696 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al. TABLE 1: Average Raman D/G Area Ratios (%) of Degassed SWNTs Subjected to Peroxides peroxide D/G (633 nm) D/G (780 nm) no. samples nonea 8.3 13.8 2 BP 14.4 19.6 5 p-MeO-BP 8.3b 12.5c 4, 2d PhP 26.7 30.9 5 TFAP 6.1 9.1 3 a Blank consisted of purified SWNTs stirred in o-DCB at 80 °C for 1 h then filtered. b A fifth sample gave D/G )17.2. c A third sample gave D/G )24.9. d Four measurements at 633 nm and two at 780 nm. TABLE 2: Average Atomic Percent of Elements in SWNTs Exposed to Peroxidesa peroxide %C %O %Cl %F no. samples none 93.3 3.2 3.6 0 2 BP 93.6 3.9 1.6 0.9 5 p-MeOBP 91.4 5.2 2.4 1.0 8 PhP 85.6 12.7 1.7 0 5 TFAP 92.4 4.4 1.2 2.0 5 a The data were obtained on 1-5 mg degassed SWNT samples exposed to amounts of peroxide ranging from 0.15 to 0.6 mmol.Figure 2. Effect of added SWNTs on the thermolysis of phthaloylperoxide (PhP) at 70 °C (solid triangles) and 80 °C (open triangles)and on the thermolysis of trifluoroacetyl peroxide (TFAP) at 40 °C To determine whether the reaction of SWNTs with BP could(solid diamonds) and 50 °C (open diamonds) for 1 h. be induced photochemically, we irradiated an o-DCB suspension with a 500-W quartz-halogen lamp through a water filter. Nofrom o-DCB to nitrobenzene (NB) to suppress the solvent- change in rate was noticed, hardly a surprising result in viewinduced decomposition of TFAP.22,23 In NB solvent at 50 °C, of the very short lifetime of excited SWNTs.25 In contrast, it7% of TFAP was consumed in 1 h; but with 5 mg SWNTs, was reported recently that visible light irradiation of SWNTsthis figure rose to 75%. Because further experiments revealed with hydrogen peroxide caused the disappearance of theirunexpected reactions with NB, it was necessary to conduct the characteristic near-infrared fluorescence.8remaining studies in o-DCB and to simply tolerate the ther-molysis of TFAP in the blank run. We found that rapid addition Product Characterizationof TFAP to o-DCB without SWNTs gave more peroxide Although the product of phenyl radical attack on SWNTsdecomposition (25-35%) than dropwise addition (15-20%) has been characterized already,4,5 we analyzed the products ofThis effect may be due to autoinduction caused by high local our peroxides with SWNTs by Raman and XPS spectroscopy.TFAP concentrations because the droplets of TFAP did not These methods were employed because SWNTs do not allowimmediately dissolve in o-DCB. The problem was minimized us the luxury of using some of the powerful analytical techniquesby dropwise addition of TFAP, in contrast to the other three applicable to the far more soluble C60.26,27peroxides, which were added all at once as solids. A few of the Raman D/G area ratios are often determined to ascertain theperoxide + SWNT rate experiments were repeated with a degree of SWNT functionalization.3,12,28,29 Such measurementsdifferent batch of SWNTs (no. 162-1). These runs showed less were carried out here using laser wavelengths of 633 and 780peroxide consumption for a given weight of SWNTs but nm on SWNT sample no. 162-1 recovered from degassedotherwise the same trends as in Figures 1 and 2. titration runs, as summarized in Table 1 and shown in more The rate acceleration seen on addition of SWNTs to peroxides detail in the Supporting Information. While 780 nm probesmight be attributed to traces of iron left in the purified SWNTs. mainly semiconducting SWNTs, 633 nm begins to see metallicHowever, such an explanation is unlikely at the outset because tubes as well. In accord with the literature on BP, we observethese SWNTs had been subjected to multiple oxidations during an increase in the Raman D band. The largest increase appearspurification17 and they contained only ∼1.5% Fe by TGA with PhP, whose high D/G ratio is comparable to other SWNTanalysis. Moreover, any residual Fe is encapsulated in a layer studies in the literature12,28,30 and implies that PhP stronglyof carbon so that we could never see an iron signal by XPS. functionalizes SWNTs. The D/G ratio at 633 nm versus that atControl experiments with deliberate addition of 2 mg of iron 780 nm is close to the value for the blank except for PhP.powder or 50 µg of Fe(II) or Fe(III) (as chlorides) to BP in Another approach to look for SWNT functionalization iso-DCB showed the same decomposition rate as the blank. X-ray photoelectron spectroscopy (XPS). The same degassedAlthough 50 µg is the approximate concentration of iron in the SWNT samples as in Table 1 were analyzed by XPS, yieldingSWNT samples, we also tried 2 mg of Fe(II) and Fe(III). These the results summarized in Table 2. Although different weightslarge doses of oxidized iron caused greatly enhanced the of SWNTs were exposed to varying concentrations of peroxides,peroxide decomposition rates but are unrealistic control experi- there was no correlation of elemental composition with thesements. We reason that if residual Fe is inaccessible to oxygen parameters. The most obvious effect is the high oxygenduring purification and to the X-ray beam of XPS it is surely incorporation with PhP, which will be discussed below.inaccessible to solution-phase peroxides. Although it is still Although the fluorine content was greatest for TFAP, thepossible that the rate acceleration is caused by an impurity in averages in Table 2 do not tell the whole story (cf. Supportingthe HiPco SWNTs, such an impurity would have to be a much Information for all data). Every TFAP-SWNT sample containedmore effective catalyst than Fe(II).24 ∼2% F, but several of the p-MeO-BP and BP samples showed
Reaction of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 112, No. 3, 2008 697 TABLE 3: Percent BP Consumed on Thermolysis of BP for 2 h at ∼86 °C in o-DCB SWNT type SWNT weight, mg % BP consumed none 0 25 SWNT 5 79 Ph-SWNT 5 52 C12H25-SWNT 5 53 SCHEME 1: Autoxidation of CumeneFigure 3. XPS C1s spectrum of SWNTs subjected to peroxides. Thetypical position of the bold, underlined carbons is shown on the plot. Reaction of SWNTs with t-Butoxy Radicalslittle or no F. This variability almost certainly arises because Highly reactive t-butoxy radicals33 are conveniently generatedthe smaller samples did not completely coat the Teflon filter by mild thermolysis of di-t-butyl hyponitrite (DTBHN).34 Tomembrane used to isolate reacted SWNTs.3 Therefore, part of check the reactivity of SWNTs toward t-BuO•, a suspension ofthe membrane was sometimes accidentally included in the 5 mg of SWNTs and 50 mg of DTBHN in 4-5 mL of benzeneanalysis. We are confident that only the TFAP-exposed SWNTs was heated under nitrogen at 55 °C for 2 days. The Raman Dcontained an elevated level of fluorine. The chlorine in all band was found to increase approximately 10-fold, indicatingsamples comes from the purification step, which involves HCl.17 considerable attack of radicals on the SWNTs. Repeating this experiment with 150 mg of DTBHN and 2 mg of SWNTs in The data in Table 1 indicate that PhP and BP functionalize ∼5 mL of benzene again increased the D band 10-fold to 44%SWNTs while Table 2 shows that same result with PhP, p-MeO- of the G band. In the breathing mode region, the pristine SWNTsBP, and TFAP. Apparently, Raman spectroscopy is not sensitive exhibited a group of three bands at 213, 224 (sh), and 232 cm-1,enough to detect the small amount (<1 CF3 per 100 carbons) but after treatment with DTBHN the lower frequency bandsof added groups from TFAP. A small shoulder at 291.2 eV on had greatly decreased relative to the one at 266 cm-1. Becausethe carbon 1s band supports the presence of a CF3 group.9 the frequency of these modes is inversely proportional to SWNTAlthough BP does not add enough phenyl groups to change diameter,13 it appears that t-BuO• is selective for larger diameterthe carbon content seen by XPS, we observe an increase in the SWNTs. The ratio of benzene to SWNTs is over 300; hence,Raman D/G ratio, in accord with previous reports.4-6 Peroxides t-BuO• must preferentially attack SWNTs35 or we would seeRCOOOCOR might introduce either R or RCOO groups, and no increase in the D/G ratio.the latter would lead to more oxygen in the recovered SWNTsthan in the blank (Table 2). Such an increase is obvious with The Effect of SWNTs on Cumene AutoxidationPhP but still occurs with the other peroxides, especially p-MeO-BP. The 5.2% oxygen found with p-MeO-BP can be explained Literature reports36,37 on the reaction of “reactive oxygenin part by p-methoxyphenyl radicals adding to SWNTs.12 In species” with fullerenes prompted us to determine whetherfact, a shoulder due to the methoxy carbon is clearly visible at SWNTs would behave similarly. We chose to study the286.5 eV. (cf. Figure 3). Because the XPS measurements were autoxidation of cumene, which proceeds by the chain mechanismdone on samples that were degassed and never exposed to air shown in Scheme 1.38 The experimental approach was to determine manometrically whether SWNTs would inhibit theuntil workup, there is little chance that atmospheric oxygen is AIBN-initiated uptake of gaseous oxygen by cumene in o-DCBcaptured by reacting SWNTs. In contrast, non-degassed samples at 70 °C. As shown in the lowest curve of Figure 4, AIBN withdid exhibit an elevated oxygen content and samples deliberately SWNTs in o-DCB exhibited an apparent rapid volume increaseexposed to O2 gave 8-9% oxygen by XPS. XPS of SWNTs + over 5 min, which we attribute to the rise in solvent vaporBP and p-MeO-BP showed a small peak at 289 eV attributed pressure after the reaction vessel was placed into the hot oilto carbonyl carbon but this peak was very clear with PhP (cf. bath. This rapid drop in the curve (volume increase) wasFigure 3). Additional support for the carbonyl group is found followed by a much slower decline due to nitrogen evolutionin the ATR IR spectrum of SWNTs that had been thermolyzed from AIBN. When AIBN was omitted (“SWNTs only”), thewith PhP, where we observed a distinct band centered at 1704 curve was flat after the equilibration period. The curve labeledcm-1 (half width 84 cm-1). “BHT” shows the typical behavior of an autoxidation inhibitor, SWNTs have been functionalized by various methods, among where BHT is 2,6-d-t-butylcresol. In this case, hardly anywhich are thermolysis with excess benzoyl peroxide5,31 and oxygen was taken up for the first 40 min. Once the inhibitorreductive alkylation with alkyl halides.32 To ascertain whether was exhausted, oxygen was absorbed at a rate of ∼9 mL/hr.previously functionalized SWNTs31,32 were capable of inducing SWNTs do not behave like BHT, for the curve shows noBP decomposition, we ran a set of four experiments in o-DCB inhibition period but instead shows the steady uptake of oxygenat ∼86 °C for 2 h, as summarized in Table 3. Although after equilibration. The same behavior was found for dodecylunfunctionalized SWNTs roughly tripled the percent of BP functionalized SWNTs.consumed, those bearing phenyl or dodecyl groups only doubled The SWNTs and C12-SWNTs that were re-isolated after thethe BP consumption. Clearly, functionalized SWNTs still induce attempted autoxidation inhibition showed no change in theirBP decomposition but not as effectively as SWNTs themselves. Raman spectra. It is therefore likely that they were not destroyed
698 J. Phys. Chem. C, Vol. 112, No. 3, 2008 Engel et al.Figure 4. Oxygen volume change in the autoxidation of cumene in o-DCB at 70 °C.and that SWNTs are not autoxidation inhibitors. Attributing this SCHEME 2: Electron-Transfer-Induced Decompositionnegative result to poor suspendability of SWNTs in o-DCB is of PhPnot a viable argument because such suspensions are stable formonths and they react nicely with peroxides.Discussion BP at 90 °C, p-MeO-BP at 80 °C , TFAP at both 40 and50 °C, but not PhP, exhibit a greater initial slope than the oneattained at higher amounts of added SWNT (cf. Figures 1 and2). This effect seems to be confined to peroxides that are alreadydecomposing at the experimental temperature. When the rateis high, autoinduction may contribute to the overall decomposi-tion rate and SWNTs may serve as radical scavengers, analogous C60.26 We do not know whether the ester moiety attached toto the decomposition of BP in aromatic solvents with added SWNT carbon suffers hydrolysis, remains intact, or whetherstyrene.19 Thus, at low SWNT levels there are multiple the SWNT cation is captured by adventitious water. Either ancompeting and interacting pathways that complicate and enhance ester group or an OH would account for the elevated oxygenthe overall rate. content in Table 2. The high efficiency of PhP in functionalizing We attribute the rate acceleration of peroxide thermolysis SWNTs might be due to electrostatic attraction between radicalcaused by SWNTs to electron-transfer-induced decomposition, anion 1 and SWNT+• because the corresponding reaction inwhich has been seen previously with electron-rich aromatics39 acyclic peroxides involves neutral radicals attacking SWNT+•s.and C60.26 For example, addition of 5 equiv of benzene to TFAP The SET mechanism proposed here is new for SWNTs plusor other perfluoroacyl peroxides causes rate enhancements of peroxides and is a likely contributor to earlier such studies.4-72.7 to 4.2.23 These peroxides have low-lying antibonding M.O.’s However, SET need not be the exclusive mechanism becausethat make them particularly good electron acceptors.40 Studies reactive radicals are known to attack SWNTs.6,9-12of PhP with polynuclear aromatic compounds41 and of SWNTs Thermolysis of di-t-butylhyponitrite (DTBHN) leads towith aryl diazonium salts3 also supported initial electron-transfer. radicals that add to SWNTs, as judged from the Raman spectra.We propose that SWNTs reduce peroxides to radical anions, This result is plausible because t-BuO• also attacks C60,42 thoughwhich immediately undergo O-O bond scission. Following SWNTs are in general less reactive. Because there is far moredecarboxylation, the radicals react rapidly with SWNT+• and benzene solvent than SWNTs, one might expect predominantlead to functionalization, probably via the pathway depicted in attack on benzene. However, the reactivity of t-BuO• isScheme 2 below for PhP. For simplicity, only one benzene ring diminished in benzene, possibly affecting selectivity.33 Theof SWNTs is shown. The experimental support for Scheme 2 question remains whether the attacking species is t-BuO• orconsists of an enhanced Raman D band (Table 1), an elevated methyl radicals arising from β-scission. The rate of β-scissionoxygen content (Table 2), and especially the carbonyl carbon can be calculated as 7.6 × 103 s-1 at 55 °C,43,44 ignoring anyseen by IR and by XPS in Figure 3. solvent effect. The reaction rate of t-BuO• with SWNTs is The mechanism for the acyclic peroxides, which is similar unknown, not to mention the problem that SWNT “solutions”to Scheme 2, has already been set forth by Yoshida et al. for are actually suspensions, making it difficult to know the
Reaction of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 112, No. 3, 2008 699concentration. An XPS spectrum of the DTBHN functionalized Cumene Autoxidation. Commercially available o-DCBSWNTs suggests that at least part of the radical attack is by (Fisher Scientific) and butylated hydroxytoluene (BHT) (Acros)t-BuO•. Thus, two measurements of the raw SWNTs used in were used without purification. Solutions of recrystallized AIBNthis experiment revealed 4.3% and 4.9% oxygen. SWNTs (0.1N) and BHT (0.1N) were prepared in o-DCB. Cumene wasthermolyzed with DTBHN showed 5.2% and 5.63% oxygen, a distilled from calcium hydride (50 mmHg at 70 °C). To ensuresmall but discernible increase that supports t-BuO• attack. uniformity, all experiments used the same size flask (25 mL) As seen in Figure 4, neither SWNTs nor C12H25-SWNTs and the same stir bar, stirring speed, and oxygen pressure (1inhibit the autoxidation of cumene. This result means that atm). Efficient stirring and constant heating bath temperatureSWNTs do not react with the chain propagating radicals cumyl were required to avoid volume fluctuations. To decrease theand cumylperoxy (cf. Scheme 1). Because cumyl radicals are partial pressure of solvent, a water chilled condenser wasboth delocalized and hindered, their inertness in not surprising. attached directly to the reaction flask. A low-power bathThe failure of SWNTs to scavenge cumylperoxy radicals stands sonicator was used to make the SWNT dispersion in o-DCB.in contrast to the behavior of C60, which traps both this radical37 In a typical procedure, a solution of AIBN in o-DCB (10and t-butylperoxy radicals.27,36,37,45 A single report46 that C60 is mL, 0.1 N, 1 mmol), and a solution of BHT in o-DCB (1 mL,inert toward alkylperoxy radicals, as determined by chemilu- 0.1 N, 0.1 mmol) was added to a 25 mL flask containing cumeneminescence quenching, stands in contrast to the aforementioned (1.322 g, 11 mmol). The flask was attached to the volumetricwork. The enhanced reactivity of C60 can be attributed to its apparatus, evacuated to 20 mmHg, and filled with oxygen threegreater curvature than SWNTs. times. The solution was immersed into an oil bath preheated to In summary, we find that SWNTs induce the thermolysis of 70 °C and the volume of oxygen consumed was monitored withdiacyl peroxides and are subject to attack by t-butoxy radicals. time. For experiments where SWNTs and C12-SWNTs are usedOn the other hand, they do not inhibit the autoxidation of instead of BHT, SWNTs were sonicated for 20 min in a solutioncumene, indicating that they are far less reactive than C6037 and of AIBN in o-DCB (10 mL, 0.1 N, 1 mmol) in a bath sonicator.are unlikely to serve as oxidation inhibitors. As a control, SWNTs (1.2 mg, 0.1 mmol) in o-DCB (10 mL) sonicated for 20 min were tested for oxygen consumption.Experimental Section SWNTs (1.2 mg, 0.1 mmol) in a solution of AIBN in o-DCB Commercial benzoyl peroxide (Luperox A98) was found to (10 mL, 0.1 N, 1 mmol) were also sonicated for 20 min andbe >99% pure by iodometric titration. p-MeO-BP and PhP were were tested for oxygen uptake.prepared according to the literature19,20,47 and were purified asneeded by multiple recrystallizations to >99%. o-Dichloroben- Acknowledgment. We gratefully acknowledge the Robertzene (o-DCB) was washed with aq. Na2S2O3 to remove any A. Welch Foundation (C-0490 and C-0499) and the Nationalperoxides, then with water, 2 M NaOH, water, saturated Science Foundation (CHE-0450085) for support of this work.NaHCO3, water, and brine. After drying over MgSO4, it was We also thank Steven Ho and Dr. Robert H. Hauge of the Ricedistilled over CaH2 at 1 atm. Carbon Nanotechnology Laboratory for the SWNT samples. The air-free peroxide titration experiments were carried outas follows. A stock solution of 100 mg of SWNTs in 1000 mL Supporting Information Available: Table of percentof 3x freeze-thaw degassed o-DCB was sonicated in a bath peroxide consumed for all runs shown in Figures 1 and 2, andsonicator for 18 h. Appropriate volumes were removed via a all Raman and XPS data shown in Tables 1 and 2. This materialvolumetric pipet and diluted with pure, degassed o-DCB to is available free of charge via the Internet at http://pubs.acs.org.produce SWNT solutions containing 0-5 mg of SWNTs. Thesesolutions were flushed with N2, sonicated for 15 min, then placed References and Notesin an oil bath at the appropriate temperature. After a 15 minequlibration period under N2, 0.3 mmol peroxide was added (1) Hirsch, A.; Vostrowsky, O. Top. Curr. Chem. 2005, 245, 193- 237.and thermolysis was carried out in a closed system. Iodometric (2) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006,titration16 was employed to determine the percent peroxide 106, 1105-1136.remaining after thermolysis. Specifically, the hot SWNT solution (3) Dyke, C. A.; Stewart, M. P.; Maya, F.; Tour, J. M. Synlett 2004,was vacuum filtered through a 0.2 µm PTFE filter, at which 1, 155.time the SWNTs received their first exposure to air. The receiver (4) Peng, H.; Reverdy, P.; Khabashesku, V. N.; Margrave, J. L. Chem. Commun. 2003, 362-363.was a 250 mL sidearm flask containing 10 mL acetic anhydride (5) Umek, P.; Seo, J. W.; Hernadi, K.; Mrzel, A.; Pechy, P.; Mihailovic,(Ac2O) and ∼0.5 g NaI. The reaction flask was rinsed with D. D.; Forro, L. Chem. Mater. 2003, 15, 4751-4755.another 5 mL ofAc2O, which was filtered as well. The filter (6) Ying, Y.; Saini, R. K. F. L.; Sadana, A. K.; Billups, W. E. Org.cake was then washed with a final 5 mL of Ac2O and stored in Lett. 2003, 5, 1471-1473.a vial under air for analysis. The resultant dark yellow-brown (7) Peng, H.; Alemany, L. B.; Margrave, J. L.; Khabashesku, V. N. J. Am. Chem. Soc. 2003, 125, 15174-15182.solution of 50 mL of o-DCB and 20 mL of Ac2O was swirled (8) Zhang, M.; Yudasaka, M.; Miyauchi, Y.; Maruyama, S.; Ijima, S.periodically for 30 min, at which time 40 mL of DI water was J. Phys. Chem. B 2006, 110, 8935-8940.added. The two-phase mixture was titrated to the water-white (9) Lee, K. M.; Li, L.; Dai, L. J Am. Chem. Soc. 2005, 127, 4122-endpoint with ∼0.07 M thiosulfate (exact concentration of 4123.thiosulfate determined by standardization with KIO3). (10) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002-4005. The re-isolated SWNTs were examined spectroscopically, (11) Nakamura, T.; Ishihara, M.; Ohana, T.; Tanaka, A.; Koga, Y. Chem.leading to the results in Tables 1 and 2. XPS characterization Commun. 2004, 1336-1337.was carried out using a Physical Electronics Quanteras with a (12) Liu, J.; Rodriguez i Zubiri, M.; Vigolo, B.; Dossot, M.; Fort, Y.;monochromated Al source (100 × 100 µm2 analysis area) while Ehrhardt, J.-J.; McRae, E. Carbon 2007, 45, 885-891.the Raman spectrometer was a Renishaw Raman Spectroscopy (13) Khare, B. N.; Wilhite, P.; Quinn, R. C.; Chen, B.; Schlingler, R. H.; Tran, B.; Imanaka, H.; So, C. R.; Bauschlicher, C. W.; Meyyappan, M.microscope. According to XPS, the filtered SWNT samples J. Phys. Chem. B 2004, 108, 8166-8172.contained the same percent oxygen after months of storage as (14) Qin, S.; Qin, D.; Ford, W. T.; Herrera, J. E.; Resasco, D. E.; Bachilo,they did when fresh. S. M.; Weisman, R. B. Macromolecules 2004, 37, 3965-3967.
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