Oxidation of C60 Fullerite by Interstitial Oxygen - The Journal of

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J. Phys. Chem. C 2008, 112, 12096–12103

ARTICLES Oxidation of C60 Fullerite by Interstitial Oxygen Y. M. Shulga,† V. M. Martynenko,† V. V. Open’ko,† A. V. Kulikov,† A. Michtchenko,‡ E. Johnson,§ M. D. Mochena,| and G. L. Gutsev*,| Institute of Problems of Chemical Physics of Russian Academy of Sciences, ChernogoloVka, Moscow District, Russia, 142432, ESIME-SEPI, IPN, Zacatenco, Mexico, D.F., C.P. 07738, Mexico, EnVironmental Sciences Institute, Florida A & M UniVersity, Tallahassee, Florida 32307, and Department of Physics, Florida A & M UniVersity, Tallahassee, Florida 32307 ReceiVed: NoVember 9, 2007; ReVised Manuscript ReceiVed: June 12, 2008

Processes induced by the heating of C60 fullerite intercalated by oxygen are analyzed using mass-spectrometry, thermogravimetry, differential scanning calorimetry, and electronic spin resonance (ESR) techniques. It was found that the primary gas produced at the heating temperatures below 100 °C is molecular oxygen while at higher temperatures up to 200 °C carbon mono- and dioxides were also observed. The heating was accompanied by an appreciable increase in the ESR signal intensity. In order to gain insight into the oxidation products that are capable to contribute to the ESR signal, we performed all-electron density functional theory computations for C58On (n ) 0-4), C59On (n ) 0-2), and endohedral complexes O2@C58, O2@C59, and O2@C60. It is found that the triplet states of C58, C58O3, O2@C58O2, O2@C58, and O2@C60 are lower in total energy than the corresponding triplet states. The singlet and triplet states of C59, O2@C59, and C602- are nearly degenerate in total energy. Thus, there are a number of species that can be responsible for the paramagnetic behavior observed in the oxidized fullerene. Introduction 1985,1

a large amount Since the discovery of C60 fullerene in of experimental and theoretical efforts was devoted to the study of various gas-phase and solid fullerenes,2 charged fullerenes3 and interactions of fullerenes with organic and inorganic species.4 C60 fullerene was found5 to interact with ozone and form C60O. A number of other fullerene oxides and ozonides were discovered6 since then. For example, highly oxygenated fullerenes C60On with 3 e n e 9 were recently prepared7 by enhanced catalytic oxidation of C60. Formation of C60O- and C58O- as predominant species was observed when studying reactions of molecular oxygen clusters with laser ablated fullerenes,8 while only odd-numbered C53-, C55-, C57-, and C59anions were observed9 during the laser desorption ionization of C60O-. A large number of different oxidation products, which include C56, C58, C58O, C58O2, and C59, were identified10 in mass spectra recorded during C60 ozonization. The C59O- and C58O2anions were observed11 during chemical ionization of C60 in the presence of nitric oxide. Oxidation of fullerite in air was studied previously12 and led to a conclusion that the oxidation product is a polycondensate with the C:O ratio of 5:1. However, oxidation of fullerite by intercalated oxygen is different from air oxidation. 13 Since the fullerite lattice is face-centered cubic, intercalated O2 molecules * To whom correspondence should be addressed. E-mail: gennady.gutsev@ famu.edu. † Institute of Problems of Chemical Physics of Russian Academy of Sciences. ‡ ESIME-SEPI. § Environmental Sciences Institute, Florida A & M University. | Department of Physics, Florida A & M University.

can occupy octahedral interstitials. The volume magnetic susceptibility of molecular oxygen at 20 °C and one atmosphere pressure (standard conditions) is 142 × 10-9 c.g.s.m units14 and, correspondingly, its molar magnetic susceptibility χM is 3183 × 10-6 cm3 (≡ c.g.s.m units). The pure C60 fullerite is diamagnetic15,16 with χM ) -250 × 10-6 cm3 at 25 °C. Therefore, an injection of more than 0.073 mol of triplet oxygen into one mol of C60 fullerite would be sufficient to observe a transition to a paramagnetic state at room temperature. However, it was found13 that the intercalant obtained is diamagnetic and disappearance of the O2 paramagnetism was interpreted13 using the results of C60O2 computations. The present work is aimed at a study of oxidation of C60 fullerite by interstitial oxygen during the heating. On the experimental side, processes accompanying the heating of (O2)0.52C60 in vacuum were carefully analyzed using massspectrometry (MS), thermogravimetry (TG), and differential scanning calorimetry (DSC) measurements. We paid special attention to the magnetic behavior of samples during the heating using the ESR measurements. On the theoretical side, we optimized geometrical structures of a large number of singlet and triplet states of oxidized C58On (n ) 0-4) and C59On (n ) 0-2) along with endohedral complexes O2@C60-x (x ) 0-2) using the all-electron density functional theory with generalized gradient approximation (DFT-GGA). Experimental Details Samples used in our measurements are of three types: (1) fullerite intercalated with molecular oxygen (O2)0.52C60; (2) fullerite intercalated with argon Ar0.75C60; and (3) fullerite of 99.98% purity. Mass spectra of gases formed during the heating

10.1021/jp710745f CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

Oxidation of C60 Fullerite by Interstitial Oxygen

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12097

Figure 1. Mass-spectra of gases produced by C60 fullerite doped with molecular oxygen during fullerite heating in the temperature range of 20-100 °C (1); 100-200 °C (2), and 200-300 °C (3).

of solid fullerite in vacuum were analyzed using an MI 1201V mass spectrometer. Our TG and DSC analyses of samples were performed with the use of Q-1500 and DSC 822 (MettlerToledo) devices, respectively. Content of the gas produced during the heating was estimated from our TG data with accounting for the corresponding mass spectra and DSC data. Our ESR spectra in the 3-cm range were recorded using an SE/ X-2544 spectrometer (Radiopan). Experimental Results Mass spectra of gases produced by (O2)0.52C60 are presented in Figure 1. As is seen, the most intense peak in spectrum 1 recorded at the temperatures from 20 to 100 °C corresponds to m/z ) 32, i.e., to the O2+ ions. The second largest intensity peak is located at m/z ) 16 and is due to the O+ ions. Peaks at m/z ) 18 (H2O+) and m/z ) 28 (N2+ and/or CO+) have a rather similar intensity. The peak at m/z ) 28 corresponds likely to a mixture of N2+ and CO+ since the spectrum contains features at m/z ) 14 and m/z ) 12 corresponding to the N+ and C+ atomic ions. The presence of molecular nitrogen can be due to its adsorption by the fullerite surface during air exposure. All other peaks in spectrum 1 have rather low intensities. The gas produced at the temperatures from 100 to 200 °C consists mainly of molecular oxygen (spectrum 2 in Figure 1). However, another intense peak appears at m/z ) 28 and the intensity of the m/z ) 44 peak corresponding to CO2+ is substantially increased. These changes with respect to spectrum 1 are to be attributed to oxidation of the fullerite by interstitial oxygen. In the temperature range from 200 to 300 °C, molecular oxygen is depleted and the gas produced consists predominantly of carbon oxides presented by peaks at m/z ) 28 and 44, (see spectrum 3). Note, that the spectra recorded at low temperatures may contain peaks correponding13 to the precipitatorsisopropyl alcohol. The peak at m/z ) 45 has the maximum intensity in the mass spectrum of pure isopropyl alcohol. But its intensity is only about 1% with respect to the main peak intensity and is less than 5% in the temperature range from 100 to 200 °C in spectrum 1. There are no traces of isopropyl alcohol at higher temperatures. No feature that could be attributed to the solvent 1,2-dichlorobenzene was observed in the whole temperature range. The major component of the gas produced by the heating of fullerite doped with argon is argon itself in the whole temperature range from 20 to 300 °C. At heating temperatures up to

Figure 2. TG curves of the mass loss for samples of C60 fullerite doped with molecular oxygen obtained at the heating rate of 10 °C/min in atmosphere of dry argon. The curve number corresponds to the sample number. Curve 3′′ was obtained when heating sample 3 in atmosphere of dry air.

100 °C, a small amount of water and nitrogen molecular species was produced. Their appearance can be attributed to the fullerite surface adsorption during air exposure. At higher heating temperatures (300-450 °C), the mass spectra contain peaks corresponding to carbon oxides. These peaks can be related to the presence of molecular oxygen or, more likely, water which have been trapped inside fullerite during its precipitation from solution since both solvents have not been specially cleaned from water. Figure 2 shows the mass loss of (O2)xC60 during its heating in the argon atmosphere. As is seen, the corresponding curve has no peculiarities, and the mass loss during the heating up to 500 °C is 3.02%. The mass loss curve for ArxC60 possesses no peculiarities as well. The total mass loss of ArxC60 (sample 2) is 4.0% while the total mass loss of pure fullerite (sample 3) is 1.42% (see curve 3). Curve 3′′ shows the mass loss of pure fullerite during its heating under air exposure. It is seen that oxidation starts at 329 °C and the total mass loss at the heating up to 500 °C is about 11%. The mass loss curves can be used for estimating the argon and oxygen content in the samples. The argon content in ArxC60 is estimated under assumptions that only argon is produced during heating and that sublimation of C60 is negligibly small. The latter assumption is justified by lacking a characteristic brown O-ring deposition on the cold part of quartz ampule in the course of the mass spectrum analysis at 500 °C. At the maximum percent ratio (4%) of argon produced from ArxC60, we found that x ) 0.75. The latter value follows from the equation 0.04 ) xMAr/(60MC + xMAr), where MC and MAr are the atomic masses of carbon and argon, respectively. It is more difficult to estimate the amount of oxygen because both CO and CO2 are produced during (O2)xC60 heating. On the basis of data obtained in this work, one cannot indicate the exact temperature at which O2 stops yielding and formation of CO and CO2 begins. However, one can estimate x in an (O2)xC60 sample assuming that only O2 is produced at temperatures up to 193 °C (∆m/m ) 0.7) and only carbon oxides CO and CO2 are produced at higher temperatures in a ratio 1:1 according to the mass spectra of gases taken for a sample heated to 300 °C. The choice of 193 °C as a threshold temperature will be clarified by the reason given below. The value of x, calculated under

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Figure 3. DSC curves of the samples obtained at the heating rate of 10 °C/min in atmosphere of dry argon. The curve number corresponds to the sample number. Curve 1′ shown by dots corresponds to sample 1 after it was heated under the DSC regime to 500 °C and then cooled to 75 °C.

this assumption, is 0.52. The real value of x is somewhat higher if the sample possesses oxygen-containing fragments after heating to 500 °C. Figure 3 displays the results of our DSC measurements. Curve 1 obtained for a (O2)0.52C60 sample shows a peak at 193 °C with the corresponding calorific effect value normalized to the sample mass (∆H) of 135 J/g. It is worth noting that we used three samples with masses of 5, 10 and 15 mg and found that the mass had no influence on the ∆H value. The peak at 193 °C corresponds to an irreversible process since it does not appear during the repeated heating (see curve 1′). The process related to this peak is most likely due to the presence of oxygen in the sample because (1) this peak is absent in curves 2 and 3 corresponding to the fullerene doped with argon and the pure fullerite, respectively; (2) for a sample with a lower oxygen content [(O2)0.2C60, the corresponding curve is not shown], the heating effect decreases and the peak maximum shifts to 206 °C. Such a behavior can be explained by a decrease in the yield of heat due to oxidation of C60 with respect to that in samples with higher oxygen content. The energy of fullerene combustion C60 + 60O2 f 60CO2 equals17 36 J/g. Assuming that the peak at the DSC curve is related to the oxidation process, one can estimate the sample content as corresponding to (O2)0.23C60. The value of 0.23 is essentially lower than that obtained from the TG data above. The discrepancy is likely related to a partial removal of O2 before oxidation reactions do occur. It is also possible that the intercalated oxygen is presented in the fullerite lattice in different states but only one state contributes to the peak at 200 °C on the DSC curve. It is generally accepted18–21 that the C60 fullerite oxidation by the gas-phase oxygen, accompanied by the CO and CO2 formation, occurs at 570 K (297 °C). We found a somewhat higher temperature of 602 K (329 °C) required for oxidation of our large-crystalline fullerite samples. Oxidation of fullerite by interstitial oxygen occurs at substantially lower temperaturess below 200 °C. Therefore, the fullerite annealing in vacuum at temperatures of 200 °C and higher used in many studies for removing the oxygen dissolved in fullerite19,22 leads to partial destruction of some fullerene units in the fullerite. We found that the intensity of the ESR signal shown in Figure 4 for an (O2)0.52C60 sample grows as the heating temperature

Shulga et al.

Figure 4. ESR spectra of the sample before (1) and after (2) heating at 300 °C in vacuum. The intensities and g-factors of the spectra are shown in Table 1.

TABLE 1: Dependence of the ESR Signal Parameters of (O2)xC60 on Heating Temperaturea annealing peak half-width radical concentration, temperature, °C (∆Hpp), mT g-factor 1016 spin/g 20 100 200 300 450 a

0.14 0.14 0.24 0.26 0.26

2.0023 2.0023 2.0024 2.0024 2.0025

3.2 4.2 1240 1310 540

Sample weight is 30 mg.

increases, which is accompanied by a decrease in the oxygen content of the sample. The intensity growth continues up to 300 °C (see Table 1). One may assume that such a temperature dependence of the ESR signal is related to the formation of species consisting of oxygen and C60-x, where x ) 0, 1 or 2. In particular, formation of C59 corresponds to releasing CO2 species while formation of C58 corresponds to the release of two CO. Computational Details. Our computations were performed using the Gaussian 0323 codes. We used the 6-311G* basis set,24 (11s5p1d)/[4s3p1d], since it is known that diffuse functions in the 6-311+G* (12s6p1d)/[5s4p1d] cause accidental overlap problems25 in linear Cn species for n > 10. For systems whose symmetry is low, it is not important because Gaussian 03 eliminates linear dependent combinations of basis functions. However, for high symmetries like Ih in C60 the overlap problem may persist. All our computations are performed using the BPW91 method, where the exchange-correlation functional is comprised of the Becke’s exchange26 and Perdew-Wang’s correlation.27 A recent comparison of the semilocal and hybrid density functionals may be found elsewhere.28 Generally, the results obtained using pure and hybrid DFT methods are rather similar.29–31 In order to estimate an anticipated difference between the results of computations performed using the 6-311G* and 6-311+G* bases, let us compare the corresponding total energies obtained for C60 (the initial coordinates of C60 (buckyball) are from the CCL archive32). Total energy of C60 (Ih,) is -2286.47442 au using the 6-311G* basis and symmetry is resolved (1A1g). Adding diffuse functions lowers the total energy to -2286.49736 au, while symmetry of core orbitals is unresolved. Harmonic


Figure 5. Optimized singlet and triplet states of C58. The cavity boundary atoms are shown in red color. The excess spin densities given for the triplet state are in electrons.

vibrational frequencies of C60 (Ih,) obtained at the BPW91/6311G* level are in good agreement with experiment: our computed first ω1-5(hg) 255.85 cm-1 and last ω170-4(hg) ) 1559.44 cm-1 harmonic vibrational frequencies are to be compared to the experimental values33–35 of 266-273 and 1572-1574 cm-1, respectively. Preliminary optimizations were performed for both singlet and triplet states using a smaller 6-31G basis followed by harmonic frequency computations. Next, the nontransition state structures were reoptimized using the 6-311G* basis. In cases where the singlet and triplet total energies were nearly matching, we performed additional optimizations using 6-311+G* basis. Excess atomic spin densities in the triplet states were computed using the Mulliken36 population analysis. Computational Results. The results of our computations of C60-xOy and O2@C60-x for x ) 0-2 and y ) 0-4 performed at the BPW91/6-311G* level are presented in Figures 5–11. Zero total energy is set to the energy of the lowest state for a given isomer series. The singlet-triplet splitting is obtained as the difference in total energies of the corresponding singlet and triplet states [∆E tot(T - S)) Etot(T) - Etot(S)], i.e., its negative value means that the triplet state is lower than the singlet state. Let us consider the results obtained for different possible oxidation products. C58. The lowest energy fullerene structure of C58 possesses37 formal C3V symmetry that lowers to Cs due to Jan-Teller distortions. In the correspondence with the Hund rule, the triplet state is lower than the singlet state by 0.07 eV (see Figure 5). Reoptimizations using the 6-311+G* basis lead to a larger value of 0.13 eV. Thus, it is unlikely that this result is an artifact of the basis set and such a fullerene formed in irradiated or oxidized fullerene will contribute to the ESR signal.


Figure 6. Optimized singlet and triplet states of C58O and C58O3. The excess spin densities are given for the triplet states.

Two adjacent C-holes in C60 can form geometrical configurations of three types.38 Two lowest energy structures are shown in Figure 5. As is seen, the singlet state whose geometrical configuration contains a 7-member ring is rather close in total energy to the ground state. One may assume that oxidation accompanied by CO formation may proceed from different interstitials that would result in two isolated C-holes as shown in the left top corner of Figure 5. There are two localized electron states in the triplet state supported by the hole boundary atoms. This state is appreciably lower in total energy than the corresponding singlet state and can contribute to the ESR signal; however, this state is above the ground-state by 7.5 eV and, therefore, its geometry should relax to that corresponding to a lower energy state. C58O. Oxygen attaches to 7- and 8-member rings of C58 in a bridge position (see Figure 6), and both isomers are close in total energies. The corresponding triplet states are appreciably higher in total energy and cannot contribute to the ESR signal. C58O2. Adding the second O atom to C58O and 2-hole C58 results in the geometrical configurations shown in Figure 7. None of the corresponding triplet state is lower or close in total energy to the corresponding singlet state. C58O3. The lowest energy singlet state found contains one bridged and two single-bonded oxygen atoms (see the bottom of Figure 6). The corresponding triplet state is appreciably lower in total energy than the singlet state and can contribute to the ESR signal. There is also an adduct complex O-C58O2 whose triplet state is significantly lower than the singlet state, which can be related with the triplet state of the O atom. Note that the BPW91/6-311G* singlet-triplet splitting in the O atom itself is 0.74 eV.


Shulga et al.

Figure 8. Optimized singlet and triplet states of C58O4. Figure 7. Optimized singlet and triplet states of C58O2.

C58O4. Adding one more O atom to the ground-state C58O3 leads to a symmetrical distribution of 4 oxygen atoms on the top of 4 carbon atoms and increases the number of carbon hole boundary atoms to 12 (see Figure 8). A similar attachment of 3 oxygen atoms was experimentally observed39 in fullerenones functionalized by different groups. The internal dissociation of O2 is highly unfavorable (see the left top corner in Figure 8). A peroxo-configurations (with a nondissociated O2) is higher in total energy by 3.41 eV. Its triplet state is nearly degenerate with the singlet state but corresponds to a transition state. During optimizations beginning with this transition state geometry modified according to the imaginary frequency mode, O2 penetrates inside C58O2 and stops in the cage center with the O2 axis oriented toward the hole center. There is no chemical bonding between O2 (R(OsO) ) 1.235 vs 1.221 Å in free O2) and the carbon frame; therefore, it should be purely electrostatic. Indeed, the Mulliken and NBO40 analyses show that the endohedral O2 carries a negative charge of -0.54 e and -0.9 e, respectively, similar to that41 in an endohedral Ag2 in C60. The triplet state of O2@C58O2 is lower than the singlet state by 0.30 eV and can contribute to the ESR signal. C59. Removal of one carbon atom from C60 may lead to the formation of two geometrical configurations38 of C59, whose hole boundaries consist of 8 and 9 carbon atoms. The lowest energy state is presented in the left top corner in Figure 9. As is seen, one of the hole boundary atoms is two coordinate, while all others are three coordinate. The Mulliken population analysis shows that this atom has the excess spin density of 1.08 e and the NBO analysis gives a close value of 0.99 e. The singlet state is below the triplet state by 0.03 eV (the 6-311G* basis)

and 0.04 eV (the 6-311+G* basis). One may anticipate that the triplet state is populated at higher temperatures and can contribute to the ESR signal. C59O. Adding an oxygen atom to C59 results42,43 in three configurations presented in Figure 9. In accordance with the previous work,43 performed at the B3LYP/6-31G* level, the lowest state has an 8-ring hole with an oxygen atom attached to an atom that was two coordinate in C59. There are no triplet states that could contribute to the ESR signal. C59O2. Its optimized configurations are shown in Figure 10. The lowest energy state geometry is formed from the 9-ring geometry of C59O by adding a bridged O atom in a position that is symmetric with respect to the single-bonded O atom. All triplet states for the C59O2 configurations corresponding to dissociated O2 are substantially higher in total energy with respect to the corresponding singlet states. The endohedral complex O2@C59 is appreciably higher in total energy and possesses by the singlet and triplet states that are degenerate in total energy. Examination of the excess spin densities on the O atoms shows that the O2 dimer retains its triplet character in the both singlet and triplet states. The excess spin density of the carbon cage is delocalized and coupled antiferromagnetically to the excess spin density of O2 in the singlet state. O2@C58 and O2@C60. Endohedral complexes of buckyball C60 formed by implantation of different chemical group inside the cage have been intensively studied.44,45 The C60 fullerene cavity is apparently too small46 for many practical purposes, except for H2 that has been experimentally implanted47 inside C60. Both O2@C60 and C60-O2 complexes were found48 to be endothermic that is in agreement with our result presented in Figure 11. The O2 dimer in the singlet state of O2@C60


Figure 9. Optimized singlet and triplet states of C59 and C59O. The excess spin densities are given for the triplet state.

corresponds to a singlet state of free O2, i.e., it carries no excess spin density while each O atom carries the excess spin density of 0.95 e in the triplet state of O2@C60. The triplet state is below the singlet state by 0.30 eV that can be compared to the BPW91/ 6-311G* singlet-triplet splitting of 0.39 eV in free O2. Thus, the O2@C60 endohedral complex will certainly contribute to the ESR signal. Figure 11 presents also two C58 isomers from Figure 5 doped with O2. They are remarkably distinct. If the oxygen dimer is in its triplet state in both singlet and triplet states of the groundstate C58 fullerene doped with O2, then the dimer has a singlet state in the C58 cage formed by removal of two adjacent carbon atoms. Such a behavior lends support to an assumption that the C60 buckyball provides the smallest cage that allow the singlet state of O2 to be sustained. Additional Sources of Unpaired Electrons. The triplet states were found to be lower in total energy than the corresponding singlet states in two isomers of C60 with removed 4 adjacent carbon atoms.49 Similar defect structures were found50 to be responsible for ferromagnetic behavior in polymerized rhombohedral C60. Another source of unpaired electrons can be due to the negatively charged species such as dianion3 C602- or oxygen-linked fullerene dimer dianion51,52 C120O2-, because the singlet-triplet splitting in these dianions is very small. According to the results of our BPW91/6-311G* computations, the triplet state of C602- is below its triplet state by 0.01 eV. Of course, with such a small splitting, the order of the states depends strongly on the method and basis used.53,54 However,


Figure 10. Optimized singlet and triplet states of C59O2. The excess spin densities are given for the triplet state.

it was recently found55 that “most of the C602- ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature.” Therefore, C602- cannot be responsible for a sustained paramagnetism. Magnetic BehaWior in C60 Polymers. Magnetism of various carbon structures was the subject of many investigations.56–58 Ferromagnetism in C60 polymers, synthesized using highpressure high-temperature method, was questioned in a recent paper,59 whose authors attributed previous observations of magnetism in the C60 polymers as due to the presence of magnetic contaminants. On the contrary, another recent paper60 reported an observation of magnetic domains in heavy ion irradiated fullerene films. Magnetic behavior was also observed for a crystalline fullerene exposed to light in oxygen atmosphere61 and C60 films irradiated in the presence of oxygen,62 while we observed in the present work the paramagnetic behavior in samples oxidized by internal oxygen. Summary Oxidation of C60 fullerite intercalated by oxygen presents a rather complicated process that starts at 193 °C and continues at substantially higher temperatures. Oxidation is accompanied by the weight loss and release of carbon oxides into the gas phase. During oxidation, the intensity of the ESR signal of solid fullerite increases by several orders of magnitude. In order to gain insight into the oxidation products that could be responsible for such a drastic increase in paramagnetism, we performed a series of simulations of the oxide products using density functional theory with generalized gradient approximation to the exchange-correlation functional.


Figure 11. Endohedral complexes O2@C58 and O2@C60. The total energies of O2@C58 isomers are given relative the corresponding lowest energy singlet state in Figure 7.

It follows from the results of our computations that there are several potential sources of unpaired electrons that could contribute to the observed ESR signal: (i) Localized carbon states whose appearance is due to formation of adjacent or isolated C-holes in C60 units during oxidation. (ii) Some C60-x fullerene isomers may have triplet states that are lower in total energy than the corresponding singlet states. C58 fullerene presents such an example where the triplet state corresponding to the lowest total energy isomer found in ref 37 is somewhat below the singlet state. (iii) Oxidation products C60-xOy whose triplet states are lower in total energy than the corresponding singlet states. Such products found are C58O3, and O2@C58O2. In the latter product, there is no barrier for penetration of a triplet oxygen dimer into the cage of the oxidized C58O2 isomer. (iv) Endohedral complexes O2@C60-x. Acknowledgment. This research was partly supported by funding from the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce to the Environmental Sciences Institute at Florida A & M University (NOAA Award No. NA05OAR4811018). Portions of this research were conducted with high performance computational resources provided by the Louisiana Optical Network Initiative (http://www.loni.org). We are greatly appreciative to Dr. De-Li Chen for providing us the coordinates of C58 fullerene and helpful discussions. We are grateful to both reviewers for their valuable remarks. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162.

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