738
Energy & Fuels 2003, 17, 738-743
The Study of Anthracene Aerosols by Solid-State NMR and ESR Mark S. Solum, John M. Veranth, Yi-Jin Jiang, Anita M. Orendt,† Adel F. Sarofim, and Ronald J. Pugmire* Department of Chemical and Fuels Engineering, and The Center for High Performance Computing, The University of Utah, Salt Lake City, Utah 84112 Received September 24, 2002
The condensed solids from three aerosols resulting from heating anthracene in a drop-tube furnace with a helium atmosphere at temperatures of 1250, 1300, and 1400 K were studied by carbon-13 solid-state NMR and by ESR methods. The 1250 K sample consisted almost entirely of unreacted anthracene. However, the proton T1 value of 17.6 s allowed the principal values of the carbon-13 chemical shift tensors to be measured by the FIREMAT experiment. The 1400 K sample appeared to be a two-component system; 9.5% (w/w) of the aerosol was extracted in dichloromethane. The extract (tar) from the 1400 K aerosol was analyzed in terms of a standard set of structural and lattice parameters. The average aromatic cluster size was found to be about 15 carbons with approximately 1.4 attachments per cluster. The NMR spectrum of the extraction residue (soot) of the 1400 K aerosol consisted of a broad featureless signal. The sample was very conductive, yielding a graphite-like factor of 0.47. The unpaired electron spin concentration was 1.2 × 1017 spins/g for the extract (tar) and 1.1 × 1020 spins/g for the residue (soot).
Introduction This paper continues our study of the early stages of soot formation by the use of solid-state NMR and ESR methods. Previous work described aerosols made from an Illinois No. 6 coal, biphenyl, and pyrene which were studied by NMR methods.1 The aerosols produced from the coal characterized the temperature range of “soot samples” for which useful solid-state NMR data can be obtained since the formation of tar and char is a wellknown characteristic of coal pyrolysis. The coal aerosol samples were made in a flat flame burner at temperatures ranging from 1180 to 1850 K. Some of the lowtemperature samples were very tar-like, while the higher-temperature samples were very carbonized and conductive. The low-temperature tar-like samples can be characterized by NMR methods in terms of a standard set of structural and lattice parameters originally developed to characterize coals.2,3 Some of these lattice parameters were then used as a coal-specific set of parameters for input into coal devolatilization modeling.4 The NMR spectra of the higher-temperature carbonized samples are usually distinguished as broad and featureless. However, they may be characterized
by a graphite-like factor that provides information on the fraction of graphite-like structures in the sample.5 These samples can also be characterized in terms of the upper limit of the aromatic cluster size.1 Pyrolysis experiments under similar conditions on solid model compounds (biphenyl, pyrene, and anthracene) exhibit behavior similar to that observed in the coal samples and the same structural analysis procedures can be employed. Additional GC-MS and high-resolution MS data6 on the biphenyl samples showed the tremendous importance of biaryl linkages (in terphenyl and quatraphenyl species) in some of the compounds formed in the early stages of soot formation. Structural units of this type were inferred by the NMR data and will be the topic of a future publication. The creation of biaryl linkages has been previously noted.7-10 However, the importance of biaryl linkages for aromatic species seems to be minimized in current soot modeling schemes.10 This paper presents data on the study of three aerosols produced from anthracene in a drop-tube furnace at temperatures of 1250, 1300, and 1400 K. The 1250 K aerosol sample appeared to be primarily unre-
* Author to whom correspondence should be addressed at the Department of Chemical and Fuels Engineering. † The Center for High Performance Computing. (1) Solum, M. S.; Sarofim, A. F.; Pugmire, R. J.; Fletcher, T. H.; Zhang, H. Energy Fuels 2001, 15, 961. (2) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (3) Orendt, A. M.; Solum, M. S.; Sethi, N. K.; Pugmire, R. J.; Grant, D. M. Advances in Coal Spectroscopy; Plenum Press: New York, 1992; Chapter 10, p 215. (4) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels 1989, 3, 175; Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54; Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1992, 6, 414.
(5) Jiang, Y. J.; Solum, M. S.; Pugmire, R. J.; Grant, D. M.; Schobert, H. H.; Pappano, P. J. Energy Fuels 2002, 16, 1296. (6) Tomczyk, N. A.; Hunt, J. E.; Winans, R. E.; Solum, M. S.; Pugmire, R. J.; Fletcher, T. H. Model Compound Study of the Pathways for Aromatic Hydrocarbon Formation in Soot. Prepr. Pap.sAm. Chem. Soc., Fuel Chem. Div. 2002, 47 (2), 731. (7) Wornat, M. J.; Sarofim, A. F.; Lafleur, A. L. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; p 955. (8) Mukherjee, J.; Sarofim, A. F.; Longwell, J. P. Combust. Flame 1994, 96, 191. (9) Badger, G. M. Prog. Phys. Org. Chem. 1965, 3, 1. (10) Appel, J.; Bockhorn, H.; Frenklach, M. Combust. Flame 2000, 121, 122.
10.1021/ef020216h CCC: $25.00 © 2003 American Chemical Society Published on Web 05/02/2003
Study of Anthracene Aerosols by Solid-State NMR and ESR
acted anthracene, but the proton T1 value was significantly shortened (17.6 s compared to hundreds of seconds, or more, in pristine anthracene) so that the first measurement of the chemical shift tensors in “anthracene” was possible. ESR work showed a great variation in the amount and type of unpaired electrons in these three aerosols. The 1400 K sample was extracted and the extract and residue were studied separately. Experimental Section Aerosol Production. The generation of the anthracenederived aerosol particles was based on methods and operating conditions developed by Wornat et al.7 To create a free-flowing feed, 20% anthracene and 80% silica gel (28-200 mesh, Aldrich Grade 12) were manually ground together using a mortar and pestle. Helium (0.095 L/min) was used as the carrier gas for conveying the feed. 5 L/min of helium was supplied to the main furnace tube, and 24 L/min of helium was used for quenching and radial flow in the water-cooled exit probe. Calculated residence time in the furnace was 0.75 s from the injector to the exit probe. The quenched products passed through an Andersen cascade impactor (preseparator and stages 0-5) that removed particles larger than 1-µm aerodynamic diameter. A diagram of the drop-tube furnace and cascade impactor has been published.8,11 The reacted products were collected on Teflon filters (Cole Parmer catalog no. 2916-66) and the deposit was recovered by scraping. Two 90mm diameter filter holders were connected in parallel with valves to allow switching filters without interrupting the furnace since multiple filters were needed to collect sufficient mass for analysis. Mass balance showed that over 95% of the silica gel was recovered in the cascade impactor and that 50% of the anthracene feed was deposited on the collection filter. Preliminary tests were conducted at 1250, 1300, 1350, 1400, and 1450 K furnace wall temperature, and nominal 0.5 g samples were generated at 1250, 1300, and 1400 K. No direct evidence is available to indicate the presence or absence of a chemical reaction between the silica gel and the anthracene. However, the ESR data and NMR spin relaxation data clearly indicated that a large number of stable free radicals have formed on the carbon atoms. Any significant reaction with the hydroxyl groups in the silica gel would have reduced the free radicals below the level detected. Hence, it is assumed that no significant amount of reaction occurred between the silica gel and anthracene. NMR. All C-13 CPMAS NMR high-speed spinning (4.1 kHz) experiments were conducted on a Chemagnetics CMX-100 spectrometer equipped with a 7.5-mm PENCIL rotor probe with a ceramic housing for reduced carbon background. The decoupling power was 62.5 kHz corresponding to a 4.0 µs 90° pulse. All variable delay experiments (variable contact time, dipolar dephasing, and saturation recovery T1 experiments) were conducted in an interleaved fashion. The contact time in the dipolar dephasing experiment was 10 ms for the 1400 K residue and 3 ms for the 1400 K extract. These contact times were chosen after observing the variable contact time curve. The pulse delay for the dipolar dephasing experiment on the 1400 K extract was 50 s and 1.0 s for the 1400 K residue. The calculation method of the structural and lattice parameters has been given in detail.1-3 The structural and lattice parameter definitions appear in the Appendix. The slow spinning chemical shift measurements on the 1250 and 1300 K anthracene aerosols were conducted on a Chemagnetics CMX-200 NMR spectrometer spinning at 501 Hz using the FIREMAT12 spinning sideband procedure. The (11) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 1.
Energy & Fuels, Vol. 17, No. 3, 2003 739 decoupling power was 62.5 kHz, the contact time was 3 ms, and the pulse delay was 20 s; there were 400 scans collected on each increment in the evolution dimension. Both of the two data sets were fit with and without the TIGER13 processing method. All four of the analyses gave results within experimental error of each other. Soot Extraction. The 1400 K aerosol sample was extracted in a Soxlet extraction apparatus with dichloromethane (DCM) for 24 h. Excess solvent was removed and the samples (extract and residue) were allowed to dry at room temperature. The extract accounted for 9.5% of the aerosol. ESR. The measurements of the unpaired electron spin concentrations were carried out on a Bruker EMX ESR spectrometer. The microwave cavity 4103TM was employed at a frequency of about 9.75 GHz. A series of standard free radical samples of BDPA (Aldrich catalog 15256-0) was prepared for the calibration of spin concentration measurements. All of the samples studied were treated under vacuum at 100 mTorr for 48 h and vacuum sealed in order to exclude the effects of adsorbed oxygen on the ESR spectra. The experiments were carried out at room temperature. Because the ESR signal is proportional to the Q value of the cavity, all experiments were performed at a constant cavity Q (the operational Q value of the cavity is about 12 500). Approximate sample volume and position were maintained inside the cavity. Samples with high conductivity that decrease the Q value of the cavity are routinely diluted in silica gel (Aldrich catalog 28859-4) to restore the required cavity Q. None of the samples in this paper required dilution.
Calculations of the Chemical Shielding Tensor The calculations of the chemical shielding tensors in anthracene were completed using the GIAO method14 as implemented15 in the GAUSSIAN 98 program.16 Calculations were done on the molecule using the X-ray coordinates17 for the carbon atoms and optimized hydrogen atom positions, as previous work has indicated that the inaccuracies in the hydrogen atom positions from X-ray structures lead to large errors in the calculated chemical shielding values.18 Calculations were done with the D95** basis set19 and all calculations were completed at the DFT levels of theory using the MPW1PW91 hybrid functional.20 A correlation between the theoretical shieldings and experimental chemical shifts, as shown in Figure 1, was used to transform the (12) Alderman, D. W.; McGeorge, G.; Hu, J. Z.; Pugmire, R. J.; Grant, D. M. Mol. Phys. 1998, 95, 1113. (13) McGeorge, G.; Hu, J. Z.; Mayne, C. L.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M. J. Magn. Reson. 1997, 129, 134. (14) Ditchfield, R. Mol. Phys. 1974, 27, 789. (15) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (17) Brock, C. P.; Dunitz, J. D. Acta Crystallogr. 1990, B46, 795. (18) Grant, D. M.; Liu, F.; Iuliucci, R. J.; Phung, C. G.; Facelli, J. C.; Alderman, D. W. Acta Crystallogr. 1995, B51, 540. (19) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; p 1. (20) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664.
740
Energy & Fuels, Vol. 17, No. 3, 2003
Solum et al. Table 1. NMR and ESR Parameters for the Anthracene Aerosols and Extraction Products sample
Ne × 1017 (spins/g)
TH 1 (s)
H T 1F (ms)
1250 K aerosol 1300 K aerosol 1400 K aerosol 1400 K extract
3.5 90 980 1.2
17.6 3.4 0.136 2.2 (54%) 9.8 (46%) 0.095
32 11
1400 K residue
1100
52
Table 2. The Chemical Shift Principal Values of the Anthracene 1250 K Aerosola carbon atom C-1,4,5,8
Figure 1. Plot of the calculated chemical shielding tensor principal values vs the experimental chemical shift tensor principal values.
C-2,3,6,7 C-9,10 bridgehead
δ11
δ22
δ33
δiso
intensityb
221.2 (226.1) 223.7 (225.1) 204.3 (213.5) 210.2 (209.4)
141.6 (135.4) 137.2 (134.6) 145.0 (139.0) 192.7 (189.2)
24.7 (23.9) 12.7 (14.2) 32.3 (35.3) -6.9 (-6.9)
129.2 (128.5) 124.5 (124.6) 127.2 (129.3) 132.0 (130.6)
0.279 (0.286) 0.277 (0.286) 0.141 (0.143) 0.303 (0.286)
a Values in parentheses are from calculations (see Experimental Section). b There are 14 total carbon atoms in anthracene. Theoretical intensities represent the fraction of magnetically equivalent carbon atoms for each resonance.
Figure 2. Carbon-13 CPMAS spectra of anthracene aerosols made in a drop-tube furnace: (a) the 1250 K aerosol, (b) the 1300 K aerosol. (c) the 1400 K aerosol, (d) the 1400 K DCM extract, (e) the 1400 K DCM extraction residue.
shieldings into the shifts that are reported in Table 2. The RMS agreement was 4.6 ppm. Results and Discussion Carbon-13 CPMAS spectra of three aerosols made from anthracene at temperatures of 1250, 1300, and 1400 K are shown in Figure 2. The 1250 K spectrum has a narrow base and shows resonances from the four types of magnetically nonequivalent carbon atoms expected in anthracene. ESR measurements on this sample, shown in Table 1, gave an unpaired electron spin count of 3.5 × 1017 spins/g. These unpaired electrons act as relaxation sinks and shortened the proton T1 value to 17.6 s. The 1300 K sample has a
similar NMR spectrum but with a slight broadening around the base of the peak, especially on the higher shift side of the line shape. This broadening is probably from the creation of a few biaryl linkages between anthracene molecules or from a small amount of soot that is starting to form. The unpaired spin concentration for this sample (see Table 1) has risen to 90 × 1017 spins/ g, and the proton T1 has been reduced to 3.4 s. The shortening of the proton T1 values for these two samples allowed shift tensor measurements to be made on these samples. Previous attempts in this laboratory to measure the shift tensor principal values of pristine anthracene have always failed because of a very long proton T1. No previous reports of anthracene principal values have appeared in the literature. The shift tensor principal values of “anthracene” have been measured for both the 1250 K sample and the 1300 K sample by the FIREMAT12 spinning sideband analysis procedure. There are two different ways to implement the FIREMAT method. One method can fit the whole 2-D data set with all four tensors included at the same time (shown in Figure 3) or one can use TIGER13 processing and extract a sideband pattern from each resonance that was fitted in the isotropic spectrum (shown in Figure 4). If one wanted to extract an “average tensor” or “average tensors” for an inhomogeneously broadened amorphous carbonaceous solid, then the whole sample must be fitted simultaneously because the TIGER procedure would be impossible. The average deviation (highest value-lowest value) for both data sets by both methods was about 1.3 ppm, with no range of principal values for any component spread larger than 4 ppm between the four data sets. The principal values from fitting the whole 2-D data set of the 1250 K sample are given in Table 2, along with theoretical values. The measured principal values are in agreement with other measurements21,22 and calculations23 on polycyclic (21) Sherwood, M. H.; Facelli, J. C.; Alderman, D. W.; Grant, D. M. J. Am. Chem. Soc. 1991, 113, 750. (22) Carter, C. M.; Alderman, D. W.; Facelli, J. C.; Grant, D. M. J. Am. Chem. Soc. 1987, 109, 2639.
Study of Anthracene Aerosols by Solid-State NMR and ESR
Energy & Fuels, Vol. 17, No. 3, 2003 741
Table 3. The Structural and Lattice Parameters for the 1400 K Extract and Residue Samples Structural Parameters compound
fa
anthracene 1400 K extract 1400 K residue
1.00 1.00 1.00
f
C a
0.00 0.00
fa′ 1.00 1.00 1.00
f
H a
0.71 0.60 0.22
f
N a
0.29 0.40 0.78
f
P a
0.00 0.00
f
S a
f
0.00 0.09 b
B a
0.29 0.31 0.78
fal 0.00 0.00 0.00
f
H al
f
0.00 0.00 0.00
/ al
0.00 0.00 0.00
Lattice Parameters compound
χb
C
σ+l
P0
B.L.
S.C.
anthracene 1400 K extract 1400 K residue
0.286 0.310 0.780
14 15 125a
0.0 1.4 0.0b
0.0 1.0 0.0
0.0 1.0 0.0
0.0 0.0 0.0
a This is a maximum value assuming all nonprotonated aromatic carbons are bridgeheads. b Not detected due to line broadening in the main aromatic peak.
Figure 3. The fitted spectrum, the experimental spectrum, and the residual of the fit for the 1250 K anthracene soot. The sideband pattern in this figure is the composite of the four sideband patterns in Figure 4.
aromatic hydrocarbons. The important δ33 value from the bridgehead tensor of -6.9 ppm is typical of cata- or linear-condensation2 and is similar to that found in naphthalene21 (-5.9 ppm) and the outer bridgehead in pyrene (-7 ppm).22 The position of this component gives information on the aromatic ring size. The δ33 component of the inner bridgehead in pyrene is -18 ppm22 and the inner (hub) bridgehead in coronene is -38 ppm.24 The bridgehead tensors reach their limit in graphite with values of δ11 ) δ22 ) 182 and δ33 about -290 and -325 for the two different crystallographic (23) Facelli, J. C.; Nakagawa, B. K.; Orendt, A. M.; Pugmire, R. J. J. Phys. Chem. A 2001, 105, 7468. (24) Orendt, A. M.; Facelli, J. C.; Bai, S.; Rai, A.; Gossett, M.; Scott, L.; Boerio-Goates, J.; Pugmire, R. J.; Grant, D. M. J. Phys. Chem. A 2000, 104, 149.
Figure 4. The isotropic spectrum (left) and the corresponding spinning sideband pattern for each isotropic resonance (right) for the 1250 K anthracene aerosol.
sites.25 The shielding tensor for graphite is a function of the very large anisotropic susceptibility found in graphite.25 Inclusion of five-membered rings in a graphite-like structure can add curvature, and this can move the δ33 components to higher chemical shift values as is observed in coranulene (6 and -10 ppm, for the inner and outer bridgeheads, respectively).24 One should also note that the fitted intensities of the individual experimental resonances are close to their theoretical values. The nonprotonated bridgehead carbon intensity is 5.6% higher than the theoretical value (4/14), while the intensity of the three protonated carbon values are slightly lower (2.5%, 3.2%, and 1.4%) than the ideal intensities (4/14, 4/14 and 2/14); this suggests that, in the latter case, proton decoupling may not have been adequately achieved. (25) Kume, K.; Hiroyama, Y. Solid State Commun. 1988, 65, 617.
742
Energy & Fuels, Vol. 17, No. 3, 2003
In Figure 2a, one observes a significant change in the spectrum of the 1400 K sample. The peak has a very broad base, similar to those observed in other aerosols,1 while the fwhh is about the same as the that of the two lower-temperature samples but with slightly different chemical shifts of the individual peaks. This sample was extracted with DCM and 9.5% was found in the soluble fraction. CPMAS spectra of the extract (tar) and residue (soot) are also shown in Figure 2, and ESR and proton relaxation data are given in Table 1. The residue is a very broad, featureless resonance. The broadening probably comes from the anisotropy in the magnetic susceptibility (as noted above for graphite) in large extended ring systems and is not removed by MAS.26,27 The extract spectrum is narrow like those of the two lower-temperature samples but with a clear shoulder at about 138 ppm on the downfield side of the region of aromatic resonance. One should notice that there are 3 orders of magnitude difference in the unpaired electron spin concentration between the residue (soot), 1.1 × 10 20 spins/g, and the extract (tar), 1.2 × 10 17 spins/g. The residue has a graphite factor5 of 0.47, indicating that the unpaired spins are dominated by electrons in conduction bands. These types of electrons seem not to be effective in H reducing T 1F so the long value of 52 ms is not unexpected. Normal model organic compounds without unH paired spins may also have very large relative T 1F H values. The 1400 K residue has a T 1 value of 95 ms, showing at least some correlation between T H 1 and the total spin concentration. This value is similar to that found in coals.2 Dipolar dephasing experiments on this sample indicated that 78% of the carbons were nonprotonated. If all of these nonprotonated carbons are bridgeheads, then the cluster size model of Solum et al.2 predicts an average aromatic cluster size of about 125 carbons per cluster. The unpaired spins in the 1400 K extract is 3 orders of magnitude less than that of the residue, and analysis of the proton spin-lattice relaxation data requires a two-component fit of 2.2 s (54%) and 9.8 s (46%). The H value was 11 ms, which is much shorter than the T 1F 52 ms value observed in the residue. The extract, unlike the residue, exhibits no conductive behavior. These results suggest that the electrons in the extract are primarily static and thus more effective in shorting H . T 1F As can be seen from the spectrum in Figure 2, no aliphatic material was observed in this sample. This implies that the shoulder, as discussed above, at about 138 ppm arises primarily from biaryl linkages. This feature also implies that there was no protonation of the aromatic rings making hydroaromatic structures as proposed by Badger in the conversion between anthracene and phenanthrene.9 Using the standard 1-D analysis work-up utilized by this laboratory for the analysis of coals,2 one estimates that the average aromatic cluster size of the extract is 15 carbons per cluster (within experimental error of the (26) Freitas, J. C. C.; Emmerich, F. G.; Cernicchiaro, G. R. C.; Sampaio, L. C.; Bonagamba, T. Solid State Magn. Reson. 2001, 20, 61. (27) VanderHart, D. L.; Earl, W. L.; Garroway, A. N. J. Magn. Reson. 1981, 44, 361.
Solum et al.
14 carbons in anthracene) with an average of 1.4 attachments per cluster. This is consistent with previous work7 that showed anthracene molecules connected together by either one or two biaryl linkages. The parameters from the standard 1-D analysis procedure are given in Table 3 along with those from an anthracene molecule and a partial set for the 1400 K residue. Conclusions Carbon-13 solid-state NMR has been successfully used to study aerosols and aerosol extraction products made from anthracene in a drop-tube furnace. This set of samples was carbonized to significantly different degrees. The 1250 K aerosol exhibited essentially no carbonization, but the creation of a few unpaired electron relaxation sinks shortened the spin-lattice relaxation time sufficiently to allow the measurement of the chemical shift tensor principal values in “anthracene”. The 1400 K aerosol was a composite of a tarlike material and a highly carbonized soot fraction. The tar fraction obtained by DCM extraction exhibited the presence of a significant number of substituted aromatic carbons and, since there was no aliphatic material observed in the spectrum, these substituted aromatic carbons must arise from biaryl linkages. This sample is thought to reflect the early stages of soot formation and shows the importance of biaryl linkages at the start of soot formation from aromatic precursors. The soot (residue) fraction from DCM extraction was conductive (graphite factor of 0.47) and had an upper limit on the aromatic cluster size of about 125 carbons per cluster. Acknowledgment. This work was supported as part of the Center For Simulation of Accidental Fires and Explosions (C-SAFE) by the Department of Energy through the Academic Strategic Alliance Program by means of a subcontract (B341493) from Lawrence Livermore National Laboratory, The Department of Energy Basic Energy Sciences through Grant DE-FG03-9414452, and the National Science Foundation under CRAEMS Grant CHE 0089133. The authors acknowledge an allocation of computer time on the Center for High Performance Computing systems. Appendix Definitions of the Structural and Lattice Parameters from the Standard 1-D Analysis Procedure. Nomenclature Structural Parameters fa ) fraction of carbon atoms that are sp2 hybridized (aromaticity) f Ca ) fraction of carbon atoms that are in carboxyl or carbonyl groups fa′ ) fraction of carbon atoms that are sp2 hybridized excluding f Ca (corrected aromaticity) fH a ) fraction of carbon atoms that are protonated aromatics fN a ) fraction of carbon atoms that are nonprotonated aromatics
Study of Anthracene Aerosols by Solid-State NMR and ESR f Pa ) fraction of carbon atoms that are aromatic with an oxygen atom attached f Sa ) fraction of carbon atoms that are aromatic with a carbon chain attached (also includes biaryl carbons) f Ba ) fraction of carbon atoms that are aromatic and a bridgehead carbon fal ) fraction of carbon atoms that are sp3 hybridized (aliphatic) fH al ) fraction of carbon atoms that are aliphatic and protonated but not methyls fal ) fraction of carbons that are aliphatic and methyls or nonprotonated
Energy & Fuels, Vol. 17, No. 3, 2003 743 Lattice Parameters χb ) mole fraction of bridgehead carbon atoms C ) average aromatic cluster size σ+1 ) average number of attachments on an aromatic cluster P0 ) fraction of attachments that do not end in a side chain (methyl group) B.L. ) average number of attachments on an aromatic cluster that are bridges or loops (A loop is a bridge back to the same cluster.) S.C. ) average number of side chains on an aromatic cluster EF020216H
