J. Phys. Chem. 1988, 92, 561-563 (in solids) and presence (in solution) of the C6hydrogen abstraction product. In two of the aqueous studies, the oxidation was by chemical means (OH attack12 and SO:- attack15). (Because the C6abstraction product was found in the HPLC plus y-irradiation ~ t u d i e s , ' ~itJ may ~ be inferred that OH attack was responsible for the oxidation in that case also. The absence of the C, abstraction product may be an artifact of the HPLC procedure.) In those cases, it was necessary for the reacting groups to approach sufficiently close to the site of attack. The geometric nature of the molecule in solution may have prevented suitable approach at other sites. On the other hand, in the crystals, the oxidation was by means of X-ray photoionization, for which approach was no problem. Thus, in the crystals, the results may be interpreted as reflecting the tendencies of the ionized amino acid molecules, while, in the spin-trapping studies, the results reflect the tendencies of the molecules toward chemical oxidation. In an amino acid in which there is no extensive *-electron system over the entire molecule (such as proline derivatives), it is reasonable to expect electron loss to lead to more than one type of product. Specifically, the electron-loss event occurs at random and creates a hole in the electronic system; subsequently, the electronic system undergoes rearrangement to establish the lowest energy configuration. However, when there is little delocalization,
561
there are relatively independent electronic regions within the molecule. While electronic rearrangement can occur within these regions, it is possible that it will not extend beyond them. (An example of this is the case of purine and pyrimidine nucleosides. Although linked by the glycosidic bond, the sugar and purine or pyrimidine respond as independent entities to electron loss.) Consequently, the variety of radicals formed will reflect the number of these regions and the probability of ionization within them (essentially, their valence electron density). The formation of radicals I and I1 as primary products in this study reflects this behavior. In contrast, electron attachment is the result of attraction by susceptible sites within the molecule and thereby can be expected to be more selective. Evidence for this behavior is clear in the case of proline derivatives since the deamination radical, which results from electron attachment to the carboxyl group, appears to occur universally (see Table V of ref 4). Acknowledgment. Support for this work was provided by the Department of Energy under Contract DE-AS05-83ER60159, and by N I H under Grant CA36810. That support is gratefully acknowledged. Registry No. Hydroxyproline, 5 1-35-4.
COMMENTS Comment on the Proposed Role of Spheroidal Carbon Clusters In Soot Formation Sir: Formation of spheroidal carbon clusters, initially reported to be observed under the conditions of laser vaporization of graphite,' has been also advanced to explain the formation and morphology of soot in hydrocarbon Zhang, O'Brien, Heath, Liu, Curl, Kroto, and Smalley (ZOHLCKS) proposed* that primary spherical soot particles, being produced rapidly in flame environment and reaching typically an average diameter of 20-40 nm,5*6are formed by a spheroidal shell growth of partially closed carbon clusters. This model, therefore, implies that prior to the onset of aggregation of the spherical particles into "chains" or "beads", their growth is independent of each other; i.e., no coagulation takes place. The experimental evidence that coagulation is an important element of soot formation process is well-d~cumented.~-'~ One (1) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature (London) 1985, 318, 162-163. (2) Zhang, Q.L.; O'Brien, S. C.; Heath, J. R.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J . Phys. Chem. 1986, 90, 525-528. (3) Kroto, H. W. In Polycyclic Aromatic Hydrocarbons and Astrophysics; Ltger, A., d'Hendccourt, L.; Boccara, N., Eds.; Reidel: Dordrecht, Holland, 1987; pp 197-206. (4) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Astroohvs. J . 1987, 314. 352-355. (5) Phmer, H. B.; Cullis, C. F. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.;Marcel Dekker: New York, 1965; Vol. 1 , pp 265-325. (6) Haynes, B. S.; Wagner, H.Gg. Prog. Energy Combust. Sci. 1981, 7 , 229-273. (7) Howard, J. B.; Wersborg, B. L.; Williams, G. C. Faraday Symp. Chem. SOC.1973, 7 , 109-119. (8) Graham, S.C. Proc. R . SOC.London, A 1981, 377, 119-145. (9) Wagner, H G g . In Soot in Combustion Systems and Its Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983; pp 171-195. (10) Prado, G.; Lahaye, J. In Particulate Carbon: Formation during Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1983; pp 143-164. (1 1) Dobbins, R. A.; Mulholland, G. W. Combust. Sci. Technol. 1984, 40, 175-191. ~~
~
~
may argue, however, that the partially closed carbon clusters may collide and adhere to each other, Le., coagulate, while their open shells continue to grow forming layers around the adducts. In fact, such an explanation would be in line with the observation that soot particles have multiple centers surrounded by concentric carbon 1 a ~ e r s . lThus, ~ the first question is how fast the spherical carbon clusters proposed by ZOHLCKS can grow. Recently, a detailed chemical reaction mechanism for the formation of polycyclic aromatic hydrocarbons (PAH) under flame conditions has been propo~ed.'~-'' The principal reaction pathways, identified by extensive computer experiments, appeared to be those leading to the formation of particularly stable PAH, like acenaphthylene, pyrene, coronene, etc., via a sequence of alternating reactions: acetylene addition to aromatic radicals and H abstraction by hydrogen atoms from forming aromatic molecules. This kinetic mechanism, developed for PAH growth, can also accommodate the formation of the spheroidal shells including the proposed C60cluster, buckminsterfullerene.' Thus, for instance, instead of H abstraction from position 5 or 6 of acenaphthylene leading to the formation of acephenanthrylene and pyrene,lsa the abstraction of an H atom from position 1 or 2 will produce, via the same sequence of reactions, fluoranthene and benzo[ghi](12) Harris, S.J.; Weiner, A. M.; Ashcraft, C. C. Combust. Flame 1986, 64, 65-8 1 . (13) Bockhorn, H.; Fetting, F.; Meyer, U.; Reck, R.; Wannemacher, G. Symp. (Int.) Combust., [Proc.] 1981,18, 1137-1 146. Bockhorn, H.; Fetting, F.; Wannemacher, G.; Wentz, H. W. Symp. (Int.) Combust., [Proc.] 1982, 19, 1413-1420. Bockhorn, H.; Fetting, F.; Heddrich, A,; Wannemacher, G. Symp. (Znr.) Combust. [Proc.] 1985, 20, 979-988. Bockhorn, H.; Fetting, F.; Heddrich, A. Symp. ( f n t . ) Combust., [Proc.],in press. (14) Lahaye, J.; Prado, G. In Particulate Carbon: Formation during Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1983; pp 33-51. (15) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S.E. Symp. ( I n t . ) Combust., [Proc]: (a) 1985, 20, 887-901; (b) in press. (16) Frenklach, M.; Clary, D. W.; Yuan, T.; Gardiner, W. C., Jr.; Stein, S. E. Combust. Sci. Technol. 1986, 50, 79-101. (17) Frenklach, M.; Warnatz, J. Combust. Sci. Technol. 1987, 52, 265-283.
0022-365418812092-0561$01.50/0 0 1988 American Chemical Society
562
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
Comments
tagonal ring containing structures, the discussion presented below fluoranthene; and indeed, the latter chemical species were idenis based on the model compound buckminsterfullerene. tified, along with planar PAH, in a number of experimental flame If we assume that the coagulation takes place between spherical studies.1s-20 Benzo[ghi]fluoranthene is the first member of a c 6 0 clusters with a diameter of 700 pm34 (or comparably sized homologous series of “bent” or “shell” polycyclic aromatic hydrocarbons. Its growth, as well as that of other shell molecules spirals), that the intracluster separation in the forming particles formed in a similar manner, can be envisioned as a sequential is that of graphite, Le. 336 pm,35 and that the c60 clusters in buildup of benzene and acenaphtha rings according to the proposed particles are arranged according the dense packing structure, the density of such carbon would be about 1.5 g/cm3, which is sigmechanism.I x2 In other words, we can run the computer code with the nificantly lower than 1.8-2.0 g/cm3,-the density of soot materia1.36,37Further structural considerations are presented below. mechanism of PAH growth15 to describe the formation of the Analysis of the diffraction patterns of soots and carbon blacks partially closed carbon clusters by simply assigning different species directly suggests the presence of stacked turbostratic benzenoid names to concentration variables of the model and changing the array^.^ Warren and Bodenstein interpreted the pattern for a values of some of the rate parameters. The most affected will probably be the rates of planar-to-bent transitions proceeding via carbon black in terms of a benzenoid structure as follows.38 The three (hM) peaks expected for a hexagonal network of bond length acetylene addition and cyclization reactionsI5-their net rates will 139-142 pm are seen: the (100) near 213 pm, the (110) near 123 be significantly lower than those of the corresponding planar pm, and the (200) near 107 pm. Detailed line width analysis reactions. The reason for that is the larger reverse rate coefficients indicates the crystallite size of the benzenoid network (denoted due to the lower thermodynamic stability of the bent molecules La, the length of a translationally periodic array of hexagons) to compared to that of planar PAH.22%23 The results of computer be of order 2-3 nm. The (002) and the (004) peaks, indicating modeling for the PAH growth revealed high sensitivity of the interference between parallel (or approximately parallel) aromatic reaction flux and the reaction pathway to the rate coefficients of networks, are observed, with crystallite sizes in the direction of these reaction For example, a lower stability of stacking (L,) of 1.4-1.7 nm. In studying a combustion tube soot ortho- and peri-jiused (cata-condenseCP5) PAH compared to that from diesel fuel, (loo), (1 lo), (002), and (004) diffraction peaks, of ortho-fused (strictly peri-conde~se8~) PAH results in that the reaction flux via the latter species dominates the g r ~ w t h . ’ ~ ~ , with ~ ~ La = 2 nm and L, = 2 nm, were observed.39 Similar results Thus, following the above arguments-lower thermodynamic have been reported by Stevenson for particulate emissions from stability and, hence, lower molecular fluxes for the bent indirect injection diesel engines.40 Electron microscopy supports the benzenoid array diffraction i n t e r p r e t a t i ~ n , ~with ~ , ~further ~ structures-the growth via shell structures should be much slower finding that the benzenoid layers are bent. The crystallite size than that via planar ones. Certainly, the formation rate of the La may be smaller than the layer length of a molecular sheet as latter provides an upper limit for the rate of the former. However, seen in m i c r o ~ c o p y . ~ ~ the growth of planar PAH at flame conditions, as described by It is evident, therefore, that results of diffraction are consistent the kinetic simulation,21 produced molecules with an average with a model of soot as primarily benzenoid arrays. To determine number of carbon atoms on the order of tens, in accord with aspects of the diffraction patterns of clusters, Debye internal experimental This implies, therefore, that interference was used to simulate the intramolecular diffraction molecular growth alone-via bent or even planar PAH-cannot explain soot particle formation in flame environment. On the other peaks of a c 6 0 cluster of diameter 698 pm of bond lengths 137.6 hand, inclusion of coagulation (and surface growth) into the kinetic and 146.5 pm. (Details of the approach have been covered model discussed above brings the average size of “young” soot elsewhere$2 and a simulation of a c 6 0 cluster of bond length 154 pm has been ~ r e s e n t e d . ~The ~ ) major diffraction peaks appear particles to the order of thousands carbon again in Thus, if we even assume agreement with experiment.s-7,12,13,1g,31-33 at 487, 285, 194, 167, 108, and 82 pm, with no intramolecular that bent structures grow with upper level rates but without interference occurring at 698/2 = 349 pm; there is thus a lack of agreement between the experimentally observed diffraction coalescence the formed molecules, whether closed shell or spiralling, will contain comparable number of carbon atoms to c 6 0 patterns and those predicted for a c 6 0 cluster. By invoking the clusters. To capture features of the growth via nonplanar penspiral cluster structure and assuming that neighboring networks are separated by ca. 350 pm, one might generate (002) and (004) peaks, but it is evident that the “intramolecular” diffraction of (18) Prado, G. P.; Lee, M. L.; Hites, R. A,; Hoult, D. P.; Howard, J. B. a curved non-benzenoid network is distinct from that of a planar Symp. (In!.) Combust, [Proc.] 1977, 16, 649-659. benzenoid network. Thus, the presence of five-membered rings, (19) Bittner, J. D.; Howard, J. B. Symp. (Int.) Combust., [Proc.] 1981, necessary for curvature in the Ca cluster or the spiralling structure, 18, 1105-1 116; In Soot in Combustion Systems and Its Toxic Properties; Lahaye, J., Prado, G., E&.; Plenum: New York, 1983; pp 109-137. Howard, breaks down the symmetry of the benzenoid (totally hexagonal) J. B.; Bittner, J. D. In Particulate Carbon: Formation during Combustion; network, and there is a lack of agreement between experimental Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1983; pp 57-91. diffraction results and predicted diffraction patterns for a curved, (20) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys. pentagon-containing network. Furthermore, assuming the spiral Chem. 1983, 87, 1067-1073. (21) Frenklach, M. Chem. Eng. Sei. 1984,40, 1843-1849. structure can lead to (002)/(004) peaks, one notes that the (22) Stein, S. E.; Fahr, A. J . Phys. Chem. 1985, 89, 3714-3725. crystallite sizes La and L, would be interrelated in a spiral structure, (23) Hites, R. A.; Simonsick, W. J., Jr. Calculated Molecular Properties unlike the situation in the turbostratic benzenoid stack. of Polycyclic Aromatic Hydrocarbons; Elsevier: Amsterdam, 1987. In part, ZOHLCKS proposed soot to be composed of clusters (24) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S.E. In Shock Waves and Shock Tubes; Bershader, D., Hanson, R., Eds.; Stanford because, as they wrote,2 of dehydrogenation reactions occurring
University Press: Stanford, CA, 1986; pp 295-301. (25) Dias, J. R. Handbook of Polycyclic Hydrocarbons. Part A. Benzonoid Hydrocarbons; Elsevier: Amsterdam, 1987. (26) Frenklach, M.; Clary, D. W.; Ramachandra, M. K. ‘Shock Tube Study of the Fuel Structure Effects on the Chemical Kinetic Mechanisms Responsible for Soot Formation. Part II,”, NASA Report CR 174880, NASA-Lewis Research Center, 1985. (27) Thomas, A. Combust. Flame 1962, 6, 46-49. (28) Homann, K. H. Symp. (Int.) Combust. [Proc.] 1985, 20, 857-870. (29) Frenklach, M.; Harris, S. J. J . Colloid Interface Sci. 1987, 118, 252-262. (30) Frenklach, M. In Modelling of Chemical Reaction Systems; Warnatz, J., Ed.; Springer-Verlag: Heidelberg, in press. (31) Prado, G.; Jagoda, J.; Neoh, K.; Lahaye, J. Symp. ( I n t . ) Combust. [Proc.] 1981, 18, 1127-1 135. (32) Flower, W. L. Combust. Sci. Technol. 1983, 33, 17-33. (33) Santoro, R. J.; Semerjian, H. G.; Dobbins, R. A. Combust. Flame 1983, 51, 203-218.
(34) Walker, P. H., Jr. Nature (London) 1957, 180, 1184-1185. (35) Heath, J. R.; O’Brien, S.C.; Zhang, Q.;Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smalley, R. E. J . Am. Chem. soc. 1985, 107, 1779-7790. (36) CRC Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Astle, M. J., Eds.; CRC: Boca Raton, FL, 1980; p B-6. (37) Nishida, 0.; Mukohara, S. Combust. Sci. Technol. 1983, 35, 157-173. ( 3 8 ) Warren, B. E.; Bodenstein, P. Acta Crystallogr. 1965, 18, 282-286. ( 3 9 ) Ebert, L. B.; Scanlon, J. C.; Clausen, C. A. Prep. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1987, 32(3), 440-447. (40) Stevenson, R. Carbon 1982, 20, 359-365. (41) Ban, L. L.; Hess, W. M. In Petroleum Derived Carbon; Deviney, M. L., O’Grady, T. M., Eds.; American Chemical Society: Washington, DC, 1976; ACS Symp. Ser. No. 169, pp 358-377. (42) Ebert, L. B.; Scanlon, J. C.; Mills, D. R. Liq. Fuels Technol. 1984, 2, 257-286.
J. Phys. Chem. 1988, 92, 563-564
at 1400-1700 K, causing polycyclic aromatic molecules to adopt pentagonal rings in preference to leaving dangling bonds. Implicit in their assumption is the low concentration of hydrogen at those conditions. However, hydrogen in various chemical forms (H, Ha, H,O, ...) is most abundant in hydrocarbon flames. The issue is not the hydrogen content but the kinetics and thermodynamics of the reaction network discussed above. In terms of the product soot, the spiralling cluster model allows for a low H / C ratio. Can we reconcile the benzenoid model for soot with the atomic H / C ratios of 0.1-0.2 typically found in "old" soots?5 A hexagonal lattice crystallite of edge 2 nm will have a periphery of 8 nm and an area of 3.46 nm2. Assuming the hydrogen edge density of acenes 245 pm/H and the graphitic carbon density 0.0260 nm2/C, we have H / C = 0.245 (32.6 H/133 C). If, however, one benzenoid array (Le., continuous covalent network) were composed of two crystallites, we have H / C = 0.18, and so on. Since microscopy shows that crystallite sizes, as determined by diffraction, are smaller than molecular arrays seen in microscopy, we see that the benzenoid array model can reconcile diffraction, microscopy, and microanalytical findings. Many models have been proposed which link benzenoid arrays to spherical m o r p h ~ l o g y ~and , ~ ,thus ~ ~ there is no compelling need to invoke spherical clusters to account for the structure of soot on a molecular, crystallite, or particle level. Although C6,-type structures proposed by ZOHLCKS may indeed be formed under certain conditions's4 including flames$5 the arguments presented in this paper, both kinetic and structural, indicate that the importance of these structures is very unlikely in soot formation. Acknowledgment. The work at Penn State was supported by the Aerothermochemistry Branch of NASA-Lewis Research Center, Grants NAG 3-477 and NAG 3-668. Registry No. C, 7440-44-0. (43) Donnet, J. B. Carbon 1982, 20, 266-282. (44) Iiiima. S.J. Phvs. Chem. 1987. 91. 3466-3467. (45) Gerhardt, Ph.; Loffler, S.;Homann; K. H.Chem. Phys. Lett. 1987, 137, 306-310.
Fuel Science Program Department of Materials Science and Engineering Pennsylvania State University University Park, Pennsylvania 16802 Exxon Corporate Research Annandale, New Jersey 08801
Michael Frenklacb*
Lawrence B. Ebert
Received: July 16, 1987; In Final Form: September 10, 1987
Ultraviolet Spectrum of Chlorine Perchlorate Sir: Chlorine perchlorate (C10C103) was synthesized in 1970 by Schack and Pilipovich,' who determined its principal physical properties including the infrared spectrum. This substance is also produced with good yield during the chlorine dioxide photolysis.24 Gaseous chlorine perchlorate in Pyrex or quartz containers at room temperature shows some instability and also proves to be very sensitive to traces of humidity. Consequently, its UV (1) Schack, C. J.; Pilipovich, D. Inorg. Chem. 1970, 9, 1387. Christe, K. 0.;Schack, C. J.; Curtis, E. C. Inorg. Chem. 1971, 10, 1589. (2) Jubert, A. H.; Sicre, J. E.;Schumacher, H. J. Presented at the 2nd Congreso Argentino de Fisicoquimica, Carlos Paz, C6rdoba, Argentina, Sept 1-5, 1980; paper 83. (3). Schell-Sorokin, A. J.; Bethune, D. S.; Lanckard, J. R.; Loy,M. M. T.; Sorokin, P. P. J. Phys. Chem. 1982, 86, 4653. (4) Barton, R. A.; Cox, R. A. Wallington, T. J. J. Chem. SOC.,Faraday Trans. 1 1984, 80,2731.
0022-3654/88/2092-0563$01.50/0
563
CI OCIO,
b
Q
I,,,,,\-
,
~
2 50
300
350
WAVELENGTH l n m l
Figure 1. Ultraviolet spectrum of chlorine perchlorate.
spectrum, free from strong UV absorbers like C102 and ClO,, is difficult to obtain. This spectrum was unknown, and its knowledge was necessary for us since large amounts of ClOClO, were produced in our experiments on the Cl02 photoly~is.~After careful handling we were able to obtain this spectrum, and as this information is necessary for a correct interpretation of some chlorine oxide reactions$,6 it seems advisable to publish it. In a quartz cylinder reactor (4 = 5 cm; I = 5 cm) 100 Torr of C102 a t 30 OC was photolyzed (A = 436 nm) until it was practically consumed. (The C10C103 yield is approximately 25%.) The low vapor pressure products, essentially C103and the dimer C1206,were eliminated by fractional condensation at -35 OC in a U-form low-volume trap connected between the reactor and the stopcock. The volatile chlorinated compounds Cl,, C102, and C10C103 were condensed in a Pyrex trap at liquid air temperature. In this trap the products of six other batches were put together. Purification was made by eliminating firstly C1, at -100 OC and secondly the small amount of C102 at -60 OC. This last part of the distillation was carefully carried out and controlled by UV spectrophotometry until practically no C102bands were detected. Large amounts of CIOCIO, were lost (of the order of 50%) because of the low vapor pressure ratio [p,(C102)/pv(C10C10,)], oc = 8.7/2.8. Pure C10C103, as a pale greenish liquid, remains at the bottom of the trap. The UV spectrum (Figure 1) was obtained at 30 "C with a Cary 14 spectrophotometer (10-cm quartz cell length). The line represents the average of four runs obtained with different samples. The ClOClO, gas pressures were 1.9, 4.3, 5.1, and 7.7 Torr. At = 234 nm, the values for In (Io/I)are 0.624, the wavelength ,A, 1.184, 1.444, and 2.372, respectively, and the calculated (Beer's cm2 law) absorption cross section is 6234nm = (94 f 7) X molecule-'. In the wavelength range 210-280 nm the error is 10% (maximum-minimum values), and it is caused by the pressure measurement uncertainty. The spectra show that CIOClO, is practically free from ClO,, ClO,, and C1207(maximum limits of impurities OS%, 1.5%, and 0.1%, respectively). However, since beyond 280 nm the absorption of the residual C10, has been deducted, the error naturally increased, and at wavelengths larger than 300 nm the reported cross section values only have a qualitative meaning. The purity of the CIOClO, was checked by IR spectrophotometry and vapor pressure measurements.' The results agree with the quoted values.' Because of the C10C103 instability, the spectra were recorded as fast as possible. For instance, with a sample of 7.7 Torr, decomposition products (mainly C10,) are noticeable after a few minutes and in an hour 8% is decomposed. Our results confirm, to a certain degree, the assignment that
( 5 ) Lopez, M. I.; Sicre, J. E.; Schumacher, H. J., to be published. (6) Molina, L. T.; Molina, M. J. J. Phys. Chem. 1987, 91, 433.
0 1988 American Chemical Society
