Theoretical Identification of Three C66 Fullerene Isomers and Related

Dec 2, 2016 - Table of Contents. Theoretical Identification of Three C66 Fullerene Isomers and Related Chlorinated ... The XPS spectra show isomer dep...
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Theoretical Identification of Three C66 Fullerene Isomers and Related Chlorinated Derivatives by X‑ray Photoelectron Spectroscopy and Near-edge X‑ray Absorption Fine Structure Spectroscopy Xiu-Neng Song,† Guang-Wei Wang,† Yong Ma,*,†,‡ Shou-Zhen Jiang,† Wei-Wei Yue,† Chuan-Kui Wang,† and Yi Luo*,‡,§ †

School of Physics and Electronics, Shandong Normal University, 250014 Jinan, People’s Republic of China Division of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden § Department of Chemical Physics, University of Science and Technology of China, 230026 Hefei, People’s Republic of China ‡

ABSTRACT: C 1s X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra for three C66 fullerene isomers and related chlorinated species have been calculated by density functional theory (DFT) method. The XPS spectra show isomer dependence for the three pristine C66 isomers but not for the chlorinated species. The NEXAFS spectra exhibit strong dependence on the structures of all the investigated molecules and thus can be well employed to identify the three C66 fullerene isomers and related chlorinated species. Both XPS and NEXAFS spectra of the chlorinated species present significant variations compared with the pristine fullerenes. The spectral components for carbon atoms of different local environments have been explored as well. The spectra for the carbon atoms connecting to chlorine atoms exhibit a significant blue shift compared to the others.

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addition of chlorine atoms outside the cage.12 In 2000, the first two IPR-violating fullerenes, Sc2@C6613 and Sc3N@C68,14 were independently reported. Shinohara and co-workers13 synthesized Sc2@C66 metallofullerene and indicated that the Sc2@C66 structure contained two pairs of 2-fold fused pentagons with two closely situated scandium atoms (a scandium dimer) encapsulated in the cage. However, in a computational study on the same system by density functional theory (DFT), Kobayashi and Nagase15 found that the previously proposed structure could not correspond to an energy minimum and proposed a much more stable endohedral structure of C66 isomer that had two sets of 3-fold fused pentagons. Subsequently, Shinohara and co-workers16 performed a more detailed analysis on the experimentally obtained Sc2@C66 metallofullerene and reconfirmed the validity of their previous Sc2@C66 structure, the C2v-#4348C66 fullerene isomer encaging a scandium dimer. It was not until 2014 that this controversy was resolved. Nagase and co-workers17 presented refined experimental evidence and analysis which strictly showed that the only known structure of Sc2@C66 was the C2v-#4059C66 isomer, including two sets of unsaturated linear triquinanes (ULTs),

INTRODUCTION Since the discovery of C60,1 fullerenes and their derivatives have been widely studied for decades. Due to their specific geometries and unique physical and chemical properties, fullerenes have been widely used to construct various functional materials, consistently prompting development in energy, medicine, and other technical fields.2−4 In basic research about fullerenes, isomer identification attracts much attention because of the rapidly increasing isomeric forms of fullerenes with the growing size of carbon cages. Here we focus on fullerene C66, which has been intensively investigated in both theoretical and experimental works over the past dozen years. According to the spiral algorithm theory,5 fullerene C66 can exist in 4478 classical isomers. However, all the possible C66 fullerene isomers violate the well-known isolated-pentagon rule (IPR),6 which indicates that stable fullerenes keep all the pentagons isolated by hexagons, and otherwise the existence of fused pentagons brings about greater local strain and decreased stability. Thus far, all pristine fullerenes synthesized outside the gas phase firmly obey the IPR rule. Nevertheless, by continuing efforts, a number of non-IPR fullerenes have been stabilized by endohedral encapsulation of metallic clusters or by exohedral derivatization with halogens.7−11 C66 is one of several C2n fullerenes (2n = 60, 66, 68, etc.) that can be obtained by insertion of a metal cluster inside the cage, as well as by © XXXX American Chemical Society

Received: September 28, 2016 Revised: November 26, 2016 Published: December 2, 2016 A

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with two scandium ions separately located within the fold of each ULT unit. Then the long-standing disputed isomer of Sc2@C66 was definitively determined to be Sc2@#4059C66.12,17 On the other side, in 2010, Xie and co-workers9 reported additional C66 fullerene isomers that were stabilized by exohedral chlorination, C66Cl6 and C66Cl10, and demonstrated that the carbon cage was the #4169C66 isomer, which was one of the first several experimentally established fullerenes featuring triple sequentially fused pentagons (TSFP). Moreover, in 2014, Xie and co-workers18 successfully synthesized the long-sought #4348 C66 isomer containing two pairs of doubly fused pentagons, C66Cl10, by exohedral chlorination, which further enriched the C66 fullerene family. In summary, up to now, three isomers of C66 fullerene, C2v-#4059C66, Cs-#4169C66, and C2v-#4348C66, have been stabilized by endohedral encapsulation of two scandium ions or by exohedral derivatization with chlorine atoms. In view of the unusual structural features and reactivity arising from the presence of fused pentagons, the IPR-violating fullerenes and their derivatives could be competently used to construct fullerene-based multifunctional materials with potential applications in advanced technologies.8,9 Further study on the structural and electronic properties of fullerene C66 and derivatives should be essential and stimulative for the utilization of this fullerene family in the future, as well as promotive for fullerene researches. In this work, we focus on isomer identification for the three experimentally captured C66 fullerene isomers and their chlorinated species. Hitherto, a number of spectroscopic techniques, including 13 C nuclear magnetic resonance (13C NMR), infrared (IR), Raman, electronic, and X-ray spectroscopies, have been used to explore the structural, electronic, and vibrational properties of the various fullerene isomers.19−24 Soft X-ray spectroscopies are known to be powerful techniques to effectively probe the electronic structure and chemical construction of molecules, complex compounds, and surfaces by core-level excitations or de-excitations.25−29 Specifically, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been intensively used to identify the isomers of fullerenes and their derivatives, such as the small fullerene C34,30 azafullerenes C48N1231 and C58N2,32 three C56 isomers and their chlorinated derivatives,33 two Ih-symmetrybreaking C60 isomers and their chlorinated species,34 two isomers of C72 and the corresponding chlorinated derivative,35 and large fullerenes C76,36 C78,23 and C82.24 XPS corresponds to the ionization process of core electrons, providing information on core orbitals; while NEXAFS spectroscopy represents excitation of the core electron to unoccupied orbitals, mostly depicting the features of virtual orbitals. According to previous research, XPS mainly displayed sensitivity to fullerene isomers with different symmetries, while NEXAFS spectroscopy showed strong isomer dependence for fullerenes and their derivatives. In the present work, we calculate the carbon K-shell (1s) XPS and NEXAFS spectra of the three experimentally captured C66 fullerene isomers and their chlorinated species, namely, C2v-#4059C66, Cs-#4169C66, C2v-#4348C66, #4169C66Cl6, #4169C66Cl10, and #4348C66Cl10, using a gradient-corrected DFT method. The purpose of this work is to investigate the relationship between spectroscopy and structure and explore the isomer dependence of spectra for C66 fullerene isomers and their chlorinated derivatives. Theoretical study on soft X-ray spectroscopies of the C66 isomers and derivatives should be basically valuable and stimulative for further research on this fullerene family.

Article

COMPUTATIONAL DETAILS

The initial coordinates of Sc2@#4059C66, #4169C66Cl6 and #4169 C66Cl10, and #4348C66Cl10 were extracted from the crystallographic data provided in previous experimental research.9,17,18 Then molecular configurations for the three pristine C66 isomers and their related chlorofullerenes studied in this work were designed by use of GaussView.37 After that, geometrical structures of the considered C66 fullerene isomers and their related chlorinated species were optimized38 with B3LYP functional39 and 6-31G(d,p) basis set40 implemented by the Gaussian09 package.41 On the basis of the obtained geometries, C 1s XPS and NEXAFS spectra of the studied molecules were calculated at the DFT level. The gradientcorrected Becke (BE88) exchange functional42 and the Perdew (PD86) correlation functional43 were employed for DFT calculations. This combination of exchange−correlation functionals proved to give excellent oscillator strength in comparison with experiments in previous studies.27,44 We used the triple-ζ quality individual gauge for localized orbital (IGLO-III) basis set45 to describe the core-excited carbon atom, while model core potentials were used for the nonexcited carbon atoms and chlorine atoms to facilitate the self-consistent field (SCF) convergence of the core-hole state. In addition, miscellaneous auxiliary basis sets were also set for all atoms.46 The C 1s ionization potentials (IPs) were calculated by using the ΔKohn−Sham (ΔKS) scheme47,48 and were taken as the energy difference between the ground state (GS) and the fully optimized core-ionized state. Relaxations caused by the introduced core-hole were fully considered in the ΔKS scheme, so the evaluated ionization energies were accurate. For the Xray absorption process, two states involved in this case were the GS (initial state) and the core-excited state (final state). The final-state rule49−51 suggested that, for finite molecular systems, accurate absorption spectra could be obtained only by the finalstate wave function. As the transition process was sudden compared to the relaxation time of the other passive electrons, the problem could be further approximated by a single-electron picture described by a pair of molecular orbitals (MOs) of the final state. Furthermore, the full core hole (FCH) approximation was used for NEXAFS spectral calculations in this work, in which the core-ionized state was chosen as a reference state to represent the core-excited states. The FCH approach was confirmed to give excellent transition moments for evaluating the NEXAFS spectral intensities and good relative energy positions of the intensities for fullerene systems.23,44,52 In addition, a double basis-set technique was used for the excited carbon atom, where the normal orbital basis set mentioned above was used for energy minimization and an augmented diffuse basis set (19s, 19p, 19d)53 was added for spectral calculations. For the transition from initial (i) to final (f) state, the absorption oscillator strength was given by54 fif =

2mεif 3ℏ2

2 2 2 (|⟨ψf |x|ψi⟩| + |⟨ψf |y|ψi⟩| + |⟨ψf |z|ψi⟩| )

(1)

in which ψi and ψf denote the core and unoccupied MOs and εif is the energy difference of the orbitals; that is, the transition energy. The average over the x, y, and z components was to account for the random orientations of molecules in reality. In order to obtain absolute energy positions of the peaks, we calculated the transition energy from C 1s to the lowest unoccupied molecular orbital (LUMO) by using the ΔKS scheme:47,48 B

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Table 1. Number of Symmetry-Independent Carbon Atoms, Relative Energies, and HOMO−LUMO Gaps for All Studied Speciesa

(2)

where the transition energy 1s → LUMO was taken as the energy difference between the GS and the fully relaxed coreexcited state with one core electron excited to LUMO. The absorption spectra were calibrated so that the first spectral feature, corresponding to the transition from 1s to LUMO, coincided with the accurate excitation energy of 1s → LUMO calculated by the ΔKS scheme. Spectral calculations were all carried out with the StoBe package.55 Moreover, a uniform correction of +0.2 eV was added to the IPs or transition energies to account for the relativistic effects for the carbon K edge.48 Finally, the XPS spectra were generated through broadening the IP values by a Gaussian function with full width at half-maximum (fwhm) of 0.2 eV. For NEXAFS spectra, we used a Gaussian function with fwhm of 0.3 eV to convolute the oscillator strengths below the IP, while the Stieltjes imaging approach56 was used to form the spectra in the continuum region. For every studied molecule, the total spectra were obtained by calculating the spectra of symmetryindependent carbon atoms57 at first and then summing up the contributions scaled by the relative abundance of each nonequivalent carbon atom.

a

molecule

Cind

C2v-#4059C66 Cs-#4169C66 C2v-#4348C66 C1-#4169C66Cl6 C1-#4169C66Cl10 Cs-#4348C66Cl10

19 35 19 66 66 34

relative energy (kcal/ mol)

HOMO−LUMO gap (eV)

55.85 0 4.77

1.409 1.956 1.181 2.693 2.996 2.940

Calculated at B3LYP/6-31G(d,p) level.

geometry, relative energies of the three C66 isomers, and energy gaps between the highest occupied molecular orbital (HOMO) and LUMO for all the studied species. Of all 4478 possible isomers of C66 fullerene, only #4169C66, #4348C66, and #4466 C66 have two pentagon adjacencies, and they are the most energetically stable isomers as suggested by previous investigations, while the other C66 isomers all contain more than two pairs of fused pentagons.5,12,58 In the present study, we focus on the two isomers #4169C66 and #4348C66 that have been experimentally captured by chlorination,8,9,18 as well as the #4059 C66 isomer experimentally stabilized by endohedral encapsulation.17 As displayed in Table 1, #4348C66 has only 4.77 kcal·mol−1 relative energy above #4169C66, yet #4059C66 is almost 56 kcal·mol−1 higher in energy than #4169C66. For #4059 C66 containing four pentagon adjacencies, the increased pentagon fusions should be responsible for its relatively high energy compared with the other two isomers, according to the pentagon adjacency penalty rule (PAPR).12,59,60 Calculated relative energies of the three C66 isomers here are essentially consistent with previous studies.12 Furthermore, we investigate the HOMO−LUMO gaps of all the studied species. The lowest-energy isomer, Cs-#4169C66, exhibits a larger HOMO− LUMO gap (1.956 eV), which further indicates its relative chemical stability compared with the other two. Generally, the HOMO−LUMO gaps of the chlorinated species are markedly enlarged in comparison with the corresponding pristine C66 fullerenes. #4169C66Cl6 and #4169C66Cl10 show gaps about 0.7 and 1.0 eV bigger than the Cs-#4169C66 isomer, respectively, while the gap of #4348C66Cl10 is increased by about 1.8 eV compared to the C2v-#4348C66 isomer. The significant enlargement of the HOMO−LUMO gaps partly reveals the changes in molecular electronic structures after exochlorination on the C66 fullerene cages. In spectral studies related to fullerene structures, it is common to group the carbon atoms of fullerenes based on the local environment.19,20,23 As shown in Figure 1b, three carbon types are commonly distinguished according to previous studies: the pyracylene site, where the carbon atom shared by two hexagons and one pentagon is connected to another pentagon through the adjacent bond of the two hexagons; the corannulene site, where the carbon atom shared by two hexagons and one pentagon is connected to another hexagon through the adjacent bond of the two hexagons; and the pyrene site, where the carbon atom is part of three hexagons. The fourth type of carbon site, known as the pen-pen site, is introduced by the existence of fused pentagons and describes the carbon atom located in the junctions of two adjacent pentagons. For chlorinated species, the carbon atoms

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RESULTS AND DISCUSSION Structures. Optimized molecular structures of the three C66 fullerene isomers and their chlorinated species are presented in Figure 1a. As demonstrated by previous research, an

Figure 1. (a) Optimized molecular structures of the three C66 fullerene isomers and their chlorinated species. Symmetries of the molecules are included in parentheses. (b) Schematic illustration for the local environment of different carbon types.

asymmetric chlorination pattern, where three of the four TSFP fusion sites are chlorinated to be sp3-hybridized whereas the fourth remains unsaturated, breaks the overall symmetry of the parental Cs-#4169C66 cage, rendering both #4169C66Cl6 and #4169 C66Cl10 chiral.8,9 Another chlorofullerene, #4348C66Cl10, has Cs symmetry, which is lower than that of its parental C 2v - #4348 C 66 cage. 18 Table 1 displays the number of symmetry-independent carbon atoms in each molecular C

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Figure 2. Calculated XPS spectra of the three C66 fullerene isomers and spectral components for different carbon types in each isomer. Each individual component is obtained by summing up the spectra of the independent carbon atoms of the same type and scaling by the relative abundance.

Figure 3. Calculated C 1s XPS spectra of the three chlorinated C66 fullerenes and decomposed spectra corresponding to carbon atoms of different local environment. Each individual component is obtained by summing up the spectra of the independent carbon atoms of the same type and scaling by the relative abundance.

around 290.18 eV. In the lower-energy region, the XPS spectrum of Cs-#4169C66 shows a weak feature a at about 289.53 eV (Figure 2b), while two weak features a1 and a2 closely arise at about 289.15 and 289.35 eV, evolving a broad line shape around this energy area, in the XPS spectrum of C2v-#4348C66 (Figure 2c). The #4059C66 isomer originates visibly distinguished XPS spectrum on both profile and energy positions compared to the #4169C66 and #4348C66 isomers, which suggests that the C 1s XPS spectra can be well employed to discern the #4059C66 isomer among the three C66 fullerene isomers. Moreover, spectral distinctions in the lower energy region between #4169 C66 and #4348C66 isomers can be roughly used to

connected to chlorine atoms are distinguished from the others, and the other carbon atoms are further classified as above. X-ray Photoelectron Spectra. Calculated C 1s XPS spectra of the three C66 isomers and the corresponding component spectra for different types of carbon atoms in each isomer are presented in Figure 2. Two main features, a and b, arise distinctly at around 289.54 and 289.97 eV in the XPS spectrum of C2v-#4059C66 (Figure 2a), where feature b at the higher energy position appears broader than feature a and the spectral line continues to broaden at about 290.24 eV. Cs-#4169C66 and C2v-#4348C66 exhibit similar photoelectron spectra characteristics in the energy region above about 289.6 eV, including a strong peak at 289.85 eV and a weak shoulder at D

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Figure 4. Calculated NEXAFS spectra of the three C66 fullerene isomers and spectral components for different carbon types in each isomer. Each individual component is obtained by summing up the spectra of the independent carbon atoms of the same type and scaling by the relative abundance.

generated by the pyracylene and corannulene sites, and the spectral shoulder or line broadening mainly stems from the pyrene sites, while the carbons connecting to chlorine atoms give rise to the peak in the higher-energy region of C 1s XPS spectra for the chlorofullerenes. In general, the carbon atoms in the chlorinated species have higher C 1s IPs than the carbons in the pristine C66 fullerenes, especially for the carbon atoms bonded with chlorine atoms, which to some extent indicates the changes in electronic structure of the C66 framework after chlorination. Furthermore, for the three chlorides, XPS spectra associated with the carbon atoms saturated by chlorination exhibit a blue shift of about 2 eV compared to the spectral components generated by the other carbon atoms, which arises from the strong pulling on electrons by the chlorine atoms with strong electronegativity. #4169C66Cl10 and #4348C66Cl10 show similar C 1s XPS spectra (Figure 3b,c). Besides, the energies and profiles of main features in the XPS spectrum of #4169C66Cl6 (Figure 3a) are not markedly distinct from the the corresponding features generated by the other two chlorides. So the XPS spectra could not allow for evident discrimination of the three chlorinated derivatives of C66 fullerene. Further study is expected to confirm the isomer dependence of C 1s NEXAFS spectra for both pristine and chlorinated C66 fullerenes. Near-edge X-ray Absorption Fine Structure Spectra. NEXAFS spectroscopy mainly describes the features of unoccupied molecular orbitals, corresponding to electron excitation from core level to virtual orbitals, which shows obvious dependence on the chemical structures of studied systems. Figure 4 presents calculated NEXAFS spectra of the three C66 fullerene isomers and the spectral contributions generated by different types of carbon atoms. In addition, the energies for significant features in the spectra are displayed in Table 2. The three C66 fullerene isomers exhibit markedly distinguished NEXAFS spectra in both profiles and energies of spectral features. We primarily focus on the energy region from 283 to 286 eV, which involves the main features in the spectra. Two distinct features, including a weak feature a at about 283.7 eV and a strong peak c at about 285.3 eV, are exhibited in the spectrum of C2v-#4059C66 (Figure 4a). The spectra of both

discriminate these two C66 isomers. By analyzing the components of XPS spectra for different carbon types, we can gain an insight into the contributions from carbons of different local environments to the total spectra and the dependence of C 1s XPS spectra on the local structures of fullerenes. As shown in Figure 2b,c, for Cs-#4169C66 and C2v-#4348C66, carbon atoms at the pyracylene site and corannulene site give rise to the highest peak b at about 289.85 eV, while shoulder c at higher energy is generated by carbon atoms of the pyrene type. Although spectral features in the lower-energy region of the #4169C66 and #4348C66 isomers show some distinctions, these features originate from the same type of carbon atoms, all of which are located at the pen-pen site. The XPS spectral components of C2v-#4059C66 (Figure 2a) show some differences from the corresponding parts of the other two isomers. For the #4059C66 isomer, the spectra originated by the corannulene sites show two distinct peaks at around 289.54 and 289.88 eV, while the pyracylene sites exhibit two features at about 289.54 and 290.02 eV. In addition, the spectra for pyrene sites in the #4059C66 isomer show a main feature at about 290.1 eV and the spectral line broadens at around 290.24 eV, which also differs from the other two C66 isomers. C2v-#4059C66, containing two TSFP moieties at the ends of the carbon cage, has a typical ellipsoid shape and the consequent widely discrepant tortuosities at different locations, which may result in noticeable discrepancies in the 1s IP values for the same types of carbon atoms and thus lead to the visibly distinguished C 1s XPS spectra compared to the others. Figure 3 displays calculated C 1s XPS spectra for the three chlorinated species of C66 fullerene and the decomposed spectra corresponding to carbon atoms of different local environments. The XPS spectrum of #4169C66Cl6 exhibits a strong peak a at 290.2 eV with a weak shoulder b at about 290.5 eV, and another distinct spectral feature c arises at around 292.32 eV (Figure 3a). In spectra of both #4169C66Cl10 and #4348 C66Cl10, a strong peak appears at about 290.3 eV and the spectral line broadens at about 290.6 eV, while a weak peak is seen at 292.47 eV (Figure 3b,c). It is noticed that, for all three chlorinated species, the strong peak at lower energies is E

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stems from the excitations of all types of carbon atoms. For Cs-#4169C66 and C2v-#4348C66 (Figure 4b,c), both features a and b arise from the pyracylene and corannulene sites, and the main feature c is primarily generated by the carbon types of pyracylene, corannulene, and pyrene. The spectra generated by pyracylene and corannulene sites in the #4059C66 isomer are visibly distinguished from the corresponding parts of the other two isomersby the absence of feature b and the lower excitation energy for main feature c in the spectrum of C2v-#4059C66. Moreover, for Cs-#4169C66 and C2v-#4348C66, the line broadening of feature b mentioned above is mainly generated by carbon atoms at the pyrene site, which also give rise to a weak feature between a and c in the spectrum of C2v-#4059C66. The distinctions between the spectra of different carbon types in the fullerenes demonstrate the sensitivity of NEXAFS spectroscopy to different local environments of carbon atoms. It is noticeable that the spectra generated by the same carbon types in different isomers also show some discrepancies, constructing important spectral features for isomer identification, which should originate from the discrepant local structural tortuosities or the diverse polygon (pentagons and hexagons) arrangements outside the first polygon layer around the carbon atoms.36 Figure 5 displays the calculated C 1s NEXAFS spectra of the three chlorinated C66 fullerenes and spectral components for carbon atoms of different local environments. The NEXAFS spectra generated by the chlorinated species are obviously different from those of the corresponding pristine fullerenes, effectively reflecting the variations in electronic structure of the carbon cages after chlorination. Energies for the marked features in the spectra are listed in Table 3. The strong peak a at about 284.3−284.5 eV commonly arises in all the spectra, while the absorption region between 285 and 287 eV exhibits visible dependence on the different molecular structures. Two close features, b and c, are seen at about 285.47 and 285.97 eV separately in the spectrum of #4169C66Cl6 (Figure 5a); feature c distinctly arises at about 285.85 eV in the spectrum of #4169 C66Cl10, while feature b is absent (Figure 5b); and in the spectrum of #4348C66Cl10, feature c appears at about 286 eV, with a weak shoulder b at about 285.53 eV (Figure 5c). Feature d arises clearly in the spectra of #4169C66Cl10 and #4348C66Cl10,

Table 2. Energies for Significant Features in the Calculated NEXAFS Spectra of the Three C66 Fullerene Isomers energy of spectral feature (eV) isomer

a

C2v-#4059C66 Cs-#4169C66 C2v-#4348C66

283.7 283.9 283.5

b

c

d

e

f

286.3

284.4 284.5

285.3 285.8 285.8

287.03 287.27 287.25

287.8 288.03 288.1

Cs-#4169C66 and C2v-#4348C66 show a clear feature b at about 284.4−284.5 eV and a main feature c at about 285.8 eV (Figure 4b,c). The absence of feature b creates the peculiar spectral shape of C2v-#4059C66 in comparison to the other two isomers, which suggests that the #4059C66 isomer can be readily identified among the three C66 isomers by NEXAFS spectroscopy. In addition, the excitation energy for feature c of C2v-#4059C66 is about 0.5 eV lower than for the corresponding features in the other spectra, which also distinguishes the #4059C66 isomer from the other two isomers. In the spectrum of Cs-#4169C66 (Figure 4b), feature a, arising at about 283.9 eV, is shown as a shoulder on the left of feature b, while for C2v-#4348C66 (Figure 4c), feature a appears at about 283.5 eV and is distinctly separated from feature b. Moreover, another shoulder arises on the right of feature b in the spectrum of Cs-#4169C66 (Figure 4b), resulting in spectral line broadening and lower height of feature b, while in the spectrum of C2v-#4348C66 (Figure 4c) , feature b is slightly broadened and its intensity is slightly weaker than that of feature c. Consequently, the different appearances around feature b, combined with the different energy separations between features a and b, can discriminate between the #4169C66 and #4348C66 isomers readily. In general, the calculated NEXAFS spectra show strong isomer dependence and should be well suited to identify the three C66 fullerene isomers. Furthermore, we investigate the spectral components generated by different types of carbon atoms in the isomers. The pen-pen sites exhibit the first absorption feature at lower energy than the others, while the first absorption feature of the pyrene sites arises at higher energy than those of the other three carbon types. It is shown that, for C2v-#4059C66 (Figure 4a), feature a originates from pyracylene and corannulene sites, and the strong peak c

Figure 5. Calculated C 1s NEXAFS spectra of the three chlorinated C66 fullerenes and corresponding spectral components for carbon atoms of different local environments. Each individual component is obtained by summing up the spectra of the independent carbon atoms of the same type and scaling by the relative abundance. F

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Table 3. Energies for Significant Features in Calculated C 1s NEXAFS Spectra of the Three Chlorinated Species of C66 Fullerene energy of spectral feature (eV) species #4169

C66Cl6 #4169 C66Cl10 #4348 C66Cl10

a

b

c

284.33 284.45 284.43

285.47

285.97 285.85 286

285.53

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respectively at about 286.65 and 286.9 eV, but it is not conspicuously present in the spectrum of #4169C66Cl6. It is noticed that the strong peak a mainly originates from the pyracylene and corannulene sites, and the absorption region corresponding to features b and c is primarily dominated by the spectra generated by the pyracylene sites, while feature d is constructed by the carbon atoms connected to chlorine. The pyracylene sites in different configurations give rise to isomerdependent spectral shapes around the energy region of 285− 286 eV, and the different appearances of features b and c in the total spectra readily distinguish the three chlorinated species from each other. Owing to the less chlorinated sites, feature d is not marked in the spectrum of #4169C66Cl6, but the carbon atoms connected to chlorine also partly contribute to the line broadening around 286.5 eV in the total spectrum. In addition, the relative intensities of features c and d are contrary in the spectra of #4169C66Cl10 and #4348C66Cl10 (Figure 5b,c), which can also be useful for discriminating these two chlorides by NEXAFS spectra. Between about 287.5 and 288.5 eV, the other two features e and f arise adjacently in all spectra, and the relative intensity of features e and f of #4348C66Cl10 (Figure 5c) is distinguished from that of #4169C66Cl6 and #4169C66Cl10 (Figure 5a,b). Moreover, in accord with the case of XPS spectra, NEXAFS spectra generated by carbon atoms connected to chlorine have a significant blue shift in comparison to spectra of the other carbon atoms. Generally, although the XPS spectra could not be employed to discriminate the three chlorinated species of C66 on account of the similar characteristics, it is readily achievable to use C 1s NEXAFS spectra to identify the three chlorinated C66 fullerenes in light of the distinct dependence of the spectra on the structures. Even for #4169 C66Cl6 and #4169C66Cl10, corresponding to the same pristine #4169 C66 isomer, the spectra of these two chlorinated species Csstill exhibit noticeable differences in some absorption regions because of the different chlorination patterns.

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d

e

f

g

286.65 286.9

287.62 287.86 287.76

288.05 288.39 288.15

290.72 291.17 290.62

AUTHOR INFORMATION

Corresponding Authors

*(Y.M.) E-mail [email protected]; phone +86 13793154301; fax +86 (0) 531 86180892. *(Y.L.) E-mail [email protected]; phone +46 (8) 55378414; fax +46 (8) 55378590. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants 21303096, 11374195, 11404193, 11674199, 61401258, and 11504208) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. We are thankful for the support of the Taishan scholar project of Shandong Province.

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REFERENCES

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SUMMARY

In summary, C 1s XPS and NEXAFS spectra of three experimentally captured C66 fullerene isomers and their related chlorinated derivatives have been calculated by the DFT method. The XPS spectra show isomer dependence for the pristine C66 fullerenes but not for the chlorinated species. Strong isomer dependence is exhibited by NEXAFS spectra for both pristine and chlorinated C66 fullerene isomers. From study of the spectral components for carbon atoms of different local environments, the origins of significant features in the spectra have been illuminated, and the dependence of the spectra on local structures of fullerenes has been clarified as well. Particularly, XPS and NEXAFS spectra generated by carbon atoms bonded with chlorine both exhibit significant blue shifts compared with spectra of the other carbon atoms. G

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