beyond BghiF toward fullerenes but also represents a major increase in curvature, from nearly flat to nearly the curvature of fullerenes. Also, corannulene, like fullerenes, has not previously been identifiedin conventional flames, therefore, the presence of corannulene in fullerene-forming flames would lend credence to the hypothesis that it is a fullerene precursor. A number of PAHs giving molecular ions of m/e 250 in mass spectrometric analyses of flame samples have been previously reported, strongly suggesting the presence of CzoH10PAHs in In our studies of fuel-rich ethylene combustion in the jet-stirred/plug-flow reactor, we identifiedseveral components whose UV and MS signatures strongly suggested them to be C20H10PAHs. Thedata obtainedpointed todicyclopentapyrenes as the most likely structures, but the lack of reference standards made unequivocal identification impossible at that time.46 Recently, Scott and co-workerssynthesizedZand graciously made availableto us a reference quantity of corannulene, thus affording us the opportunity to search for this suspected fullereneprecursor in this and in other samples. To obtain reference data for the identification of corannulene in flame samples, we ran a battery of chemical analyses including high performanceliquid chromatographywith spectrophotometric diode-array detection (HPLC/DAD) and gas chromatography coupled with mass spectrometry (GC/MS) and Fourier transform infrared spectrometry (GC/FTIR). With these reference data in hand, it soon became apparent that none of our previously identified C ~ O HPAHs I O could have been corannulene; however, the reference data made possible the unequivocal identification of corannulene in flame samples. Figure 3 shows an HPLC chromatogram obtained for a PAH fraction obtained by pooling toluene extracts from benzene flame samples generated under a range of fullerene-formingconditions as described earlier. The corannulene peak (Crl), as later determined, is situated between those of pyrene (Pyr) and cyclopenta[cdlpyrene (CPP) in the 25-30-mL elution interval. As a minor component in a complex PAH mixture, corannulene is difficult to resolve, even on Vydac 201TP HPLC columns which are optimized for PAH separation. In Figure 3, it is barely visible,

Lafleur et al.

13542 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993

m.0 89.8

89.8

8

89.7

6 QQ.6

.u

E 896

.f:

e

c

89.4 c corannubnc [lower troce)

89.3 89.2 99.1 QQ.0 4

elution volulme [mL] F i p e 4 . HPLC chromatogram of a corannulene-enrichedfraction from a toluene extract of condensed material from fullerene-formingflames. Abbreviations: AP, acephenanthrylene;Fla, fluoranthene;Pyr, pyrene; PAK, polycyclic aromatic ketone; CPP, cyclopenta[cdlpyrene; BghiF, benzo[ghi]fluoranthene. 3M3

I

wavenumber [cm-)] Figure 6. GC/FTIR data for HPLC peak C confirming the structural assignment as corannulene.

TABLE I: Summary of Coranndene Data for Flame Samples

c/o

combustor 1 premixed-flame

bumeP

atomic pressure tem corannulene fuel ratio [Torr] [Kf C ~ O(crl/pyr)c benzene 0.960 40 2000 yes 6.3%

2 plug-flow combusto+ ethylene 0.790

760 760

3 plug-flow combustorb ethylene 0.733

1630 no 1630 no

0.8% 2.4%

a Fullerene-generating flame; see refs 18-20 and 41. b Jet-stirred/ plug-flow reactor; see refs 4244. Mass corannulene/masspyrenc (96).

450 4M1 550

1 ,

[I = 236-500 nm]

I

c CPPtBghiF

wavelength [nm] Figure 5. UV spectrum of a corannulene standard compared with that of HPLC peak C.

appearing as a small shoulder on the pyrene peak. To obtain adequate resolution for conclusive identification, we found it necessary to isolate a corannulene subfraction from our original PAH fraction. The corannulene isolate was analyzed by HPLC/DAD using a narrow-bore Vydac 218TP52 column as detailed in the Experimental Section, and results are shown in Figure 4. Here, corannulene is sufficiently resolved so that clean spectral data can be obtained. Corannulene (Crl, peak C) exhibits a short HPLC retention time for a C20 PAH, eluting between pyrene, a C16PAH, andcyclopenta[cdJpyrene,aCla PAH. Thisapparently anomalous elution is to be expected for nonplanar PAHs and for PAHs with compact molecular shapes, and corannulene possesses both of these characteristic^.^^ Also visible in the chromatogram are three C16HlO PAHs, fluoranthene, acephenanthrylene (AP), and pyrene; two C l ~ H l oPAHs, cyclopenta[cd]pyrene (CPP) and benzo[ghi]fluoranthene(BghiF), and a polycyclicaromatic ketone (PAK) were found to give molecular ion at m/e 254 when isolated and analyzed by GC/MS. Figure 5 shows the UV spectrum of a corannulene reference standard compared with HPLC peak C (Figure 4). and a close match is observed. Peak C (Figure 4) was isolated by HPLC, concentrated, and exchanged into dichloromethane for analysis by GC/MS and GC/FTIR. The mass spectrum with a molecular ion at m/e 250 and M++ion at m/e 125 was identical to that of corannulene as was its GC retention time. Additional confirmation was provided by its FT'IR spectrum, shown in Figure 6, which is seen to be idelltical to that of corannulene. In order to determine whether corannulene might also be present in flames not producing fullerenes, we analyzed extracts of

elution volume [mL] Figure 7. HPLC chromatogram of an ethylene combustion samples PAH isomers. showing the presence of C~OHIO

condensed material and soot from two well-characterized flames in which fullerenes were clearly absent. Corannulene was found in both cases, as summarized in Table I. The quantity of corannulene is given as a percent of the amount of pyrene in the sample. The maximum mole fraction of pyrene is 8 X 10-6 for the premixed flames41and 5 X 10-6 in the plug-flow comb~stor.~3 Additional work is currently underway to determine the distribution profiles of corannulene and fullerenes in our flames. Overall, the data indicate that corannulene is more prevalent under conventional combustion conditions than would be expected on the basis of its not having been previously detected in flames. There also appears to be significant differences in the distribution profiles for C2oHlo PAHs between samples from the fullerene-producing benzene flames and those from the ethylene combustor. An interesting contrast to the fullerene-soot chromatograms shown in Figures 3 and 4 is the HPLC chromatogram for an ethylene combustion sample shown in Figure 7. In this sample, corannulene (not visible in thechromatogram) was present at barely detectable levels; however, three other C ~ O HPAHs IO are seen to be quite abundant, a result which is opposite to that

Corannulene Identified in Flames obtained with the fullerene flame sample. The isolation of a corannulene subfraction and further analysis by HPLCIDAD and GC/MS permitted the unequivocal identification of corannulene. The other CzoHlo PAHs, thought to be dicyclopentapyrene isomers, are abundant in other combustion and pyrolysis samples and are presently the focus of study in our laboratories. The characteristicHPLC elution behavior and UV and IR spectral properties of corannulene allow it to be distinguished from other C20H10 PAH isomers; however, as we have shown, the mere finding of C ~ O H PAHs I O in a flame does not necessarily indicate the presence of corannulene. There are at least two reasons why the presence of corannulene in conventional flames, represented by the jet-stirred/plug-flow reactor, has gone undetected until now despitedecadesof chemical analysis utilization in combustion research. One is the elusiveness of corannulene in the complex mixtures of PAHs encountered in flamesamples. The other is thegeneral scarcityof PAH reference compounds. Reference quantities of corannulene were generally unavailable until the recent work of Scott and co-workers. The results in Table I show that the presence of corannulene in a flame is not necessarily indicative of fullerene production. However, this observation does not weaken the hypothesis that corannulene is a fullerene precursor, since there is a well-known precedence for such an apparent lack of correlation between flame generated carbon and its PAH precursors. Thus, just as high molecular weight planar PAHs, the generally accepted precursors of flame-generated graphitic carbon (soot), are present in fuelrich but nonsooting so is the curved PAH corannulene, the hypothesized precursor of flame-generated fullerene carbon, present in fuel-rich but not fullerene-formingflames. Accordingly, the present data would imply that corannulene can be formed in detectable quantities under conditions in which growth from corannulene on to fullerenes does not compete successfully with oxidativeor pyrolytic destruction reactions that consume fullerene precursors. In the analogous known behavior of the planar PAH/ soot system, competition between molecular weight growth and destructive oxidation and pyrolysis reactions under fuel-rich conditions determines whether or not soot production occurs. The soot precursors are present in either case. The present data are consistent with the hypothesis that corannulene is a fullerene precursor. If the hypothesis is correct, the newly discovered prevalence of corannulene in flames, both with and without fullerene formation, would imply that at least some fullerene formation chemistry may be more generally operative in flames than had been previously known. Acknowledgment. The combustion research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences,Office of Energy Research, US.Department of Energy, under Grant No. DE-GF02-84ER13282. The chemical analysis work was supported by National Institute of Environmental Health Sciences Center Grant NIH-5P30-ES02109-13 and Health Effects of Fossil Fuels Combustion Program Grant NIH5P01-ES01640-14. L. T. Scott graciously provided a reference quantity of corannulene. Laboratory assistance was provided by M. K. Chung, L. M. Giovane, D. G. Goldenson, Y.Makarovsky, R. McKern, K. Taghizadeh, R. Marino, and L. W. Theiss. References and Notes (1) Lawton, R. G.; Barth, W. E. J. Am. Chem. SOC.1971, 93, 1730. (2) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J . Am. Chem. Soc. 1991, 113,7082. (3) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (4) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354.

The Journal of Physical Chemistry, Vol. 97, No. 51, I993

13543

(5) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J. Chem. SOC.,Chem. Commun. 1990, 1423. (6) Ajie, H.; Alvarez, M. M.; Anz. S. J.; Beck, R. D.; Diederich, F.; Fostiropulos, K.; Huffman, D. R.; Kritschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetton, R. L. J. Phys. Chem. 1990, 94, 8630. (7) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, N. E.; Flanagan, S.;Haley, M. M.; O’Brien, S.C.; Pan, C.; Xiao, Z.; Billup, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990, 94, 8634. (8)

Zhang,Q.L.;OBrien,S.C.;Heath,J.F.;Liu,Y.;Curl,R.F.;Kroto,

H. W.; Smalley, R. E. J. Phys. Chem. 1986, 90, 525. (9) Kroto, H. W.; McKay, K. Nature 1988, 331, 328. (10) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (1 1) Kroto, H. Science 1988, 242, 1139. (12) Kroto, H. W.; Allaf, A. W.; Balm, S.P. Chem. Rev. 1991,91,1213. (13) Kroto, H. W. J. Chem. Soc. Faraday Trans. 1991, 87(18), 2871. (14) Kroto, H. W. Angew. Chem., Inr. Ed. Engl. 1992, 31, 111. (15) Gerhardt, Ph.; Loffler, S.;Homann, K. H. Chem. Phys. Lett. 1987, 137, 306. (16) Gerhardt, Ph.; Loffler, S.;Homann, K. H. Proceedings, TwentySecond Symposium (Inr.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1989; p 395. (17) Loffler, S.;Homann, K. H. Proceedings, Twenty-Third Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1991, p 355. (18) Howard, J. B.; McKinnon, J. T.; Makarovsky, Y.; Lafleur, A. L.; Johnson, M. E.; Nature 1991, 352, 139. (19) Howard, J. B.; Johnson, M. E.; Makarovsky, Y.; Lafleur, A. L. J. Phys. Chem. 1992, 96,6657. (20) Howard, J. B.; Lafleur, A. L.; Makarovsky, Y.; Mitra, S.;Pope, C. S.;Yadav, T. K.; Carbon 1992, 30, 1183. (21) Anacleto, J. P.; Perreault, H.; Boyd, R. K.; Pleasance, S.;Quilliam, M. A.; Sim, G. P.; Howard, J. B.; Makarovsky, Y.; Lafleur, A. L. Rapid Commun. Mass Spectrom. 1992, 6, 214. (22) Anacleto, J. F.; Boyd, R. K.; Pleasance, S.;Quilliam, M. A,; Howard, J. B.; Lafleur, A. L.; Makarovsky, Y. Can. J. Chem. 1992, 70,2558. (23) Rotello, V. M.; Howard, J. B.; Yadav, T.; Conn, M. M.; Viani, E. Giovane, L. M.; Lafleur, A. L. Tetrahedron Lett. 1993, 34, 1561. (24) Anacleto, J. F.; Quilliam, M. A.; Boyd, R. K.; Howard, J. B.; Lafleur, A. L.; Yadav, T. Rapid Commun. Mass Spectrom. 1993, 7 , 229. (25) Howard, J. B. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 933. (26) McKinnon, J. T.; Howard, J. B. Combust. Sei. Technol. 1990, 74, 175. (27) Pope, C. J. Ph.D. Thesis, Massachusetts Institute of Technology, 1993. (28) Scott, L. T.; Roelofs, N. H. J. Am. Chem. SOC.1987, 109, 5461. (29) Stone, A. J.; Wales, D. J. Chem. Phys. Lett. 1986, 128, 501. (30) Schmalz, T. G.; Seitz, W. A,; Klein, D. J.; Hite, G. E. J. Am. Chem. SOC.1988, 110, 1113. (31) Ebbesen, T. W.; Tabuchi, J.; Tanigaki, K. Chem. Phys. Lett. 1992, 191, 336. (32) Haymet, A. D.J. Am. Chem. SOC.1986,108, 319. (33) Curl, R. F.; Smalley, R. Sci. Am. 1991 (October), 265, 54. (34) Borchardt, A.; Fuchicello, A.; Kilway, K. V.;Balridge, K. K.; Siegel, J. S.J. Am. Chem. SOC.1992, 114, 1921. (35) Kroto, H. W. Nature 1987, 329, 529. (36) Frenklach, M.; Ebert, L. B. J. Phys. Chem. 1988, 92, 561. (37) Ebert, L. B. Science 1990, 247, 1468. (38) Chang, T.-M.; Naim, A.; Ahmed, S.N.; Goodloe, G.; Shevlin, P. B. J . Am. Chem. SOC.1992, 114, 7603. (39) Bittner, J. D. Ph.D. Thesis, Massachusetts Institute of Technology, 1981. (40) Bittner, J. D.; Howard, J. B. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981, p 1105. (41) McKinnon, J.T. Ph.D. Thesis,Massachusctts InstituteofTechnology, 1989. (42) Nenninger, J. E. Ph.D. Thesis, Massachusetts Instituteof Technology, 1983. (43) Lam, F. W. Ph.D. Thesis, Massachusetts Institute of Technology, 1988. (44) Vaughn, C. B. Ph.D. Thesis, Massachusetts Institute of Technology, 1988. (45) Lafleur, A. L.; Gagel, J. J.; Longwell, J. P.; Monchamp, P. A. Energy Fuels 1988, 2, 709. (46) Lafleur, A. L.; Longwell, J. P.; Monchamp, P. A.; Shirname-Mor& L.; Peters, W. A,; Plummer, E. F. Energy Fuels 1990, 4, 307. (47) Lafleur, A. L.; Longwell, J. P.; Marr, J. A.; Monchamp, P. A.; Plummer, E. F.;Thilly, W. G.; Mulder, P. P.Y.;Bffire, B. B.; Cornelisssc, J.; Lugtenburg, J. Environ. Health Perspect. 1993, 101, 146. (48) Smedley, J. M.; Williams, A.; Bartle, K. D. Combust. Flame 1992, 91, 71. (49) Wise, S. A. In Handbook of Polycyclic Aromatic Hydrocarbons; Bjarseth, A., Ed.; Marcel Dekker: New York, 1983; p 183.