JPC02SWNTs

Interaction of Oxidized Single-Walled Carbon Nanotubes with Vaporous Aliphatic Amines

Elena V. Basiuk,*, Vladimir A. Basiuk, Jose-Guadalupe Banuelos,Jose-Manuel Saniger-Blesa, Valeriy A. Pokrovskiy, Taras. Yu. Gromovoy,Aleksandr V. Mischanchuk, and Boris G. Mischanchuk

Centro de Instrumentos, UniVersidad Nacional Autonoma de Mexico, Apdo. Postal 70-186,Mexico, D.F. 04510, Mexico, Instituto de Ciencias Nucleares, UniVersidad Nacional Autonoma de Mexico,Apdo. Postal 70-543, Mexico, D.F. 04510, Mexico, and Institute of Surface Chemistry, National Academy ofSciences of the Ukraine, Prospekt Nauki 31, UA-03680 KieV, Ukraine

ReceiVed: May 24, 2001; In Final Form: NoVember 28, 2001

The gas-phase derivatization procedure was employed for direct (i.e., without chemical activation of terminalcarboxylic groups) amidization of oxidized single-walled carbon nanotubes (SWNTs) with simple aliphaticamines. The procedure includes treatment of SWNTs with amine vapors under reduced pressure and atemperature of 160-170 C. Applicability of infrared (IR) spectroscopy and temperature-programmeddesorption mass spectrometry (TPD-MS) for chemical characterization of the derivatized SWNTs was analyzed.It was concluded that IR spectra of oxidized SWNTs treated with amines under different conditions (describedhere and elsewhere) cannot correspond to amide derivatives on SWNT tips because of the very lowconcentration of the terminal groups relative to the whole sample mass, which implies a negligible contributionto the IR spectra. The bands detectable in the case of long-chain amines correspond to amine moleculesphysisorbed because of strong hydrophobic interactions of their hydrocarbon chains with SWNT walls.Energetically preferable adsorption sites are the channels inside SWNTs, according to MM+ molecular-mechanics modeling. TPD-MS provided additional information on the chemical state of the amines. Heatingof the amine-treated SWNTs at >200 C causes cleavage of alkenes from the amine residues: nonene andpentene form in the case of nonylamine and dipentylamine, respectively. For the short-chain amine(dipentylamine), only one chemical form was detected, whereas two forms (amide and physisorbed amine)can be distinguished for the SWNTs treated with nonylamine. The content of physisorbed nonylamine isabout 1 order of magnitude higher than the amide content. According to the results of two-level ONIOMquantum-chemistry-molecular-mechanics calculations, the direct formation of amides on armchair SWNTtips is more energetically favorable than that on the zigzag tips, although the activation barriers are ofapproximately equal height.

Introduction

In the studies of the interaction of carbon nanotubes (CNTs)with organic compounds, amines have attracted specialattention.1-9 Aspects of particular interest are the covalentfunctionalization of CNT probe tips for chemical force micros-copy,1,2 increasing the solubility of single-walled CNTs(SWNTs),3-6 their self-assembly on gold substrates,7 plasmaactivation of CNTs for chemical modification,8 and chemicalgating of individual semiconducting and metallic SWNTs.9Among these, the most extensively explored is the formationof amide derivatives between carboxylic groups on oxidizedCNT tips and long-chain amines.1-5 The reaction is currentlyperformed through chemical activation of the carboxylic groupswith thionyl chloride or carbodiimides in an organic solventmedium.1-5,7

From common considerations, for the functionalization ofCNT tips, one can use the same chemical approaches that havebeen developed for other poorly soluble inorganic materials,for example silica materials. A decade ago we performedsystematic studies on the use of the gas-phase chemicalderivatization for the synthesis of chemically modified silicas,mainly for liquid-chromatographic applications.10-14 Among thederivatizing reagents tested were compounds that are not volatileunder ambient temperature and pressure, such as polyaza-macrocyclic ligands,10,12 pyrimidine bases,12,14 and solid car-boxylic acids.11,13 However, decreasing the pressure to amoderate vacuum and, on the other hand, increasing thetemperature to >150 C provided efficient formation of thechemically bonded surface derivatives. In particular, the reactionbetween silica-bonded aminoalkyl groups and vaporous car-boxylic acids to form surface amides proceeds smoothly at 150-180 C without chemical activation of the carboxylic groups, itis relatively fast (0.5-1 h), and it provides high yields of theamide derivatives (>50% yield based on the starting surfaceconcentration of aminoalkyl groups). Excess derivatizing reagentis spontaneously removed from the reaction zone. In addition,there is no need to use a (organic) solvent medium; this feature

* To whom correspondence should be addressed. E-mail: [email protected].

Centro de Instrumentos, Universidad Nacional Autonoma de Mexico. Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de

Mexico. National Academy of Sciences of the Ukraine.

1588 J. Phys. Chem. B 2002, 106, 1588-1597

10.1021/jp0120110 CCC: $22.00 2002 American Chemical SocietyPublished on Web 01/24/2002

is attractive not only from an ecological point of view, but alsoin that it helps to avoid undesirable particle aggregation of thematerial derivatized.

With the above advantages of the gas-phase derivatizationin mind, in the present study, we attempted to apply thisprocedure to oxidized SWNTs containing carboxylic groups ontheir tips, in other words, to verify whether the amide derivativescan be synthesized directly according to the following generalscheme:

where HNR1R2 is an aliphatic amine. Nonylamine, dipentyl-amine, ethylenediamine, and propylenediamine were used astest compounds. Applicability of infrared (IR) spectroscopy andtemperature-programmed desorption mass spectrometry (TPD-MS) for chemical characterization of the derivatized SWNTswas analyzed. Molecular-mechanics and quantum-chemistry-molecular-mechanics calculations were employed to gain insightinto the mechanisms of SWNT-amine interactions.

Experimental Section

Materials. Nonylamine, dipentylamine, anhydrous ethylene-diamine, and propylenediamine from Aldrich were used asreceived, without further purification. SWNTs from MERCorporation were purified/oxidized, following the reportedprocedures.15,16

Gas-Phase Derivatization. The procedure was performedusing the custom-made Pyrex glass vacuum manifold presentedin Figure 1. In a typical experiment, 100 mg of oxidized SWNTswas placed into the bigger reactor 14. To remove volatilecontaminants from the SWNTs, the reactor was pumped out toa vacuum of ca. 10-2 Torr (valves 1 and 4 open; 2, 3, and 5closed), and its bottom was heated for 0.5 h at 100-120 C bymeans of a heating mantle. Then, the reactor was cooled andopened and ca. 50 mg of amine was dropped to the bottomcontaining the SWNTs. After pumping the reactor out to ca. 1Torr at room temperature, its valve 4 was closed, and the bottomwas heated at 160-170 C for 1 h. During this procedure, amineevaporated and reacted with SWNTs, and its excess condensed

a few centimeters above the heating mantle. After the procedurewas finished, valve 4 was opened again for ca. 15 min to removevolatiles. Then, the heating mantle was removed, and the reactorwas cooled and disconnected from the manifold. Beforeunloading the SWNTs, the upper reactor part with the condensedexcess amine was wiped with cotton wool wet with ethanol.For milligram-scale syntheses, the narrow reactor 13 can be usedin a similar way.

Infrared Spectra. IR spectra of SWNTs were recorded inKBr pellets (4 mg of SWNTs per 130 mg of KBr) on a Nicolet5SX FTIR spectrometer.

Atomic Force Microscopy. Atomic force microscopy (AFM)measurements were performed using an AutoProbe CP instru-ment from Park Scientific Instruments (tapping mode; typicalforce constant of 2.1 N/m; typical resonance frequency of 80kHz). SWNTs were dissolved in tetrahydrofurane with sonica-tion and deposited onto a freshly cleaved mica surface.

Temperature-Programmed Desorption Mass Spectrom-etry. TPD-MS measurements were performed using a custom-made system, the general scheme of which is shown in Figure2. The system was based on a MX-7304A mass spectrometer(Sumy, Ukraine) with a mass range of 1-400 Da and asensitivity of 10-8 g. The temperature ramp was variable in arange of 0.05-30 C min-1. SWNT samples (0.1-1 mg) wereplaced into the quartz-molybdenum tube and the tube wasevacuated to 10-1 Pa and then attached to the mass spectrometerinlet. The reactor-to-mass spectrometer interface included a high-vacuum valve with a 5-mm orifice and a 20-cm long tube, whichwas kept at 150 C. The sample tube was opened in the ion-source direction, and under the heating rate used (about 0.1 Cs-1), the observed intensity of the ion current is expected to beproportional to the desorption rate so that diffusion inhibitionmay be neglected. We assumed quasi-stationary conditions whenthe shape and position of desorption peaks do not depend onthe temperature of the spectrometer interface, sample mass, andits geometric characteristics. An average experiment durationwas 1 h. More details on TPD-MS methodology and datainterpretation were described in our previous papers.17-19

Theoretical Section

Molecular Mechanics. For purely molecular mechanicsmodeling, the MM+ method was used, implemented in theHyperChem version 5.1 package (by HyperCube Inc., Canada).In all calculations, full geometry optimization was performed

Figure 1. Vacuum manifold used for the gas-phase derivatization ofSWNTs: (1-5) Teflon valves; (6-9) 10-mm i.d. O-ring joints; (10-12) 41.4-mm i.d. O-ring joints; (13) milligram-scale reactor; (14) gram-scale reactor.

SWNT-COOH + HNR1R2 fSWNT-CO-NR1R2 + H2O (1)

Figure 2. Custom-made system used for temperature-programmeddesorption mass spectrometric measurements.

Oxidized Single-Walled Carbon Nanotubes J. Phys. Chem. B, Vol. 106, No. 7, 2002 1589

with the Polak-Ribiere conjugate gradient algorithm and a rmsgradient of 0.001 kcal -1 mol-1. For some bigger molecularsystems (SWNTs larger than 15- diameter), the latter con-vergence criteria were difficult to meet because of a largenumber of shallow local minima; in such cases, the calculationswere stopped when the total energy did not change anymoreafter >500 cycles.

Quantum Chemistry/Molecular Mechanics. Reaction mech-anisms were studied using a two-level ONIOM approach20,21implemented in the Gaussian 98W suite of programs.21,22 Theuniversal force field23 (UFF) was used for the low-leveltreatment, and the Beckes three-parameter hybrid method24 withthe exchange functional of Lee, Yang, and Parr25 (B3LYP) wasused for the high-level description, in conjunction with the4-31G basis set by Pople et al.26-28 The usefulness of thistheoretical approach to study chemical reactions of SWNTs wasdemonstrated recently.29 A search for transition states wascarried out using the QST2 procedure. The stationary pointgeometries were fully optimized and characterized as minima(0 imaginary frequencies) or first-order saddle points (1imaginary frequency) by calculations of vibration frequencies.All of the optimizations met the default convergence criteriaset in Gaussian 98W.

Results and Discussion

IR spectra of SWNTs treated with organic amines have beenmeasured by different research groups.3,4,6,7 Unfortunately, thedata reported so far do not allow an unambiguous interpretationof the spectral changes observed because they turn out to bequite contradictory. In particular, after transforming oxidizedSWNTs into their octadecylamide derivatives, Chen et al.3 foundtwo CdO (amide I) bands at 1663 and 1642 cm-1 of almostequal intensity and not one band as one normally expects.Afterward, the same group4 performed a similar SWNT deriva-tization with 4-dodecylaniline; now, they found two bands at1655 and 1598 cm-1 and assigned them to CdO (amide I) andNH (amide II) vibrations. This is a reasonable assignment forthe waVenumbers. However, the band intensities must equallybe taken into account: in the IR spectrum of SWNT 4-dodecyl-anilide,4 the intensity of the NH band is several times higherthan that of the CdO band, whereas in amides the carbonyl bandis always more intense (usually by about a factor of 2) than theamide II band. In addition, in both cases, the intensities of thebands appearing after amide formation turn out to be muchhigher than those of the bands typical for oxidized SWNTs (at1200, 1590, and 1720 cm-1), thus giving the impression that inthe synthesized samples amine content is higher than SWNTcontent. Finally, according to Liu et al.,7 SWNTs derivatizedwith cysteamine exhibit a band around 1600 cm-1, which wasassigned to amide I vibrations: this interpretation can hardlyagree with the preceding data.

Bearing the above observations in mind, we were not veryoptimistic about the possibility of providing a straightforwardIR-spectral characterization for our samples. Still, the IR spectraobtained appeared even worse than we expected. Of four amines(nonylamine, dipentylamine, ethylenediamine, and propylene-diamine) used for the gas-phase treatment of oxidized SWNTs,only nonylamine treatment caused obvious changes in the IRspectra (Figure 3a,b). In the other three cases, the changesdetected were at the noise level. If we suppose that all of theamines studied form amide bonds with carboxylic groups atSWNT tips (there are no logical reasons to expect that this isnot the case), such spectral behavior is hard to explain: forsmaller amine molecules, the efficiency of amide formation

should be higher than for longer-chain molecules (such asnonylamine) because of better accessibility to the reactingcarboxylic groups of the SWNTs.

The IR spectrum of nonylamine-treated SWNTs (Figure 3b)contains several new bands due to nonylamine at 1358 (C-Nstretch.), 1462 (C-H def.),30 2856 (sym. C-H stretch.), 2927(asym. C-H stretch.), and 1586 cm-1. Assignment of the lastband is the most important to characterize a chemical state ofnonylamine, but obviously its frequency is too low to attributethis band to CdO (amide I) vibrations; it should be assigned toNH vibrations of the NH2 group. Along with the above bands,there is one more at 1715 cm-1, having a shoulder at 1750cm-1. However, before trying to suggest a further explanation,it is worthwhile to analyze in more detail what kind ofinformation on the terminal organic groups can be expected fromIR spectra of typical SWNTs.

The first thing to remember is that in some respect NTs aresimilar to chemically derivatized inorganic adsorbents (silica,

Figure 3. IR spectra of oxidized SWNTs (a), SWNTs after gas-phasetreatment with nonylamine (b), and SWNTs after liquid-phase impreg-nation with nonylamine from ethanol solution with an SWNT-nonylamine mass ratio of 3:1 (c) and 1:1 (d). Samples used were 3%SWNT samples in KBr; spectra are presented without baselinecorrection.

1590 J. Phys. Chem. B, Vol. 106, No. 7, 2002 Basiuk et al.

alumina, amorphous carbon, graphite, etc.). For the latter, thecontribution of surface organic groups to the whole IR spectrumis relatively low because of an overwhelming mass fraction ofthe adsorbent itself; for CNTs, the same effect should beexpected because of their typically large aspect ratios. EvenSWNTs called short, prepared by oxidation and purifica-tion in strong acids, are on average 100-300 nm in length, ata diameter of 1.4 nm.3,4,6,7,15,31,32 To demonstrate the implica-tions of these parameters, we performed the following experi-ment.

To evaluate the SWNT-nonylamine mass ratio in the gas-phase-treated sample, we impregnated oxidized SWNTs withthe amine from ethanol solution taking two different proportions,3:1 and 1:1 (w/w). The samples were dried in a vacuum, andIR spectra were measured for them under the same conditions(3% SWNTs in KBr) as for oxidized SWNTs and the gas-phase-treated sample (Figure 3c,d). The resulting spectra are qualita-tively similar to each other, as well as to the spectrum of thegas-phase-treated sample (Figure 3b). Comparing the transmit-tance scale and amine-band intensities, one can conclude thatthe latter sample has a closer resemblance to the impregnatedsample of the 3:1 SWNT-nonylamine mass ratio.

Implications of the above observations can be understood interms of SWNT aspect ratios. For illustrative purpose, one cantake a (20,0) zigzag SWNT backbone, containing 1920 carbonatoms (Figure 4) of 1.55-nm diameter and 10-nm length. Sucha SWNT can have about 10 carboxylic groups at each tip (ahigher density of COOH groups at the tip is unlikely for stericreasons). As a result, it can form amide bonds with 20nonylamine molecules, and this stoichiometry would correspondto a SWNT-nonylamine mass ratio of 8.4:1. Thus, even if wesuppose that oxidized SWNTs are on average 10 nm in length,to obtain the IR-band intensities shown in Figure 3b,c, onewould have to have a nonylamine content roughly triple that ofthe stoichiometric one. The sample would have to contain >60%nonylamine in a physically adsorbed form, and correspondingly>60% of the IR absorption would be due to the chemicallyunbound fraction. It should be obvious that for 100-300-nm-long SWNTs, the amide contribution to the IR absorption willbe even lower, by 10-30 times, and such bands are almost

impossible to distinguish (especially given the generally poorquality of the CNT spectra). SWNTs gas-phase treated withnonylamine have dimensions of the same order of magnitude,as can be seen from the AFM microphotograph in Figure 5.Consequently, the data presented here and elsewhere3,4,6,7 reflectmainly amines strongly physisorbed because of hydrophobic

Figure 4. Schematic representation of a (20,0) zigzag SWNT of 1.55-nm diameter and 10-nm length, containing 10 carboxylic groups at eachtip.

Figure 5. AFM micrograph of SWNTs gas-phase treated withnonylamine.

Figure 6. TPD-mass spectrum (a) of volatile products evolved at 325C from SWNTs gas-phase treated with nonylamine and experimentalthermograms (b) for selected hydrocarbon peaks at m/z ) 42, 43, 55,57, and 69.

Figure 7. Logarithm of desorption rate k for three values of reactionorder (0sfirst order, )ssecond order, Osthird order) as a functionof inverse temperature ) (KT)-1, calculated from eq 6 in ref 19.Temperature dependence is linear for first-order reaction; values ofactivation energy E and preexponential factor k0 are E ) 79.6 kJ mol-1and k0 ) 105 s-1.


interactions between their long-chain hydrocarbon radicals andCNT side walls.

As we already mentioned, in the IR spectrum of the gas-phase-treated sample (Figure 3b), there is one more band at 1715cm-1 with a shoulder at 1750 cm-1. This band (observed inthe 1700-1735-cm-1 range by different authors) is believed tocorrespond to the terminal COOH groups.3,4,6,7,32,33 A higher-frequency absorption was reported as well; in particular, for

SWNTs after oxidation with gaseous ozone (i.e., no liquid-phasetreatment with strong acids), the band at 1739 cm-1 wasassigned to ester CdO vibrations.34 A band at 1750 cm-1 wasfound also for CNTs after boiling in concentrated HNO3;35 inthis case, however, the existence of ester groups must beexcluded: in strong acid media, they are hydrolyzed into COOHgroups. In our case, the shoulder at 1750 cm-1 cannot be dueto ester absorption, for the same reason. One might suggest theformation of anhydrides through thermal condensation ofneighboring carboxylic groups; however, carboxy anhydridesare extremely reactive, especially with amines. Besides that,we did not detect this absorption in IR spectra of oxidizedSWNTs heated to 150-180 C in a vacuum in the absence ofamines.

One more observation that is difficult to explain is that inthe liquid-phase-impregnated samples (Figure 3c,d) the bandat 1720 cm-1 significantly increased as compared to that in the

Figure 8. Experimental thermogram (solid line) for the peak at m/z )43 for SWNTs gas-phase treated with nonylamine as a superpositionof two first-order effects: (a) E1 ) 81; (b) E2 ) 93.5, k0 ) 105.8 s-1.

Figure 9. A (12,0) zigzag SWNT-COOH (a) as an example of (n,0)SWNT models used to simulate adsorption of nonylamine on the insideand outside walls (carboxylic O-atoms are highlighted) and effect of non total SWNT energy (b) calculated with the MM+ force field.

Figure 10. Mutual orientation (a) of a zigzag (n,0) SWNT-COOH((20,0), as an example) and the nonylamine molecule inside it in MM+molecular mechanics simulation (carboxylic O-atoms and amine N-atomare highlighted) and effect of n on the difference between total energies(b) of nonylamine adsorption on the outside and inside SWNT wall.


gas-phase-treated sample. This band is apparently due to SWNTsand not to nonylamine because it does not increase in intensityto the same degree as the bands corresponding to nonylaminedo. To summarize, we believe that some of the existinginterpretations of IR spectra of CNTs are premature and needfurther extensive studies.

Looking for other techniques that could be more informativefor chemical characterization of SWNT-amine systems, wetested temperature-programmed desorption mass spectrometry(TPD-MS), which is widely used to study organic groups andmolecules on inorganic adsorbent surfaces,17-19 including carbonmaterials.36-39 A first feature found in the mass spectra ofSWNTs gas-phase treated with nonylamine is a series ofhydrocarbon peaks appearing at temperatures of 200 C(Figure 6a) at m/z ) 27 (C2H3), 29 (C2H5), 41 (C3H5), 42 (C3H6),43 (C3H7), 55 (C4H7), 56 (C4H8), 57 (C4H9), 67 (C5H7), 69(C5H9), 70 (C5H10), 71 (C5H11), 79 (C6H7), 81 (C6H9), 83(C6H11), 84 (C6H12), 85 (C6H13), 97 (C7H13), 98 (C7H14), and99 (C7H15). Thermograms (i.e., plots of peak intensities vstemperature) for all of these peaks have a similar shape; theypass through a maximum at 325 C (as an example, Figure6b shows thermograms for selected peaks at m/z ) 42, 43, 55,57, and 69), and total disappearance of the hydrocarbonfragments is observed at 400 C. (No hydrocarbon peaks werefound in TPD mass spectra of SWNTs without amine treatment,recorded for comparison under the same conditions, in the sametemperature interval.) Taking into account the consistency oftheir behavior, one can infer a common origin of the hydro-carbon fragments. Moreover, by searching for similar massspectra in the Wiley 138 K Mass Spectral Library40 (includingabout 138 000 spectra), we found that both the mass numbers

and the peak intensity distribution correspond to nonene. Thesmooth curve of decreasing intensities (Figure 6a) is charac-teristic of straight-chain hydrocarbons and not of branched ones.At the same time, location of the double bond is difficult, asusual for acyclic olefins, because of its facile migration in thefragments.41 We suggest it to be at position 1 and explain theformation of 1-nonene by pyrolysis of nonylamide terminalgroups in SWNTs according to the following scheme:

Apparently, nonylamine molecules strongly adsorbed in SWNTsdecompose in a similar way, forming nonene and ammonia.The presence of two forms of nonylamine can be confirmed bythe following results.

For a detailed analysis of the thermal evolution of thehydrocarbon fragments, we selected the peak at m/z ) 43 asone of the most abundant and illustrative peaks. According tothe analytical procedure described in ref 19, for this peak, weplotted the logarithm of the desorption rate k as a function ofinverse temperature, ) (KT)-1, supposing first, second, andthird reaction orders (Figure 7). The dependence appears to belinear for the first-order reaction. The calculated values ofactivation energy and preexponential factor (E ) 79.6 kJ mol-1and k0 ) 105 s-1, respectively) are very low, suggesting theexistence of an activation-energy distribution for the processof destruction of the nonylamine species. An attempt to fit theobserved dependence with a rectangular distribution of activationenergies18 gave unsatisfactory results. The reason becomes clearif we consider the shape of the thermodesorption curve (for m/z

Figure 11. TPD mass spectrum (a) of volatile products evolved at 252 C from SWNTs gas-phase treated with dipentylamine; experimentalthermograms (b) for selected hydrocarbon peaks at m/z ) 42, 55, 56, 57, and 70.

SWNT-CO-NH-(CH2)8-CH3 fSWNT-CO-NH2 + H2CdCH-(CH2)6-CH3 (2)


) 43, as well as for other hydrocarbon peaks); it is noticeablyasymmetric. Good agreement with the experimental curve wasobtained for two activation energies, admitting the existence oftwo types of nonylamine species in the SWNTs, with E1 ) 81and E2 ) 93.5 kJ mol-1 (Figure 8). This agrees well with theIR spectral results if one assumes that one of them is the terminalnonylamide derivative (due to the first thermodesorptionmaximum at 250 C and lower abundance of hydrocarbondecomposition products) and that the other, more abundant, isnonylamine physisorbed on SWNTs because of strong hydro-phobic interactions (due to the second thermodesorption maxi-mum at 320 C and higher abundance of hydrocarbondecomposition products). Their abundance ratio estimated fromthe maxima of curves a and b is 1:5.

Where exactly in the SWNTs are nonylamine molecules mostlikely to physisorb? Evidently, there should be a well-manifestedenergetic difference between adsorption outside and inside CNTsbecause of a cooperative wall effect in the latter case. Weattempted to estimate until what SWNT size this difference canbe observed, using MM+ molecular mechanics. As SWNTmodels, zigzag species were usedsvariable in diameter (startingwith n ) 12) but of the same length (Figure 9a), sufficient toaccommodate one nonylamine molecule oriented along theSWNT axis. For narrower SWNTs (n ) 12 and 13), which aremore strained because of a stronger distortion from the plaingraphite sheet, the calculated total energies were positive; thevalues become negative starting with the (14,0) SWNT, as canbe seen from Figure 9b. Because of the high hydrophobicity ofnonylamine, it sticks strongly at any site on the SWNT walls,and depending on that, the total calculated energies can varywithin up to 2 kcal mol-1. To obtain more uniform andcomparable results, in all simulations, we chose the same startinggeometry, in which the following three criteria are met: (1)the amino group of nonylamine is placed close to the SWNTcarboxylic group, (2) the two molecules are parallel to eachother, and (3) the C-chain plane is parallel to the closest SWNTwall. For narrower SWNTs, in the calculated inside com-plexes, the molecules tended to remain parallel, but the nonyl-amine gradually rotated with increasing SWNT diameter, as canbe seen, for example, for a (20,0) SWNT (Figure 10a). In theoutside complexes, this effect was already observed for n )12, for which the angle between the SWNT and the nonylamineaxes was 10. For a (30,0) SWNT (the biggest considered inthe present study), the angles for the inside and the outsidenonylamine orientation both reached 30. For even bigger n(e.g., 40 and 60), the angles increase further, although we havenot been able to finish the geometry optimization because ofconvergence problems. Nevertheless, this trend is evident, andone can conclude that hydrophobic interactions contribute moresignificantly to long-chain amine adsorption on a SWNT thaninteraction between the polar NH2 and COOH groups does.

As regards the energetic differences between adsorptionoutside and inside a SWNT (plotted in Figure 10b), it wasbiggest (28 kcal mol-1) for a (12,0) SWNT because of thegreater effect of other walls. Then, Eout/in drops rapidly withincreasing n up to 20, and its further decrease becomes veryslow: even for a (30,0) SWNT (of which the diameter is 2.3nm for the MM+ geometry), Eout/in is almost 5 kcal mol-1.Thus, the energetic difference discussed should be significantfor a wide size range of CNTs (both single- and multiwalled),and the preferential sites for adsorption of long-chain amines(and most likely of other organic molecules) are inside SWNTs.

As was mentioned before, we found no IR-spectral mani-festations of interaction of gaseous dipentylamine with oxidized

SWNTs. However, the TPD-MS method was able to provideuseful information on the reaction products. In some respect,its behavior is similar to that of the nonylamine sample. Themass spectra of the desorbed species (Figure 11a, for 252 C,as an example) contain a series of peaks due to hydrocarbonfragments, but the highest mass number detected was 70. Thispeak, as well as the peaks at lower m/z, corresponds to pentene(molecular weight 70); this identification was made by searchingin the Wiley 138 K Mass Spectral Library.40 The formation ofpentene can be explained by thermal decomposition of thedipentylamide groups on the SWNT tips, by analogy with theprevious case [ reaction 2]:

Thermodesorption curves (Figure 11b) for different hydro-carbon peaks have similar profiles. However, a big differencebetween the two samples is that the curves are almost symmetricfor the dipentylamide derivative. Their maximum was found at250 C; this approximately coincides with the first maximumfor the nonylamine sample. Evolution of the hydrocarbon speciesceased at

Rather surprising was the absence of any hydrocarbonfragments in TPD mass spectra of the SWNT samples treatedwith vaporous ethylenediamine and propylenediamine under thesame (and even more harsh, up to 220 C) temperatureconditions. Chemical inertness of aliphatic amino groups in thesecompounds is deemed totally impossible. The only remainingexplanation is that the terminal amide species formed undergofurther in situ chemical transformations and cleave off of theSWNT tips. An argument in its favor is the formation of acolorless, sticky (apparently polymeric) substance on the upper,cold wall of the reactor. This finding is of special interest forus and will be studied in more detail in the future.

All of the molecular-mechanics calculations presented aboveinvolved zigzag SWNTs. Nevertheless, both zigzag and arm-chair species are apparently present in CNTs grown by mostmethods. The different reactivity of terminal functional groupson different CNT species is an important aspect of theirchemistry, and we have attempted to address it in connectionwith the direct amide formation reaction 1. From commonconsiderations, depending on whether the nanotubes have anarmchair or zigzag structure, the reactivity of carboxylic groupsshould change because of their different spatial orientation withrespect to the SWNT axis.

To verify whether the differences in geometric and energeticreaction parameters can be substantial, we performed a theoreti-cal study of the reaction of monocarboxylated short fragmentsof (10,0) zigzag and (5,5) armchair SWNTs (Figure 12) withthe simplest aliphatic amine, methylamine. Because the wholemolecular systems considered turn out to be too large to allowtheir treatment at a sufficiently high (ab initio or DFT)theoretical level, we employed a two-level ONIOM approach.The hybrid B3LYP functional (in conjunction with the 4-31G

basis set) was used to describe the high level consisting ofmethylamine and carboxylic groups along with their adjacentC and H atoms, whereas the universal force field (UFF)molecular mechanics was used for the low-level treatment ofthe remaining part of the nanotubes. Although the total numberof atoms (and nanotube length) did not coincide in the twoSWNT models, important parameters such as the numbers ofB3LYP-treated atoms and of terminal H atoms were kept thesame.

Optimized full geometries and fragments treated at theB3LYP/4-31G theoretical level of the reaction complexes,transition states, and products for the gas-phase reaction of a(10,0) zigzag and a (5,5) armchair SWNT with methylamineare presented in Figures 13 and 14, respectively. As could beexpected, there are noticeable differences in the geometries atevery stationary point. For example, for the reaction (van derWaals) complexes, in the case of the (5,5) armchair SWNT,there is a weak hydrogen bond between the amino andcarboxylic groups (2.229 ; Figure 14a), whereas the NHOdistance in the case of the (10,0) zigzag is far too long to qualifyit for H bonding (3.600 ; Figure 13a). For the transition statesand products, the differences are less prominent and do notexceed 0.14 for interatomic distances. In terms of energies(relative to the reactant level), their differences for the reactioncomplexes and transition states are minor: for the zigzagSWNT, -5.8 (reaction complex) and 36.3 kcal mol-1 (transitionstate); for the armchair SWNT, -4.2 (reaction complex) and37.4 kcal mol-1 (transition state). In other words, the tworeactions have activation barriers of approximately equal height(42.1 and 41.6 kcal mol-1, respectively). Where a big differencecan be observed is in the energy of the products: -7.6 kcalmol-1 for the zigzag SWNT vs. -19.4 kcal mol-1 for the

Figure 13. Optimized full geometries (upper row) and fragments treated at the B3LYP/4-31G theoretical level (lower row) of the reaction complex(a), transition state (b), and products (c) with selected interatomic distances and calculated B3LYP energies (in kcal mol-1, relative to the reactantlevel) for the model gas-phase reaction of a (10,0) zigzag SWNT (Figure 12a) with methylamine.


armchair SWNT. Thus, at least at the present computationallevel, the direct formation of amides on armchair SWNT tipsis more energetically favored than that on the zigzag tips.

Conclusions(1) IR spectra of oxidized SWNTs treated with amines under

different conditions (described here and elsewhere3,4,6,7) cannotcorrespond to amide derivatives on SWNT tips, because thevery low concentration of the terminal groups relative to thewhole sample mass results in a negligible contribution to theIR spectra. The bands detectable in the case of long-chainamines correspond to amine molecules, physisorbed due to thestrong hydrophobic interactions of their hydrocarbon chains withSWNT walls. Energetically preferable sites for the adsorptionof amine molecules (and most likely of other organic molecules)are the channels inside SWNTs, according to MM+ molecular-mechanics modeling.

(2) Temperature-programmed desorption mass spectrometrycan provide additional information on the chemical state of theamines. Heating of the amine-treated SWNTs at >200 C causescleavage of alkenes from the amine residues: nonene andpentene form in the case of nonylamine and dipentylamine,respectively. For the short-chain amine (dipentylamine), onlyone chemical form was detected, whereas two forms (amideand physisorbed amine) can be distinguished for the SWNTstreated with nonylamine. The content of physisorbed nonylamineis about 1 order of magnitude higher than the amide content.

(3) According to the results of two-level ONIOM quantum-chemistry-molecular-mechanics calculations, the direct forma-tion of amides on armchair SWNT tips is energeticallypreferable to that on the zigzag tips, although the activationbarriers are of approximately equal height.

(4) The gas-phase technique described can be considered asa simplified and convenient method of the derivatization ofcarboxylated CNTs with aliphatic amines. It requires nochemical activation of the carboxylic groups, it is relatively fast(0.5-1 h), excess of the derivatizing reagent is spontaneouslyremoved from the reaction zone, and there is no need to use a(organic) solvent medium (this feature not only is attractive froman ecological point of view, but also helps to avoid undesirableparticle aggregation of the material derivatized).

Acknowledgment. Financial support from the NationalAutonomous University of Mexico (Grant DGAPA IN-106900)and from the Science and Technology Center of Ukraine (ProjectNo. 2196) is greatly appreciated. The authors would like to thankMr. Salvador Ham for making the glassware.

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Figure 14. Optimized full geometries (upper row) and fragments treated at the B3LYP/4-31G theoretical level (lower row) of the reaction complex(a), transition state (b), and products (c) with selected interatomic distances and calculated B3LYP energies (in kcal mol-1, relative to the reactantlevel) for the model gas-phase reaction of a (5,5) armchair SWNT (Figure 12b) with methylamine.


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