ARTICLE pubs.acs.org/JPCC
Photoelectron Spectroscopy of Sc3N@C78 Shojun Hino,*,† Masashi Zenki,† Takeyuki Zaima,† Yusuke Aoki,† Sosuke Okita,† Tomona Ohta,† Hajime Yagi,† Takafumi Miyazaki,† Ryohei Sumii,‡,§ Haruya Okimoto,|| Yasuhiro Ito,|| and Hisanori Shinohara|| †
Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan UVSOR, Institute for Molecular Science, Okazaki 444-8585, Japan § Research Center for Materials Science, Nagoya University, Nagoya 464-8602 Japan Graduate School of Science, Nagoya University, 464-8602 Japan
)
‡
ABSTRACT: Ultraviolet photoelectron spectra (UPS) and X-ray photoelectron spectra (XPS) of Sc3N@C78 are measured. The upper valence band UPS differ significantly from those of Ti2C2@C78 or La2@C78, although their cage symmetry is the same D3h. Chemical shift of the XPS of Sc 2p supports a much less oxidation state than +3, possibly +1 state, and that of N 1s suggests excess electrons on the N atom, presumably N1 oxidation state. Simulation spectra calculated by density functional theory on an optimized structure starting from D3h geometry reproduces the UPS very well, which supports the theoretically proposed structure of Sc3N@C78.
and the charge distribution of Sc+0.44 and N 0.54, namely, (Sc3N)+0.78@C78 0.78, was proposed.19,20 The calculations also pointed out the existence of a strong interaction between the entrapped cluster and the C78 cage. Experimental determination of the electronic structure as well as the oxidation state of Sc3N@C78 is one of the important issues to be investigated in the field of endohedral fullerenes. In this article, the UPS of Sc3N@C78 obtained using synchrotron radiation will be presented and compared with the UPS of La2@C78 and Ti2C2@C78. The XPS of Sc3N@C78 is also presented to evaluate the oxidation state of the entrapped species and the fullerene cage. The UPS are discussed with the aid of DFT calculations and the most plausible structure of which electronic structure reproduces the UPS will be presented. Spectral differences among these three C78 caged endohedral fullerenes are also discussed with respect to electron transfer from the entrapped species to the cage.
1. INTRODUCTION Fullerene cages entrap metal clusters,1,2 metal nitrides,3,4 and metal carbon clusters (metal carbides),5,6 as well as single metal atoms.7 Entrapped metal atoms often donate electrons to the cage. Extensive ultraviolet photoelectron spectroscopic studies of single metal atom entrapped metallofullerenes, M@C82, established an empirical rule that the cage structure principally governs the electronic structure of these metallofullerenes, and a difference in the amounts of transferred electrons to the cage is another factor to modify it.8 11 In other words, single metal atom containing endohedral fullerenes of the same cage symmetry with an incorporated metal atom of the same oxidation state give essentially the same ultraviolet photoelectron spectra (UPS). A similar relationship was found in the UPS of endohedral fullerenes encapsulating multiple atoms, such as Sc2C2@C82,12 Y2C2@C82,13 and M3N@C80 (M = Sc, Tm, and Dy).14 However, the UPS of La2@C78 and Ti2C2@C78 were significantly different,15 even though their cage symmetry was the same D3h (78:5). (Numbers in parentheses correspond to the nomenclature of Fowler and Manolopoulos.16) This difference was attributed to the nature of the Ti atoms. Namely, the wave functions of Ti and C atoms form hybrid orbitals, and the Ti C bonds in Ti2C2@C78 have both covalent and ionic characters.15 However, the difference in the amounts of transferred electrons in La2@C78 and Ti2C2@ C78 was not considered in ref 15. Density functional theory (DFT) calculations on Ti2C2@C78 suggested the (Ti2)4+ and (C2)2 states,17 and that of La2@C78 suggested the (La)6+@C786 state.18 Isolation of another C78 cage endohedral fullerene, Sc3N@C78, has also been reported.4 Since its absorption spectrum is significantly different from those of La2@C78 and Ti2C2@C78,4 it is highly plausible that their electronic structure might be mutually different. DFT calculations on Sc3N@C78 were also performed r 2011 American Chemical Society
2. EXPERIMENTAL AND CALCULATION METHODS The synthesis and isolation of Sc3N@C78 is reported in ref 4. The method adopted to obtain Sc3N@C78 was slightly different from the procedure used in ref 4. Details of isolation were basically the same as reported in ref 5. Soot containing Sc3N@C78 was produced by direct-current arc heating of a Sc2O3/graphite composite rod in a mixed He/N2 atmosphere. Sc3N@C78 was extracted using o-xylene from the soot and isolated using multiple-stage high performance liquid chromatography with toluene as an eluent. Samples for the photoelectron measurements were prepared by Received: August 9, 2011 Revised: November 1, 2011 Published: November 09, 2011 165
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vacuum sublimation of Sc3N@C78 onto a gold-deposited molybdenum disk (for the UPS measurement) and a gold-deposited gold disk (for the XPS measurement). Sublimation was conducted using a resistive heating quartz crucible in a preparation vacuum chamber directly attached to a photoelectron measurement chamber. The temperature of the crucible during Sc3N@C78 sublimation was about 850 K. The pressure of the chamber during the deposition increased to 1.1 10 6 Pa (base pressure before the deposition was less than 2.0 10 7 Pa). The reading of the quartz thickness monitor located beside the molybdenum disk during the deposition of Sc3N@C78 was about 10 nm, but this did not seem to reflect the actual thickness of the deposited film because the crucible was so collimated. Since the gold Fermi edge was not observed after repeated sample deposition, the thickness of the film might be several tens of nm. The Sc3N@C78 film on the gold disk was thin enough to allow photoelectron penetration from the substrate through the film. The UPS were measured using a photoelectron spectrometer at BL8B2 of UVSOR (Ultraviolet Synchrotron Orbital Radiation Facility) at the Institute for Molecular Science. The resolution of the spectrometer was 110 meV. Energy calibration of the UPS was carried out using the Fermi edge of a gold-deposited sample disk before the measurement. The UPS were referenced against the Fermi level. The base pressure of the measurement chamber was e5.0 10 8 Pa, and the pressure during the measurement was about 5.5 10 8 Pa. The XPS were measured using a photoelectron spectrometer that consisted of a SCIENTA SES100 electron analyzer, a MB Scientific L-1 and M-1 ultraviolet light source and a VG SCIENTA XR3E2 dual anode X-ray source. Energy calibration of the XPS was carried out using the Au 4f7/2 peak, and the pressure during the measurement was 2 10 7 Pa or less. Molecular orbitals of Sc3N@C78 were calculated with a Gaussian03 program module. The geometry of Sc3N@C78 was optimized at the Hartree Fock level using the 6-31G basis set. Simulation spectra generated by broadening the calculated Eigen values at the Hartree Fock level with Gaussian functions of 0.2 eV full width at half-maximum differed significantly from the observed UPS. The DFT calculation was performed on the optimized Sc3N@C78 structure using the B3LYP hybrid functional to obtain the Kohn Sham orbital energies. Basis sets to obtain the Eigen values of Sc3N@C78 were 6-31G-(d) for C atoms and the TK/NOSeC-V-TZP function21,22 for Sc atoms. Simulation spectra obtained by the same procedure described above using Kohn Sham orbital energies reproduced the UPS far better than those obtained from the Hartree Fock calculation. Therefore, the simulation spectra in the following text are the result of the DFT calculation. Mulliken charges and natural population analysis (NPA) charges of Sc3N@C78, La2@C78, and Ti2C2@C78 were calculated on the Hartree Fock level optimized structure using B3LYP and Lanl2dz functions.
Figure 1. Incident photon energy dependent ultraviolet photoelectron spectra of Sc3N@C78. Numbers beside each spectrum indicates the energy of the incident photon. Approximate peak positions are indicated with dotted lines.
3. RESULTS AND DISCUSSION
Figure 2. Ultraviolet photoelectron spectra of Sc3N@C78, Ti2C2@C78, and La2@C7815 obtained with hν = 40 eV photon. Cage symmetry of these endohedral fullerenes is the same D3h (78:5), but their electronic structures differ significantly.
3.1. UPS of Sc3N@C78. Figure 1 shows the valence band UPS of Sc3N@C78 obtained with hν = 20 60 eV photon energy. The spectral onset was 0.75 eV below the Fermi level. There are 8 explicit structures labeled A to H in the UPS of Sc3N@C78. Approximate peak positions of the structures are indicated with dotted lines. As were observed in the UPS of other fullerenes,8 11,13,15,23 the relative intensity of these structures oscillates when the energy of the incident photon is tuned.
Because of this intensity oscillation, peak positions deviate slightly in accordance with the incident photon energy change. The UPS obtained with hν = 50, 55, and 60 eV reveal a substructure at the lower binding energy side of structure C. As will be shown later, more than 8 Kohn Sham orbitals compose structure C. 166
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their electronic structures was attributed to hybridization between the molecular orbitals derived from the entrapped species and the fullerene cage.15 Titanium has a tendency to form carbides, and the basic idea was that hybridization took place in Ti2C2@C78. However, scandium does not have such a tendency. The present findings seem to be another exception to the empirical rule. Hence, a comprehensive explanation why these three endohedral fullerenes have different upper valence band electronic structures is required. The oxidation state of the fullerene cage or the amounts of transferred electrons from the entrapped clusters might be one of the reasons. UPS measurements alone are not sufficient to elucidate the amounts of transferred electrons in the endohedral fullerenes. XPS chemical shift of core level ionization potentials is helpful to elucidate the oxidation state or the amounts of transferred electrons so that the XPS of Sc3N@C78 was measured. 3.2. XPS of Sc3N@C78. Scandium 2p and N 1s XPS of Sc3N@C78 and Sc3N@C8025 are shown in Figure 3 together with Sc 2p XPS of Sc metal and Sc2O3.26 Peak position of Sc 2p3/2 of Sc metal, Sc3N@C78, Sc3N@C80, and Sc2O3 is 398.5, 400.4, 401.2, and 401.7 eV, respectively. The scandium 2p chemical shift suggests that the oxidation state of Sc in Sc3N@C80 is less than +3 and larger than 0. Assuming a linear relationship between the oxidation state and the amounts of chemical shift of the Sc 2p level, the most plausible oxidation state of Sc in Sc3N@C80 is +2, and that in Sc3N@C78 is less than that, presumably +1, although the Sc2+ state cannot be completely excluded. Peak position of N 1s of Sc3N@C78 is located at 395.8 eV, which is much shallower than that of Sc3N@C80 (396.9 eV)25 and those of other N atom containing inorganic compounds such as 396.6 eV of CrN.27 The fact that the N 1s peak of Sc3N@C78 locates at lower binding energy than that of Sc3N@C80 indicates that the N atom in Sc3N@C78 carries more electrons than the one in Sc3N@C80. It is remarkable that the addition of only two carbon atoms to the C78 cage induces a significant charge population change on the entrapped nitrogen atom. As for the chemical shift of the N 1s level, a linear relationship between the ionization potential of N 1s and the theoretically calculated amounts of electrons on the N atom has been established.27 The respective charges on the N atom of Sc3N@C78 and Sc3N@C80 estimated by the relationship between the N 1s binding energy and calculated charge using the extended H€uckel method are 2.32 and 1.95, and those using the CNDO method applied to neutral molecules are 0.44 and 0.37. As there was too large of a difference in these two estimated charges on the N atom, we calculated the charges on the nitrogen atom of several nitrogen containing organic molecules using Gaussians03 with B3LYP level and 6-311 g(d) basis sets. The plots of the binding energy of nitrobenzene,27 pyridine N-oxide,28 pyrrole,28 and aniline29 as a function of calculated Mulliken charges and NPA charges show good linear relationships, and those of pyridine28 and benzonitrile27 are slightly off-lined. Assuming the above linear relationships can be applicable to Sc3N@C78, Mulliken and NPA charges on the nitrogen atom are determined to be 0.80 and 1.50, respectively. Thus, the plausible oxidation state of Sc3N@ C78 might be either (Sc+)3N1 @C78 2 or (Sc+)3N2 @C78 . The same procedure on Sc3N@C80 gives 0.75 (Mulliken) and 1.34 (NPA) charges on the N atom. 3.3. Geometry Optimization. Geometry optimization was performed on a D3h (78:5) fullerene cage with an entrapped Sc3N cluster. Two initial structures of entrapped atoms, a planar structure (Sc atoms at the vertices of a regular triangle with the N atom at their center), and a triangular pyramidal structure (three
Figure 3. X-ray photoelectron spectra of Sc3N@C78, Sc3N@C80,25 Sc metal, and Sc2O3.26 Their Sc 2p peak position is 400.4, 401.7, 398.5, 401.2, and 401.7 eV, respectively. The nitrogen 1s peak is also observed at 396.1 eV in the XPS of Sc3N@C78.
This finding indicates that even the orbitals having very close energies show different intensity dependence on the incident photon energy. The UPS of La2@C78, Ti2C2@C78, and Sc3N@C78 obtained with hν = 40 eV photon energy are shown in Figure 2. These three endohedral fullerenes have the same cage symmetry of D3h (78:5).4,15,17,18,24 For the electronic structures located in the 5 11 eV binding energy region, there is a fairly good correspondence among these three UPS, although there are slight deviations in the corresponding structure peak positions (indicated by dotted lines) and slight difference in their relative intensity. Since the structures in this region are due to σ-electrons, the present findings suggest that the σ-electronic structures of these three endohedral fullerenes do not differ significantly. The corresponding peak positions of Sc3N@C78 of this energy region are located about 0.5 eV deeper than those of La2@C78 and about 0.3 eV deeper than those of Ti2C2@C78. As will be discussed later, this shift might be attributed either to the difference in the amounts of electrons transferred from the entrapped clusters or to the distortion of the C78 cage induced by cluster encapsulation. However, there is a significantly large difference in the upper valence band UPS (between the Fermi level and BE = 5 eV). There is an empirical rule for monometal atom encapsulated endohedral fullerenes that their electronic structure is essentially governed by the cage structure (symmetry) and the amounts of electrons transferred from the entrapped species.8 11 Thus, if one obtains different UPS from endohedral fullerenes composed of specific carbon atoms, either their cage structure (symmetry) or the amounts of transferred electrons is different. The same empirical rule seems to be held in multiple atom entrapped fullerenes.12 14 However, an exception was found in the C78 cage endohedral fullerenes. When the UPS of La2@C78 was found to be critically different from those of Ti2C2@C78 in spite of their same cage symmetry D3h (structure), the reason for the disagreement in 167
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Figure 5. Ultraviolet photoelectron spectrum of Sc3N@C78 obtained with hν = 30 eV photon and three simulation spectra theoretically obtained from optimized geometry assuming D3h (78:5) cage symmetry. Bars under each simulation spectrum indicate the energy of calculated ionization potentials. Details of geometry optimization are in the text.
Sc3N plane of isomer 3 was not perpendicular to the fullerene C3 axis but tilted slightly, so the overall symmetry dropped to Cs, which does not agree with the NMR data. Therefore, the geometry of isomer 3 might be a metastable one, and Sc3N@C78 might not adopt this geometry. From a viewpoint of formation energy, the most plausible geometry of Sc3N@C78 is isomer 2 since it has the smallest formation energy among them (those of isomers 1 and 3 are larger than that of isomer 2 by 25 and 475 kcal/mol, respectively). 3.4. Comparison of the UPS with Simulation Spectra. Figure 5 shows the simulation spectra obtained from these geometry optimized endohedral fullerenes together with the UPS of Sc3N@C78 obtained with hν = 30 eV photon energy. The abscissa of the simulation spectra was shifted by 4 eV so that they could be compared easily with the UPS. The bars in Figure 5 indicate the calculated Kohn Sham orbital energies. As was described above, the simulation spectra generated from the Hartree Fock calculation were not able to reproduce the UPS. However, the simulation spectra deduced from isomer 2 reproduces the UPS very well in terms of the peak positions and relative intensity of the structures appearing in the upper valence band region, and those of isomers 1 and 3 also gives reasonably good correspondence. The simulation spectra generated from the DFT calculation on Y2C2@C82 also reproduces the UPS of endohedral fullerenes.30 Although there is no theoretical correspondence between Kohn Sham orbital energies and ionization potentials defined from Koopmans’ theorem at the moment, there seems to be an empirical relationship between them. Further reasoning might be required to relate them. In general, theoretical calculations reproduce the upper valence band very well but give rather poor correspondence in the deeper
Figure 4. Top and side view of three structures obtained by the geometry optimization of Sc3C@C78.
Sc atoms forming a regular triangle with the N atom above it), were adopted. The initial Sc Sc separation distance in the planar structure was chosen to be 0.34 nm. The adopted initial Sc N distance in the pyramidal structure was 0.2 nm. The initial positions of the entrapped species were adjusted to meet C3 symmetry. When the initial form of the entrapped cluster was planar, the overall symmetry was D3h, and when the pyramidal cluster was entrapped, the overall symmetry was C3v. The calculations using these arrangements converged, and the optimized structure of the entrapped cluster was found to be planar even if the pyramidal initial structure was assumed. Optimization gave two forms with respect to the relative position of the fullerene cage and the entrapped cluster. Figure 4 shows these optimized structures. One form has three Sc atoms above the adjacent hexagon ring (isomer 1), and the other has three Sc atoms on the pyracylene unit (isomer 2). The symmetry of these isomers was found to be D3h, which agreed with the NMR4 of Sc3N@C78. Another arrangement with the C3 axis of the pyramidal or triangular cluster perpendicular to the fullerene C3 axis was also calculated and yielded a converged result with a planar Sc3N structure (isomer 3). As is shown in Figure 4, the 168
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Table 1. Calculated Charges of Entrapped Species present results XPS Mullikena Sc3N@C78
Sc N Sc3N
Ti2C2@C78
NPAa
1 0.84 2.16
1.50 1.5
Ti C Ti2C2
La2@C78 a
Campanerab
calculated charges
Krausec
Mulliken
NPA
Mulliken
atomic charge
0.819
1.386
0.88
0.44
1.256
1.54
1.201
2.618
0.385
0.64
0.248 0.274
1.41 1.23
0.54 0.78
0.31 0.67
La
1.287
NA
La2
2.574
NA
Estimated from the linear relationship between N 1s binding energy and calculated charges. b From ref 31. c From ref 19.
valence band region. It should be noted that remarkable correspondence in terms of the energy difference and intensity ratio of structures, D G is also observed in the deeper valence band region. Because of the good correspondence between the simulation spectrum of isomer 2 and the UPS together with the results of the formation energy calculation, Sc3N@C78 seems to take isomer 2 geometry, which is in good agreement with the crystallographic results of Sc3N@C78 3 [Co(oep)] 3 1.5 C6H6 3 0.3 CHCl324 and the theoretically proposed structure.31 Mulliken charges and charges calculated with natural population analysis (NPA) of entrapped species of isomer 2 are collected in Table 1. Charges on Sc and N atoms estimated by the XPS chemical shifts and calculated charges of entrapped species of Ti2C2@C78 and La2@C78 (obtained with the Hartree Fock level geometry optimized structure with the B3LYP level single point DFT calculation using 6-31 g(d) for carbon atoms and Lanl2dz for Ti and La atoms) are also collected in the table. NPA charges on La2@C78 could not be obtained because the calculation did not converge. Previously reported Mulliken charges on Sc and N atoms by Campanera and co-workers31 as well as atomic charges by Krause and co-workers19 were also collected in the table. Presently obtained results do not completely coincide with the previously reported results, which could be due to the basis sets used in the calculation, since calculated charges often depend on the using basis sets. However, there seems to be a correlation among them. Charges estimated from the XPS chemical shift of N 1s are very close to our present results and Campanera’s ones. The amounts of calculated charges on the Sc atom are about 60 80% of those on the N atom with opposite sign, which suggests the Sc+ state rather than Sc2+. That is, the formal charge distribution might be either (Sc+)3N1 @C78 2 or (Sc+)3N2 @C78 . There is a clear difference in the calculated charges of entrapped species in these three endohedral fullerenes. This difference is directly connected to the charge on the C78 cage of each endohedral fullerene. The cage of La2@C78 bears the most abundant electrons followed by that of Sc3N@C78. This difference might be one of the reasons for their different electronic structure, although they have the same cage symmetries. The total number of the occupied states of D3h-C78 (78:5) depends on the amounts of transferred electrons, which is the driving force to rearrange the occupied molecular orbitals. As the result, these endohedral fullerenes have different π- electronic structures. During the geometry optimization process of these endohedral fullerenes, it was found that the size of various clusters of the
encapsulated C78 cage differed from that of the empty D3h-C78 (78:5) cage; change was up to (0.2 Å cage elongation and/or contraction in a specific axis direction. This cage distortion might be another reason for the difference in electronic structures.
4. CONCLUSIONS The intensity of the structures appearing in the UPS of Sc3N@C78 oscillates in accordance with the incident photon energy change, which means that this molecule has analogous geometry to other fullerenes. The UPS of Sc3N@C78 are completely different from those of endohedral fullerenes, Ti2C2@C78 and La2@C78, both have the same cage symmetry. Comparison of the UPS with theoretically generated simulation spectra indicates that the most plausible structure of Sc3N@C78 is isomer 2, which is in concords with the crystallographic data of the Sc3N@C78 adduct. Chemical shift in binding energies of Sc 2p3/2 and N 1s of Sc3N@C78 and charge population analysis suggest either the (Sc+)3N @C782 or (Sc+)3N2 @C78 oxidation state. ’ AUTHOR INFORMATION Corresponding Author
*Fax: +81-89-927-9924. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Professor Shigeru Nagase of the Institute for Molecular Science for his valuable discussion. This study was conducted as part of a Joint Research Program of UVSOR, Institute for Molecular Science. This work was supported by a Grant-in-Aid for Scientific Research in the Priority Area Molecular Conductors (no. 15073203) as well as a Grant-in-Aid for Basic Scientific Research (no. 18350068) from the Ministry of Education, Science, Sports, and Culture, Japan. This work is also supported by a Subsidy to Create Center of Excellence from Ehime University. ’ REFERENCES (1) Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y.; Kodama, T.; Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (2) Takata, M.; Nishibori, E.; Sakata, M.; Inakuma, M.; Yamamoto, E.; Shinohara, H. Phys. Rev. Lett. 1999, 83, 2214. 169
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(3) Stevenson, S.; Rice, G.; Glass, T.; Harich, M.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55. (4) Olmstead, M. M.; de Bettencourt-Dias, A.; Duchamp, J. C.; Stevenson, S.; Marciu, D.; Dorn, H. C.; Balch, A. L. Angew. Chem., Int. Ed. 2001, 40, 1223. (5) Inoue, T.; Tomiyama, T.; Sugai, T.; Okazaki, T.; Suematsu, T.; Fujii, N.; Utsumi, H.; Nojima, K.; Shinohara, H. J. Phys. Chem. B 2004, 108, 7573. (6) Cao, B.; Hasegawa, M.; Okada, K.; Tomiyama, T.; Okazaki, T.; Suenaga, K.; Shinohara, H. J. Am. Chem. Soc. 2001, 123, 9679. (7) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843 and references therein. (8) Hino, S.; Umishita, K.; Iwasaki, K.; Miyamae, T.; Inakuma, M.; Shinohara, H. Chem. Phys. Lett. 1999, 300, 145. (9) Iwasaki, K.; Wanita, N.; Hino, S.; Yoshimura, D.; T. Okazaki, T.; Shinohara, H. Chem. Phys. Lett. 2004, 398, 389. (10) Hino, S.; Wanita, N.; Iwasaki, K.; Yoshimura, D.; Ozawa, N.; Kodama, T.; Sakaguchi, K.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K. Chem. Phys. Lett. 2005, 402, 217. (11) Miyazaki, T.; Sumii, R.; Umemoto, H.; Okimoto, H.; Ito, Y.; Sugai, T.; Shinohara, H.; Hino, S. Chem. Phys. 2010, 378, 11. (12) Pichler, T.; Hu, Z.; Grazioli, C.; Legner, S.; Knupfer, M.; Golden, M. S.; Fink, J.; de Groot, F. M. F.; Hunt, M. R. C.; Rudolf, P.; Follath, R.; Jung, C.; Kjeldgraad, L.; Br€uhwiler, P.; Inakuma, M.; Shinohara, H. Phys. Rev. B 2000, 62, 13196. (13) Hino, S.; Wanita, N.; Iwasaki, K.; Yoshimura, D.; Akachi, T.; Inoue, T.; Ito, Y.; Sugai, T.; Shinohara, H. Phys. Rev. B 2005, 72, 195424. (14) Shiozawa, H.; Rauf, H.; Pichler, T.; Grimm, D.; Liu, X.; Knupfer, M.; Kalbac, M.; Yang, S.; Dunsch, L.; Buchner, B.; Bachelor, D. Phys. Rev. B 2005, 72, 195409. (15) Hino, S.; Kato, M.; Yoshimura, D.; Moribe, H.; Umemoto, H.; Ito, Y.; Sugai, T.; Shinohara, H.; Otani, M.; Yoshimoto, Y.; Okada, S. Phys. Rev. B 2007, 75, 125418. (16) Fowler, P. W.; Manolopoulos, D. E. An Atlas of Fullerenes; Oxford Press: Oxford, U.K., 1995. (17) Tan, K.; Lu, X. Chem. Commun. 2005, 4444. (18) Cao, B.; Wakahara, T.; Tsuchiya, T.; Kondo, M.; Maeda, Y.; Rahman, G. M. A.; Akasaka, T.; Kobayashi, K.; Nagase, S.; Yamamoto, K. J. Am. Chem. Soc. 2004, 126, 9164. (19) Krause, M.; Popov, A.; Dunsch, L. ChemPhysChem 2006, 7, 1734. (20) Popov, A.; Dunsch, L. J. Am. Chem. Soc. 2007, 129, 11835. (21) Koga, T.; Yamamoto, S.; Shimazaki, T.; Tatewaki, H. Theor. Chem. Acc. 2002, 108, 41. (22) Osanai, Y.; Sekiya, M.; Noro, T.; Koga, T. J. Mol. Phys. 2003, 101, 65. (23) Binning, J.; Martins, J. L.; Weaver, J. H.; Chibante, P. F.; Smalley, R. E. Science 1991, 252, 1417. (24) Olmstead, M. M.; de Bettencourt-Dias, A.; Duchamp, J. C.; Stevenson, S.; Marciu, D.; Dorn, H. C.; Balch, A. L. Angew. Chem., Int. Ed. 2001, 40, 1223. (25) Alvarez, L.; Pichler, T.; Georgi, P.; Schwieger, T.; Peisert, H.; Dunsch, L.; Hu, Z.; Knupfer, M.; Fink, J.; Bressler, P.; Mast, M.; Golden, M. S. Phys. Rev. B 2002, 66, 035107. (26) Takahashi, T.; Ito, A.; Inakuma, M.; Shinohara, H. Phys. Rev. B 1995, 52, 13812. (27) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1969, 8, 2642. (28) Casanovas, J.; Ricart, J. M.; Rubio, J.; Illas, F.; Jimenez-Mateos, J. M. J. Am. Chem. Soc. 1996, 118, 8071. (29) Lindberg, B. J.; Hedman, J. Chem. Scr. 1975, 7, 155. (30) Hino, S.; Miyazaki, T.; Aoki, Y.; Wanita, N.; Kato, M.; Sumii, R.; Akachi, T.; Inoue, T.; Ito, Y.; Sugai, T.; Shinohara, H. Bull. Chem. Soc. Jpn. 2009, 82, 963. (31) Campanera, J. P.; Bo, C.; Olmstead, M. M.; Balch, A. L.; Poblet, J. P. J. Phys. Chem. A 2002, 106, 12356.
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