Effect of Different Functional Groups on the Free Radical Scavenging Capability ofSingle-Walled Carbon Nanotubes

Ana Martınez,*,† Misaela Francisco-Marquez,‡ and Annia Galano‡

Departamento de Materia Condensada y Criogenia, Instituto de InVestigaciones en Materiales, UniVersidadNacional Autonoma de Mexico, Circuito Exterior S. N., Ciudad UniVersitaria,CP 04510, Mexico D. F. Mexico, and Departamento de Quımica, UniVersidad AutonomaMetropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina. Iztapalapa.C. P. 09340, Mexico D. F. Mexico

ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: July 6, 2010

The effect of different functional groups on the free radical scavenging capability of single-walled carbonnanotubes (SWCNT) is investigated using density functional theory calculations. The reaction mechanismthat is considered in this study is the radical adduct formation. The reactions of a •OH radical with eightdifferent functionalized SWCNTs (armchair and zigzag) were studied. All of them were found to be exothermicand exergonic. Different sites of reaction were considered, and the increase on reactivity with respect topristine tubes was found to be site-dependent. The presence of functional groups on the sidewalls of SWCNTsis predicted to increase their OH radicals scavenging activity. To enhance this property, the best functionalgroups are -CH2-OH for the armchair (3,3) SWCNT and -CONH-CH3 for the zigzag (5,0) fragment. Themajor conclusion from this work is that functionalized SWCNTs, in addition to be soluble in different media,are also very good free radical scavengers, being the armchair tubes better than the zigzag ones, when theyare ultrashort fragments.

Introduction

Single-walled carbon nanotubes (SWCNTs) have been provento possess very unique properties that make them perfectcandidates for diverse nanotechnological applications. They havebeen proposed as nanopipets,1 nanotweezers,2 nanocontainers,3

free radical scavengers,4-11 and field emission devices,12-14 aswell as for chemical detection,15,16 drug delivery and diagnos-tic,17 gas storage,18-20 and membrane separation,21,22 amongmany other applications. However, their lack of solubility limitsthe use of SWCNTs. Actually pristine tubes are insoluble in allorganic solvents and aqueous solutions.23 Chemical function-alization is one of the possible strategies to overcome thisproblem. Depending on how the structural modification takesplace, functionalizations of SWCNTs can be categorized in threegeneral groups:23 (a) sidewall covalent attachment of chemicalgroups; (b) noncovalent exohedral adsorption (wrapping) ofvarious functional molecules; and (c) endohedral filling of theinner empty cavity.

In the particular case of covalent attachment to sidewalls,different chemical groups have been successfully used tofunctionalize carbon nanotubes and increase their solubility invarious solvents.23-27 Since the sidewalls of SWCNTs areexpected to be inert, fluorination was initially chosen becauseit had been used in the chemical transformation of graphite.28

Afterward, and since fluorinated SWCNTs can be dissolved inalcohols by ultrasonication,29,30 they can be chemically trans-formed using wet chemistry. The fluorine atoms can beefficiently substituted for alkyl groups by treatment with alkyllithium or Grignard compounds.31 Functionalization can also

be achieved using the reactivity of surface carboxyl groups,which arise from previous oxidation of the tubes with strongacids, or from purification. The anchoring of the desired moietyis then accomplished by amidation, esterification, and Friedel-Crafts reactions, among others.23,26,32,33 Direct sidewall func-tionalization of SWNTs with organic groups is also possibleby chemical reactions with reactive species such as nitrenes,carbenes, and radicals.34 SWCNTs have also been directlyfunctionalized using 1,3-dipolar cycloadditions yielding modifiednanotubes that are remarkably soluble in most organic solventsand even in water.35 Diazonium salts are also common precur-sors for covalent functionalization processes;36-39 in particular,aryl diazonium salts have been successfully applied to themodification of carbon surfaces.36,40 In this case, SWCNTsfunctionalized with diverse phenyl moieties, depending on theused salt, are attached to their walls, and they show increasedsolubility in different organic solvents. Other functionalizationapproaches are based on peroxides,42-45 perfluoroalkyl iodides,34,46

perfluoro azo compounds,47,48 hydroxilation,44 thiolation,49 mi-crowave discharge of ammonia,50 and alkoxy radicals.51

Thus, depending on the particular process, SWCNTs carbonnanotubes can be functionalized with a large variety of chemicalgroups. In addition, it has been shown that structural modifica-tions alter the physical52-64 and chemical65-72 properties ofSWCNTs, and even their toxicity.73,74 In a previous work fromour group, the free radical scavenging activity of SWCNTsfunctionalized with -COOH groups was investigated, and itwas concluded that carboxylated SWCNTs are at least as good,or even better, free radical scavengers than their nonfunction-alized partners.10 However, different functional groups mighthave different effects on the target property. Therefore, the mainpurpose of the present work is to investigate such effects onthe free radical scavenging activity of SWCNTs for a varietyof funcionalizations. This is still an emerging area of research

* To whom correspondence should be addressed. E-mail: martina@iim.unam.mx.

† Universidad Nacional Autonoma de Mexico.‡ Universidad Autonoma Metropolitana-Iztapalapa.

J. Phys. Chem. C 2010, 114, 14734–1473914734

10.1021/jp1033382 2010 American Chemical SocietyPublished on Web 08/16/2010

with numerous unanswered questions.9 One of them is just iffunctionalized SWCNTs are as good free radical scavengers astheir nonmodified partners, and how each particular function-alization affects this desired activity. To answer these questions,the reactions of OH radicals with finite fragments of (3,3)armchair and (5,0) zigzag SWCNTs, functionalized with eightdifferent functional groups (Table 1) have been studied andcompared with those involving their pristine partners. Differentsites of reactions have been modeled using density functionaltheory (DFT), considering the radical adduct formationmechanism.

Computational Details

Electronic structure calculations have been performed withthe Gaussian 0375 package of programs. Full geometry optimi-zations and frequency calculations were carried out for all thestationary points using the B3LYP hybrid HF-density functionaland the 3-21G basis set. No symmetry constraints have beenimposed in the geometry optimizations. Singlet and triplet spinmultiplicities were used for functionalized SWCNTs. Doubletis the spin state of the adduct products with OH radical. Theenergies of all the stationary points were improved by singlepoint calculations at the B3LYP/6-311+G(d) level of theory.Thermodynamic corrections at 298.15 K were included in thecalculation of relative energies. Local minima were identifiedby the absence of imaginary frequencies. This methodology waspreviously validated.6,8,10,11 As was reported, the effect ofincreasing the basis set for geometry optimizations and fre-quency calculations is only minor. In the particular case of theSWCNTs, the effect of increasing the basis set on the ∆E, ∆H,and ∆G of reaction is less than or about to 0.5 kcal/mol.10

The stationary points were first modeled in gas phase(vacuum), and solvent effects were included a posteriori bysingle point calculations using a polarizable continuum model,specifically the integral-equation-formalism (IEF-PCM)76-79 atthe B3LYP/6-311+G(d) level of theory, with benzene and wateras solvents for mimicking nonpolar and polar environments,respectively. The solvent cage effects have been included inthe calculations of the Gibbs free energies of reaction accordingto the corrections proposed by Okuno,80 taking into account thefree volume theory.81 These corrections are in good agreementwith those independently obtained by Ardura et al.82 and havebeen successfully used by other authors.83-87 This correction isimportant, since the packing effects of the solvent reduce theentropy loss associated with any addition reaction.

Results and Discussion

The addition reaction of •OH with double functionalized finitefragments of (3,3) armchair and (5,0) zigzag SWCNTs, about11 Å long, have been studied. Eight different functional groupshave been taken into account (Table 1). For each of them, twofunctionalization sites were considered: with the two groups atthe middle of the tubes’ walls or at their open ends (Figure 1).Hydrogen atoms were used to saturate the dangling bonds at

the ends of the SWCNTs in order to avoid unwanted distortions.Two spin multiplicities were also considered (singlet and triplet).In most of the systems, singlet states are more stable thantriplets. Some exceptions (indicated in Table 1S of the Sup-porting Information) were found for zigzag (5,0) SWCNTs withthe two groups at their open ends. The reactions with a OHradical had been computed in gas phase, water, and benzene,aiming for environmental and biological applications.

The schematic representation of Figure 2 shows the fourdifferent sites of reaction that were considered when function-alization of SWCNT occurs at the middle of the fragment. Thesites of reaction that have been considered are at the C atom(1) next to that where the functional group is attached to, (2)two C-C bonds apart from the closest functionalization site,(3) three C-C bonds apart from both functionalization sites, inthe direction perpendicular to the tube axis, and (4) at the para

TABLE 1: Functional Groups Considered in This Study

Figure 1. Fragments of SWNCT functionalized with F1 (-CO-NH2)as example, (3,3)-M and (5,0)-M with the two groups at the middle ofthe tubes, and (3,3)-E and (5,0)-E with the functional groups at theends of the tubes.

Figure 2. Schematic representation of the studied sites of the reaction,for SWCNTs functionalized at the middle of the fragment.

Free Radical Scavenging Capability of SWCNs J. Phys. Chem. C, Vol. 114, No. 35, 2010 14735

site with respect to the closest functional group. All the productsof the reaction of (3,3) functionalized with F7 are shown inFigure 3 to illustrate this. For SWCNTs functionalized at theend of the fragment, only the addition at the central hexagonof the nanotube was considered. Otherwise, unwanted interac-tions with the terminal H atoms could affect the results. In thiscase, the addition product is labeled E-p.

The values of the calculated enthalpies and Gibbs freeenergies of reaction are reported in Table 1S of the SupportingInformation. All the studied reactions were found to beexothermic and exergonic at room temperature. The calculatedvalues are in all the cases large enough to overcome anyinaccuracy of the used methodology. This indicates that func-tionalized nanotubes are expected to be good free radicalscavengers in water and benzene. Since the results in water aremore relevant than the results in benzene due to the solubilityof these functionalized nanotubes in polar solvents, we will focusthe further discussion on the water results.

To analyze if functionalized SWCNTs are as good free radicalscavengers as their nonmodified partners, a comparison of theGibbs free energies of the reaction is reported in Figures 4-6.For simplification, the comparison between the enthalpies is not

included, but as can be expected, the effects on the enthalpyare similar to those on the Gibbs free energy. In order to helpvisualizing the effects of the studied functionalizations on thethermochemistry of the •OH addition processes to SWCNTs,the variations of ∆G of reaction with respect to those involvingnonfunctionalized SWCNTs have been plotted in those figures.The ∆G values for pristine nanotubes were reported before10

and are included in Table 1S of the Supporting Information.When comparing the values of ∆G for the reactions involving

functionalized SWCNT with those of their pristine partners(Figure 4-6), the first thing that highlights is that, in general,the presence of functional groups increases the OH radicals

Figure 3. Products of the reaction of a OH radical with (3,3) SWCNTfragments functionalized with F7 as an example. Red sphere representsthe oxygen atom of the free radical while the green one is the chlorineatom of the functional group.

Figure 4. Variation in the Gibbs free energies (in water) of reactionsinvolving SWCNTs functionalized with F1, F2, and F3 groups, in kcal/mol, with respect to those involving the corresponding pristineSWCNTs.

Figure 5. Variation in the Gibbs free energies (in water) of reactionsinvolving SWCNTs functionalized with F4, F5, and F6 groups, in kcal/mol, with respect to those involving the corresponding pristineSWCNTs.

Figure 6. Variation in the Gibbs free energies (in water) of reactionsinvolving SWCNTs functionalized with aryl groups (F7 and F8), inkcal/mol, with respect to those involving the corresponding pristineSWCNTs.

14736 J. Phys. Chem. C, Vol. 114, No. 35, 2010 Martınez et al.

scavenging activity of SWCNTs, considering that the reactionstake place to yield the products that are lowest in energy. Eventhough there are reactions at some sites of the functionalizedtubes that are less exergonic than those involving pristine tubes,in all the studied systems, there is at least one site of reactionin the functionalized SWCNTs at which the •OH additions ismore thermochemically favored. Therefore, it can be concludedthat the presence of the studied functional groups would increasethe free radical scavenging activity of SWCNTs, provided thatthe functionalization occurs in such extent that there are enoughfree space on the walls for the reactions to take place.

The feasibility of the OH radical addition reaction is differentfor each functionalized nanotube. Gibbs free energy of thereactions yielding M-p1, M-p3, and M-p4 adducts of most ofthe (3,3) functionalized SWCNT fragment are more negativethan the corresponding value of the pristine (3,3) nanotubes.The exceptions are for functionalizations with F5 at site 3 andwith F3 at site 4. The least exergonic addition is at site 2 of(3,3) SWCNT for most of the systems, with the exception ofSWCNTs functionalized with F3 and F8 groups. The •OHreaction with SWCNTs functionalized at their open ends is ingeneral less exergonic than the corresponding reaction forpristine nanotubes. For F8, the products of the •OH reactionwith centrally functionalized (3,3) SWCNT fragments havesimilar values of ∆G at different reaction sites. Apparently, for(3,3) nanotubes, functionalized with F8, the viability of theaddition does not depend on the site of the reaction. In general,•OH reactions with (3,3) nanotubes functionalized at the middleof the tubes were found to be more exothermic and moreexergonic than those involving SWCNTs functionalized at theend of the fragment.

The increase in exergonicity due to functionalization is lessimportant for the zigzag (5,0) fragments than for the armchair(3,3) nanotubes. Site 1 is the only reaction site that leads tomore negative values of the Gibbs free energy than the pristine(5,0) nanotube for all the functionalized SWCNTS that we reportin this work. The radical additions leading to the formation ofadducts M-p2, M-p3, M-p4, and E-p of zigzag (5,0) fragmentsare less exergonic than those involving the nonfunctionalizedSWCNT.

The adduct formation reactions of a OH radical with armchair(3,3) SWCNT fragments were found to be, in general, moreexergonic than those with zigzag (5,0) nanotubes. Moreover,the zigzag (5,0) SWCNT fragments have only one site withincreased reactivity, where the reaction is more exergonic thanthe corresponding reaction with their pristine partners. Thearmchair (3,3) SWCNT fragments, on the other hand, have two,three, or four sites of reaction leading to ∆G values that aremore negative than those involving nonfunctionalized nanotubes.Apparently, the increment in the free radical scavenging capacityof functionalized SWCNTs, with respect to the pristine nano-tubes, is larger for (3,3) functionalized nanotubes than for (5,0)ones, since in the first case, the number of positions with greaterreactivity is larger than in the second one. Notwithstanding thesedifferences, both armchair and zigzag functionalized nanotubesare proposed to be better free radical scavengers than theirpristine partners.

In comparing the effects of the functional groups, it seemsthat F4 would be the best functionalization for (3,3) SWCNTs,if the tubes are intended to be used as free radical scavengers.The reaction with a OH radical is more exergonic for nanotubeswith this functionalization than with any other, among thestudied set. In addition, this particular functionalization leadsto the highest number of sites with increased reactivity, that is,

it has four sites of higher reactivity. Moreover, it is the onlyfunctionalized (3,3) SWCNT fragment for which the Gibbs freeenergy associated with the formation of E-p is more negativethan the corresponding value of the pristine nanotube. For afunctionalized (5,0) SWCNT, the functionalization leading tothe highest free radical scavenging capability is F3. F8 is alsoa functional group that was found to be particular efficient forthe intended purpose of SWCNTs. It also leads to four positionswith increased reactivity in the (3,3) SWCNTs. Moreover the∆G value of the reaction yielding M-p1 and involving the (5,0)fragment functionalized with F1 and F8 is similar to thoseinvolving SWCNTs with F3. In general, SWCNTs functional-ized with chemical groups that contain only carbon, nitrogen,and oxygen were found to be better OH radical scavengers thanthose with chlorine and sulfur in their structure.

In summary, the energy release associated with the adductformation depends on the helicity of the nanotubes, on thefunctional group, and on the reactions site. Apparently, F1, F3,F4, and F8 are the best functional groups in order to increasethe free radical scavenger capacity of SWCNT fragments. Thethermochemical viability of the •OH scavenging processes byfunctionalized SWCNTs increases more when functionalizationtakes place in armchair (3,3) than when it occurs in zigzag (5,0).It should be noticed here that in the present study the modeledSWCNTs correspond to ultrashort fragments; and therefore,none of the studied SWCNT fragments have actual metalliccharacter. For tubes with length . diameter, the proposed trends,between armchair and zigzag SWCNTs, might change.

Additionally, it has been explored if geometrical parametersare directly related with the exergonicity of the studied reactions.To this end, the distance of the formed C-O bond through •OHadditions on the walls of the modeled SWCNTs has beenanalyzed. The optimized values of the C-O bond distance arereported as Supporting Information (Tables 2S and 3S). Theyare within the range of 1.42-1.49 Å, and they are not correlatedwith the Gibbs free energy values. Apparently, the differencesin the reactivity are not directly reflected on this geometricalparameter. The frequencies of the infrared (IR) bands corre-sponding to the O-H and C-O stretching vibrations of theproducts of •OH additions to the modeled SWCNTs are alsoprovided as Supporting Information (Tables 2S and 3S). Thereported frequencies have been scaled using a scaling factor of0.9627, as recommended by Irikura el al.88 The vibrationscorresponding to the O-H stretching mode appear within therange 3053-3499 cm-1. There are several bands that correspondto the C-O vibration in an interval of 900-1100 cm-1 thatmight be useful for the experimental characterization of thereaction products.

In this paper, two small-diameter SWCNTs were used in orderto analyze the influence of functionalization on the free radicalscavenger capacity. For nonfunctionalized SWCNTs, it waspreviously reported8 that armchair nanotubes become lessreactive toward the OCH3 free radical as their diameter increases.The reactivity of zigzag nanotubes, on the other hand, wasdescribed to be less sensitive to the tubes diameter. Accordingly,similar trends are expected for functionalized SWCNTs as well.The results reported in the present work show that the presenceof functional groups on the tubes’ walls increases the viabilityof the free radical scavenging activity of SWCNTs. Therefore,it can be predicted that wider armchair nanotubes can still havesuch activity when functionalized, compared to pristine tubes.Accordingly, sidewall covalent functionalization is predicted toincrease the antioxidant activity of SWCNTs for wide distribu-tions of tubes diameter and chirality.

Free Radical Scavenging Capability of SWCNs J. Phys. Chem. C, Vol. 114, No. 35, 2010 14737

Conclusions

The presence of functional groups is predicted to increasethe free radical scavenging capacity of SWCNT fragmentscompared to their pristine partners. There is a significantinfluence of the helicity of the nanotube and also of the func-tional group on the free radical scavenger capacity of thefunctionalized SWCNT. The thermochemical feasibility of the•OH antiradical process by functionalized nanotubes increasesmore on armchair (3,3) nanotubes than when the functional-ization takes place in zigzag (5,0) SWCNT fragments. Thefunctionalized SWCNTs that lead to more exergonic adductformation reactions with •OH radical are those functionalizedwith F3 and F4 chemical groups. Apparently, the best freeradical scavengers are those functionalized SWCNTs that haveonly carbon, nitrogen, and hydrogen. This could be importantfor further applications.

Acknowledgment. This study was made possible due tofunding from the Consejo Nacional de Ciencia y Tecnologıa(CONACyT), as well as resources provided by the Instituto deInvestigaciones en Materiales IIM, UNAM. The work wascarried out, using a KanBalam supercomputer, provided byDGSCA, UNAM, and the facilities at Laboratorio de Super-computo y Visualizacion en Paralelo of UAM Iztapalapa. A.M.is grateful for financial support from DGAPA-UNAM-Mexico.

Supporting Information Available: ∆H and ∆G values forall the studied reactions, C-O bond distances in the additionproducts involving (3,3) and (5,0) SWCNTs, and frequenciesof selected vibrational modes. This material is available free ofcharge via the Internet at http://pubs.acs.org.

References and Notes

(1) Hong, M. H.; Kim, K. H.; Bae, J.; Jhe, W. Appl. Phys. Lett. 2000,77, 2604.

(2) Kim, P.; Lieber, C. M. Science 1999, 286, 2148.(3) Koshino, M.; Tanaka, T.; Solin, N.; Suenaga, K.; Isobe, H.;

Nakamura, E. Science 2007, 316, 853.(4) Watts, P. C. P.; Fearon, P. K.; Hsu, W. K.; Billingham, N. C.;

Kroto, H. W.; Walton, D. R. M. J. Mater. Chem. 2003, 13, 491.(5) Fenoglio, I.; Tomatis, M.; Lison, D.; Muller, J.; Fonseca, A.; Nagy,

J. B.; Fubini, B. Free Radical Biol. Med. 2006, 40, 1227.(6) Galano, A. J. Phys. Chem. C 2008, 112, 8922.(7) Lucente-Schultz, R. M.; Moore, V. C.; Leonard, A. D.; Price, B. K.;

Kosynkin, D. V.; Lu, M.; Partha, R.; Conyers, J. L.; Tour, J. M. J. Am.Chem. Soc. 2008, 131, 3934.

(8) Galano, A. J. Phys. Chem. C 2009, 113, 18487.(9) Galano, A. Nanoscale 2010, 2, 373.

(10) Francisco-Marquez, M.; Galano, A.; Martınez, A. J. Phys. Chem.C 2010, 114, 6363.

(11) Galano, A.; Francisco-Marquez, M.; Martınez, A. J. Phys. Chem.C 2010, 114, 8302.

(12) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179.(13) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Nordlander, P.; Colbert,

D. T.; Smalley, R. E.; Lou, L.; Kim, S. G.; Tomanek, D. Science 1995,269, 1550.

(14) Saito, Y.; Uemura, S. Carbon 2000, 38, 169.(15) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.;

Cho, K.; Dai, H. Science 2000, 287, 622.(16) Snow, E. S.; Perkins, F. K.; Houser, E. J.; Badescu, S. C.; Reinecke,

T. L. Science 2005, 307, 1942.(17) Prato, M.; Kostarelos, K.; Bianco, A. Acc. Chem. Res. 2007, 41,

60.(18) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.;

Dresselhaus, M. S. Science 1999, 286, 1127.(19) Lee, S. M.; An, K. H.; Lee, Y. H.; Seifert, G.; Frauenheim, T.

J. Am. Chem. Soc. 2001, 123, 5059.(20) Cao, D.; Zhang, X.; Chen, J.; Wang, W.; Yun, J. J. Phys. Chem. B

2003, 107, 13286.(21) Hinds, B. J.; Chopra, N.; Rantell, T.; Andrews, R.; Gavalas, V.;

Bachas, L. G. Science 2004, 303, 62.(22) Kalra, A.; Garde, S.; Hummer, G. Proc. Natl. Acad. Sci. U.S.A.

2003, 100, 10175.

(23) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006,106, 1105.

(24) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853.(25) Sun, Y. P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35,

1096.(26) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen,

R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105.(27) Tessonnier, J. P.; Villa, A.; Majoulet, O.; Su, D. S.; Schlogl, R.

Angew. Chem., Int. Ed. 2009, 48, 6543.(28) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.;

Hauge, R. H.; Margrave, J. L. Chem. Phys. Lett. 1998, 296, 188.(29) Kelly, K. F.; Chiang, I. W.; Mickelson, E. T.; Hauge, R. H.;

Margrave, J. L.; Wang, X.; Scuseria, G. E.; Radloff, C.; Halas, N. J. Chem.Phys. Lett. 1999, 313, 445.

(30) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.;Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys.Chem. B 1999, 103, 4318.

(31) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson,L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave,J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367.

(32) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund,P. C.; Haddon, R. C. Science 1998, 282, 95.

(33) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.;Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834.

(34) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes,M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002.

(35) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger,M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760.

(36) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley,R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536.

(37) Dyke, C. A.; Stewart, M. P.; Maya, F.; Tour, J. M. Synlett 2004,1, 155.

(38) Marcoux, P. R.; Hapiot, P.; Batail, P.; Pinson, J. New J. Chem.2004, 28, 302.

(39) Zhao, J.-x.; Ding, Y.-h. J. Phys. Chem. C 2008, 112, 13141.(40) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley,

R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536.(41) Peng, H.; Alemany, L. B.; Margrave, J. L.; Khabashesku, V. N.

J. Am. Chem. Soc. 2003, 125, 15174.(42) Peng, H.; Reverdy, P.; Khabashesku, V. N.; Margrave, J. L. Chem.

Commun. 2003, 362.(43) Umek, P.; Seo, J. W.; Hernadi, K.; Mrzel, A.; Pechy, P.; Mihailovic,

D. D.; Forro, L. Chem. Mater. 2003, 15, 4751.(44) Liang, F.; Beach, J. M.; Rai, P. K.; Guo, W.; Hauge, R. H.; Pasquali,

M.; Smalley, R. E.; Billups, W. E. Chem. Mater. 2006, 18, 1520.(45) Zhang, M.; Yudasaka, M.; Miyauchi, Y.; Maruyama, S.; Ijima, S.

J. Phys. Chem. B 2006, 110, 8935.(46) Lee, K. M.; Li, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 4122.(47) Nakamura, T.; Ishihara, M.; Ohana, T.; Tanaka, A.; Koga, Y. Chem.

Commun. 2004, 1336.(48) Liu, J.; Rodriguez i Zubiri, M.; Vigolo, B.; Dossot, M.; Fort, Y.;

Ehrhardt, J. J.; McRae, E. Carbon 2007, 45, 885.(49) Denis, P. A. Int. J. Quantum Chem. 2009, 109, 772.(50) 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. J.Phys. Chem. B 2004, 108, 8166.

(51) Engel, P. S.; Billups, W. E.; Abmayr, D. W.; Tsvaygboym, K.;Wang, R. J. Phys. Chem. C 2008, 112, 695.

(52) Ebbesen, T. W.; Takada, T. Carbon 1995, 33, 973.(53) Charlier, J. C.; Ebbesen, T. W.; Lambin, Ph. Phys. ReV. B 1996,

53, 11108.(54) Crespi, V. H.; Cohen, M. L. Phys. ReV. Lett. 1997, 79, 2093.(55) Chico, L.; Sancho, M. P. L.; Munoz, M. C. Phys. ReV. Lett. 1998,

81, 1278.(56) Orlikowski, D.; Nardelli, M. B.; Bernholc, J.; Roland, C. Phys.

ReV. Lett. 1999, 83, 4132.(57) Ouyang, M.; Huang, J. L.; Cheung, C. L.; Lieber, C. M. Science

2001, 291, 97.(58) Ouyang, M.; Huang, J. L.; Lieber, C. M. Acc. Chem. Res. 2002,

35, 1018.(59) Charlier, J. C. Acc. Chem. Res. 2002, 35, 1063.(60) Watts, P. C. P.; Hsu, W. K.; Kroto, H. W.; Walton, D. R. M. Nano

Lett. 2003, 3, 549.(61) Ding, F. Phys. ReV. B 2005, 72, 245409.(62) Kotakoski, J.; Krasheninnikov, A. V.; Nordlund, K. Phys. ReV. B

2006, 74, 245420.(63) Galano, A.; Orgaz, E. Phys. ReV. B. 2008, 77, 045111.(64) Xue, B.; Shao, X.; Cai, W. J. Chem. Theory Comput. 2009, 5, 1554.(65) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul,

P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias,F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998,280, 1253.

14738 J. Phys. Chem. C, Vol. 114, No. 35, 2010 Martınez et al.

(66) Bom, D.; Andrews, R.; Jacques, D.; Anthony, J.; Chen, B.; Meier,M. S.; Selegue, J. P. Nano Lett. 2002, 2, 615.

(67) Fan, Y.; Burghard, M.; Kern, K. AdV. Mater. 2002, 14, 130.(68) Grujicic, M.; Cao, G.; Singh, R. Appl. Surf. Sci. 2003, 211, 166.(69) Grujicic, M.; Cao, G.; Rao, A. M.; Tritt, T. M.; Nayak, S. Appl.

Surf. Sci. 2003, 214, 289.(70) Chakrapani, N.; Zhang, Y. M.; Nayak, S. K.; Moore, J. A.; Carroll,

D. L.; Choi, Y. Y.; Ajayan, P. M. J. Phys. Chem. B 2003, 107, 9308.(71) Li, X.; Niu, J.; Zhang, J.; Li, H.; Liu, Z. J. Phys. Chem. B 2003,

107, 2453.(72) Lu, A. J.; Pan, B. C. Phys. ReV. Lett. 2004, 92, 105504.(73) Fenoglio, I.; Greco, G.; Tomatis, M.; Muller, J.; Raymundo-Pinero,

E.; Beguin, F.; Fonseca, A.; Nagy, J. B.; Lison, D.; Fubini, B. Chem. Res.Toxicol. 2008, 21, 1690.

(74) Muller, J.; Huaux, F.; Fonseca, A.; Nagy, J. B.; Moreau, N.; Delos,M.; Raymundo-Pinero, E.; Beguin, F.; Kirsch-Volders, M.; Fenoglio, I.;Fubini, B.; Lison, D. Chem. Res. Toxicol. 2008, 21, 1698.

(75) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,M. A.; Cheeseman, J. R.; Montgomery, Jr. J. A.; Vreven, T.; Kudin, K. N.;Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;

Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,Revision D.01; Gaussian, Inc: Wallingford, CT, 2004.

(76) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107,3032.

(77) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151.(78) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101,

10506.(79) Tomasi, J.; Mennucci, B.; Cances, E. THEOCHEM 1999, 464, 211.(80) Okuno, Y. Chem.sEur. J. 1997, 3, 210.(81) Benson, S. W. The Foundations of Chemical Kinetics; Krieger:

Malabar, Florida, 1982.(82) Ardura, D.; Lopez, R.; Sordo, T. L. J. Phys. Chem. B 2005, 109,

23618.(83) Alvarez-Idaboy, J. R.; Reyes, L.; Cruz, J. Org. Lett. 2006, 8, 1763.(84) Alvarez-Idaboy, J. R.; Reyes, L.; Mora-Diez, N. Org. Biomol. Chem.

2007, 5, 3682.(85) Galano, A. J. Phys. Chem. A 2007, 111, 1677.(86) Galano, A.; Cruz-Torres, A. Org. Biomol. Chem. 2008, 6, 732.(87) Galano, A.; Alvarez-Idaboy, J. R. Org. Lett. 2009, 11, 5114.(88) Irikura, K. K.; Johnson, R. D., III; Kacker, R. N. J. Phys. Chem. A

2005, 109, 8430.

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