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Stability of Metal-encapsulating Boron Fullerene B Wei Fa, Shuang Chen, Seema Pande, and Xiao Cheng Zeng

J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b07173 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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The Journal of Physical Chemistry

Stability of Metal-Encapsulating Boron Fullerene B40 Wei Fa*, † Shuang Chen, ‡,§ Seema Pande, ‡ and Xiao Cheng Zeng*, ‡ †

Group of Computational Condensed Matter Physics, National Laboratory of Solid State

Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, China ‡

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA

§

Kuang Yaming Honors School, Nanjing University, Nanjing 210093, China

ABSTRACT Structural stability of MB40 (M = Li, Na, K, Ba, and Tl) is investigated on the basis of densityfunctional theory calculations at the PBE0 level. Particular attention is placed on the relative stability between the endohedral and exohedral configurations of metalloborospherenes. It is found that the Na and Ba atoms can be stably encapsulated inside the B40 cage, while the Li, K, and Tl atoms favor the exohedral configuration where the dopant caps one of heptagons of B40 cage. In-depth analysis of the endohedral versus exohedral configurations with different dopants suggests that besides the comparable atomic size with the cage size, another key factor that can affect stability of endohedral versus exohedral configuration is the interaction between the dopant and B atoms. The infrared (IR) spectra of the endohedral C2v Na@B40 and exohedral Cs Na&B40 clusters are also computed, from which some useful spectra indictors may be used for identification of the structures in the future experiments. KEYWORDS: metal doping, boron cluster, endohedral and exohedral metalloborospherenes, density-functional calculations

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I. INTRODUCTION Stable all-boron fullerenes belong to a new family of elemental cage molecules,1-15 which not only give rise to intriguing chemistry that differs from the carbon fullerene family, but also can serve as potential building blocks for fabrication of new hybrid nanostructures with novel properties. The most eye-catching theoretical prediction of boron fullerenes is B80, which is a structure analogue of C60 with 20 additional B atoms capping the 20 hexagons. However, this B80 cage is shown to be a high-energy isomer compared to the core-shell isomers according to density-functional theory (DFT) results.3, 5, 6 Very recently, experimental detections of B39 and B40 fullerenes have been reported.16,

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The neutral D2d B40 cage is composed of interwoven

double chains with two hexagons at the top and bottom, and four heptagons in the side surface,16 which is notably lower in energy than other low-lying isomers. The simulated optical spectra show dramatic differences between the D2d B40 cage and other quasi-planar structures.18 More importantly, its nearly spherical geometry endows a sizable hollow space with a diameter of ~6.2 Å, suggesting possibility of encapsulating either a metal atom or a small molecule inside. If confirmed in the laboratory, the intriguing endohedral metalloborofullerenes may be exploited for potential applications in nanoboronchemistry. For example, encapsulating a lithium atom into the B40 fullerene may significantly enhance the CO2 adsorption.19 Due to the electron-deficient nature of boron, there exists competition between three-center and two-center bonding in boron sheets, boron nanotubes and boron clusters.20 This competition is also manifested in the B40 fullerene via the formation of two hexagonal and four heptagonal rings. These polygonal rings (or holes) provide another possibility to bond with metal atoms in an exohedral manner. A recent theoretical study suggests that the Ti-decorated B40 fullerene is a promising hydrogen storage material with high capacity.21 Hence, it is of fundamental interest to


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examine all possible binding configurations of metal atoms with the B40 cage. Bai et al recently reported the first DFT computation of the B40 fullerene doped with either a Be, Mg, Ca, or a Sr atom.22 They found that both Ca@B40 and Sr@B40 exhibit endohedral structures as the global minima, while BeB40 and MgB40 favor the exohedral geometries with the dopant face-capping a heptagon on the side surface (η7–M).22 Their study suggests that the atomic radii of metal dopants are a key factor in determining stability of the endohedral configuration of complexes.22 In another recent theoretical study, endohedral derivatives, M@B40 (M = Sc, Y, and La), have been predicted.23 Compared to Ca@B40 which has a slightly off-center dopant Ca, or to the highly symmetric endohedral Sr@B40,22 the Sc, Y, and La atoms all favor the off-center location within the B40 cage, even though the size of Y (1.80 Å) is same as that of Ca (1.80 Å).24 Note also that the Mg (1.50 Å) dopant is slightly smaller than the Sc dopant (1.60 Å),24 but Mg favors the η7–M exohedral location while Sc favors the endohedral off-center location. Therefore, it will be useful to determine additional factors that can be used to identify the preference of endohedral or exohedral doping for the metal atoms. In addition, in view of numerous reports on endohedral carbon fullerenes, an extension of doping a metal atom in the boron fullerenes is of importance as well. In this study, we investigated structure stability of MB40 (M = Li, Na, K, Ba, and Tl) by using first-principles DFT calculations and first-principles molecular dynamics (FPMD) simulations. Our results suggest that the Na and Ba atoms can be stably encapsulated inside the B40 cage. We describe our computational details in Section II and present our results and discussions in Section III. Some concluding remarks are given in Section IV.


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II. COMPUTIONAL METHODS The theoretical simulations start from an unbiased search for the global minima of MB40 (M=Li, Na, and K) using the basin-hopping algorithm coupled with DFT optimization.25, 26 The Accelrys’ DMol3 package27 was employed in the DFT calculations, in which a semi-core pseudopotential28 and a double-numerical-polarized basis set with a real-space cutoff of 5.5 Å were chosen to compute the electronic structure. For the exchange-correlation functional, the spin-polarized generalized-gradient approximation with the Perdew-Burke-Ernzerhof (PBE) formulation was used.29 Several different low-lying isomers of B40 with a metal atom, such as quasi-planar and double-ring tubular configurations, were used as the initial input, besides the endohedral and exohedral metalloborospherenes. Over 2000 stationary points on the potential surface were evaluated. Low-lying isomers were then re-optimized fully and their relative energies were computed at the level of hybrid functional PBE0 and the 6-311+G* basis set,30, 31 which has been tested extensively in prior studies as a reliable DFT method for boron clusters and metalloborospherenes.16, 17, 22 To examine DFT functional effect on the predicted lowestenergy structures, the low-lying isomers of NaB40 was also re-examined at the hybrid metal functional TPSSh/6-311+G* level.32 For M = Ba and Tl, isomer structures of BaB40 and TlB40 were constructed using low-lying structures of M = Li, Na, and K. These isomer structures were optimized using PBE0 functional with the Stuttgart/Dresden (SDD) effective core potential valence basis.33, 34 Finally, harmonic vibrational frequencies were computed at PBE0 level of theory to affirm that the lowest-energy structures do not show imaginary frequencies. All the PBE0 and TPSSh computations were performed using the Gaussian09 program package.35 To examine thermal stability of the endohedral M@B40 (M= Na and Ba) found here, the FPMD simulations are performed using the QUICKSTEP program implemented in the CP2K


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software package.36 Within the framework of the Kohn-Sham formulation of DFT and the Gaussian plane-wave method,37 the core electrons are described by the Goedecker-Teter-Hutter normconserving pseudo-potential,38, 39 and the wave functions of valence electrons are expressed by a combination of the polarized double-ξ quality Gaussian basis40 and a plane-wave basis set (with an energy cutoff of 330 Ry). The PBE-D3 method is employed here to account for weak van der Waals interactions. The spin-polarized computations are also applied. The endohedral M@B40 clusters are put in a cell with volume of 3 nm × 3 nm × 3 nm. The FPMD simulations are performed in the constant-volume and constant-temperature ensemble with temperature controlled at 500 K and 1000 K, respectively. For the stability tests, 10-ps simulations are carried on with the time step of 1.0 fs. Statistical analysis of the root mean square derivation (RMSD) and bond length derivations (BLD) for both M-B and B-B bonds of M@B40 with respect to its corresponding global minimum (initial) is done. The RMSD is defined as RMSD =

1 ( ri , t − ri , ini ) 2 ∑ n n

where ri, t denotes the distance of the i-th atom to the center of cluster mass at simulation time t. The BLD is defined as BLD M − B/B − B =

1 ∑ ( d M − B/B − B, t − d M − B/B − B, ini ) 2 n n

where dM-B/B-B, t indicates the M-B/B-B bond length of M@B40 cluster at simulation time t. III. RESULTS AND DISCUSSION Configurational energy spectra of LiB40 and NaB40 computed at the PBE0/6-311+G* level are shown in Figure 1, where the selected low-lying isomers are depicted according to their relative energies. The ground state of LiB40 adopts the η7-M exohedral configuration as shown in Figure 1a. This isomer is lower in energy than another exohedral isomer with Li capping a hexagon


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either on the top or at the bottom (η6-M), due to formation of one more bond with B atoms. The next three low-energy isomers, all lying higher by at least 0.4 eV than the lowest-energy isomer, and they all adopt the endohedral configurations. Among these three isomers, the off-center doping isomer is energetically more favorable. Beyond the three isomers, higher-lying isomers typically exhibit quasi-planar or triple-ring configurations with the Li dopant. Other isomer configurations such as double-ring, tube-like, and flat-cage are well separated from the ground state by more than 2 eV. For the NaB40 cluster, the structures of low-energy isomers (with relatively higher energies than the ground-state isomer by 0.6 eV or more) are similar to those of LiB40 cluster. The most striking difference between the Li and Na dopant is that Na favors the endohedral configuration Na@B40 with the C2v point-group symmetry as Na being only 0.001Å off the cage center along the C2 axis. The next two low-lying isomers of NaB40 adopt the η7-M and η6-M exohedral configurations, respectively. Since the energy differences between the endohedral and exohedral isomers of NaB40 are small, their formation energies are calculated with considering the basis set superposition error. It is found that the C2v Na@B40 still has the lowest formation energy. We have also computed relative energies, at the TPSSh level,32 of various NaB40 isomers within 0.7 eV energy difference from the lowest-energy isomer. As shown in Figure 1b, trends in relative stabilities among different isomers are consistent with one another by using either the TPSSh or PBE0 level of theory, especially in predicting the relative stability of endohedral versus exohedral isomers. These results suggest that the two strong competitors for the lowestenergystructures of Li@B40 and Na@B40 are most likely either the endohedral or the η7-M exohedral configuration, due in part to the higher stability of the B40 cage that is well separated from other B40 isomers by an energy gap of about 0.5 eV (at the PBE0 level).16


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Figure 1. Configurational energy spectra of (a) LiB40 and (b) NaB40 at the PBE0/6-311+G* level. The energies of the global minima are taken to be zero. B, Li, and Na atoms are depicted by the pink, blue, and purple balls, respectively. The symmetric point group and the relative energy (in eV) with respective to the global minimum are listed below each isomer. For the lowlying isomers of NaB40 within 0.7 eV from the ground state at the PBE0 level, the relative energies (in eV) computed at the TPSSh/6-311+G* level are also listed in the right side of each isomer as those shown in red brackets. Cartesian coordinates of their global minima are given in Table S1.


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To gain additional insights into relative stabilities between the endohedral and exohedral structures of metalloborospherenes, we have performed a scan of potential energy by using the dopant as a probe to move along the path connecting the B40 center and a heptagon center but keeping the B40 cage intact. As illustrated in Figure S1a, the energy barriers for the Na dopant are 10.09 and 9.95 eV for its moving “in” and “out” through the heptagon ring, respectively. The values decrease for the Li dopant due to its smaller size. We also notice that these potential energy curves exhibit two local minima: One corresponds to the endohedral structure while another corresponds to the η7-M exohedral structure. For the Li dopant, the exohedral minimum is lower in energy than the off-center endohedral minimum by about 0.31 eV, whereas, for the Na dopant, the endohedral minimum is 0.14 eV lower in energy than the exohedral minimum (see the inset of Figure S1a). These results are consistent with the full structural optimizations shown in Figure 1, suggesting that the potential-energy scan may serve as a quick computational indicator to be helpful in exploring relative stability between the endohedral and exohedral configurations of metalloborospherenes. To further confirm this simple indicator, we have examined other metal dopants (M=K, Ba, and Tl) and shown the scanning results in Figure S1, from which it may expect that Ba can be stably encapsulated in the B40 cage. The more elaborated DFT computation results on the MB40 (M=K, Ba, and Tl) clusters are shown in Figure 2a, b, c. Among the three clusters, only the Ba-dopant in BaB40 favors the endohedral configuration. This is somewhat surprising in view of the atomic radius of Ba (2.15 Å) being very close to that of K (2.20 Å).24 Based on our calculations and previously published results, it seems that the relative stabilities of endohedral and exohedral configurations for different metal dopants in the B40 cage are not only depending on the size of dopants, but also the specific interaction between the dopant and boron cage. For metal dopants with similar sizes, the


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Figure 2. Configurational energy spectra of (a) KB40, (b) BaB40, and (c) TlB40 clusters at the PBE0 level. B, K, Ba, and Tl atoms are depicted by the pink, dark purple, green and red balls, respectively.

dopant-boron interaction plays an important role, as illustrated by the comparison of stable endohedral M@B40 (M = Na, Ca, and Sr) vs exohedral Tl&B40. Although the size of Tl (1.90 Å) only differs by 0.1 Å from that of Na (1.80 Å), Ca (1.80 Å), and Sr (2.00 Å),24 the MB40 (M = Na, Ca, and Sr) prefers the endohedral configuration,22 whereas the TlB40 favors the exohedral one,


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as shown in Figure 2c. For alkali metals (M = Li, Na, and K), the RMSD values of the endohedral configurations are 0.0114, 0.0088, and 0.0093 Å, respectively. Among them, the Na@B40 possesses the smallest extent of distortion of the B40 cage. The Na doping prefers the endohedral configuration largely due to favorable size match, whereas the Li atom is notably under size and K atom is over size. The bonding pattern of C2v Na@B40 analyzed by the adaptive natural density partitioning (AdNDP) method41 is illustrated in Figure S2a, which shows forty three-center two-electron (3c2e) σ bonds, eight 6c-2e σ bonds, four 5c-2e π bonds, four 6c-2e π bonds, four 7c-2e π bonds, and a 40c-2e π bond. Such a bonding pattern is very similar to that of the C2v Ca@B40 metalloborospherene.22 The electron localization results shown in Figure S2b support the abovementioned AdNDP analysis. Also, no electron localization can be found between the Na dopant and the boron atoms in the electron localization function plots, indicating that the bonding between the metal dopant and B is predominantly ionic. For the endohedral C2v Na@B40, the calculated natural atomic charge is +0.89 |e| for Na with the electronic configuration Na [Ne]3s0.11, which is different from that of the Ca(Sr)-encapsulated metalloborospherenes (+1.60 |e| for Ca and +0.58 |e| for Sr, respectively).22 Therefore, the amount of the charge transfer depends on the metal dopants, which induces different interaction between the dopant and boron cage.

For both newly found endohedral Na@B40 and Ba@B40 clusters, their thermal stabilities are further evaluated by statistical analyses of RMSD, BLDM-B, and BLDB-B with respect to the corresponding global minima based on 10-ps FPMD simulations at 500 K and 1000 K, shown in Figure 3a, b, c and Figure 3d, e, f respectively. The average RMSD values for the endohedral Na@B40 versus Ba@B40 are almost the same at 500 K, while the average RMSD value of Ba@B40 is even smaller than that of Na@B40 at 1000 K. This result can be attributed to the larger atomic radius of Ba than that of Na. The BLDM-B of Na@B40 in Figure 3b fluctuates more


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strongly at 1000 K, which may be due to the smaller atomic sizeof Na dopant than Ba dopant. For BLDB-B shown in Figure 3c, f, the two M@B40 (M = Na and Ba) exhibit no obvious difference. In addition, higher temperature can induce larger BLDB-B, indicating the B40 cage is quite flexible to hold different-sized metal atoms and even to expand at higher temperature.

Figure 3. Statistical analyses of RSMD, BLDM−B, and BLDB−B for the endohedral C2v Na@B40 (a~c) and D2d Ba@B40 (d~f) clusters. The results are based on FPMD simulations with temperature controlled at 500 K (black line) and 1000 K (red line). The highlighted average values for RSMD, BLDM−B, and BLDB−B in blue are estimated based on the last 6 ps (equilibrium stage) of each FPMD simulation. To further understand the contribution of metal dopant to the host B40 cage, molecular-orbital analyses of electronic structures for the endohedral C2v Na@B40 and exohedral Cs Na&B40 clusters have been made in Figure 4. It can be seen that the frontier-orbital configurations of C2v


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Na@B40 resemble those of D2d B40 since they are mainly contributed by the B40 cage. For example, the alpha single-occupied molecular orbital (α-SOMO) of the C2v Na@B40, represented in Figure 4, is fully located on boron atoms without any contribution from the Na dopant, which is totally different from the exohedral Cs Na&B40. In the Cs Na&B40, the 3s orbital of the Na dopant plays a substantial role in its α-SOMO. The electronic structures of the other endohedral and exohedral metalloborospherenes, such as the lowest-energy D2d Ba@B40 and Cs Li&B40, Cs K&B40, Cs Tl&B40, are very similar to those of the endohedral C2v Na@B40 and exohedral Cs Na&B40, respectively, as shown in Figure S3. Considering that encapsulating a metal atom into the B40 cage does not significantly alter the electronic or atomic structure of B40, it would also be interesting to dope the boron cage with size-match magnetic metal atom, which may lead to magnetic M@B40 clusters. This work is undertaken.

Figure4.

Frontier

molecular

orbital

energy

levels

of

endohedral

and

exohedral

metalloborospherenes C2v Na@B40 and Cs Na&B40, as compared with those of the pure D2d B40 cage. Some molecular orbital pictures and their symmetries are indicated.


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The lowest-energy Cs Li&B40, C2v Na@B40, Cs K&B40, D2d Ba@B40, Cs Tl&B40 clusters have the negative formation energies (Ef = E(MB40) - E(B40) - E(M)) of -49.8, -34.1, -37.1, -86.9, and -35.2 kcal/mol, respectively, at the PBE0 level. For comparison, the formation energies for the C2v Ca@B40, D2d Sr@B40, CsBe&B40, and Cs Mg&B40 series are -82.2, -78.5, -20.0, and -67.1 kcal/mol, respectively, at the same level of theory.22 These results imply the stabilization of the metal-doped borofullerenes found here with respect to the metal atoms and the empty B40 fullerene. The calculated vertical ionization potentials and vertical electron affinities of the ground-state MB40 (M=Li, Na, K, Ba, and Tl) are (5.53, 5.69, 5.14, 6.19, 5.66) eV and (2.18, 2.43, 1.85, 2.39, 2.42) eV, respectively, which differ from those of the D2d B40 cage (7.49 and 2.20 eV, respectively, at the same level). Therefore, doping different metal atom may offer chemical versatility for fine tuning the electronic properties of borofullerenes. The far-infrared multiple-photon dissociation (FIR-MPD) seems the only size-selective experimental technique available to determine the structure of neutral metal clusters in the gas phase.42 The IR spectra of the pure D2d B40 cage, the endohedral C2v Na@B40, and the exohedral Cs Na&B40 are shown in Figure 5 for comparison. Encapsulating the metal dopant Na in the B40 cage does not significantly change the IR absorption peaks of the host B40, resulting in similar well-resolved peaks. However, the band in the low frequency region broadens, and some peaks in the high frequency region (such as those at 1104 and 1184 cm-1) are enhanced, giving another indicator to identify the endohedral dopant. On the other hand, a η7–M exohedral doping of Na results in substantial influence of the IR spectra of the host B40 cage, showing some new IR absorption peaks in the low-frequency region (< 300 cm-1). Since the energy difference between the endohedral and exohedral isomers is small for the Na dopant (~0.1 eV), there is a possibility to observe both geometries in experiment. Such notably differences in FIR spectra between the


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endohedral and exohedral configurations will be useful to identify their structures in the future experiment. The simulate IR spectra for the other ground-state MB40 (M = Li, K, Ba and Tl) have been illustrated in Figure S4.

300

IR activity

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250

D2d B40 cage

200

Cs Na&B40

C2v Na@B40

150 100 50 0 0

300

600

900

1200

1500

-1

Wavenumber (cm ) Figure 5. Computed IR spectra of the pure D2d B40 cage, the endohedral C2v Na@B40 and the exohedral Cs Na&B40 clusters, as shown by the black solid, red dashed, and blue dotted lines, respectively.

IV. CONCLUSION We have investigated a series of metal-doped B40 clusters, MB40 (M=Li, Na, K, Ba, and Tl). Among these clusters, endohedral doping of the B40 cage by Na and Ba is predicted. In addition to the prerequisite of matching atomic size with the cage size, we find that the dopant-boron interaction appears to be another key factor that can be used to analyze relative stabilities among


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many doped boron, particularly whether the dopant can be encapsulated inside the boron cage. As an example, although the metal atoms Na and Tl have nearly the same atomic size, the endohedral doping of the B40 cage is favored by Na atom, whereas it is unfavored by Tl atom. Lastly, we suggest that the computed IR spectra may be used as a fingerprint to identify the lowest-energy structure of metalloborospherenes. Our results provide valuable insights into the possible existence of endohedral metalloborospherenes and are of broad interest to the on-going research of boron clusters.

ASSOCIATED CONTENT Supporting Information. Cartesian coordinates of the optimized ground-state MB40 (M = Li, Na, K, Ba, and Tl); potential energy scan by moving the metal dopant from the cage center through a heptagon center of the B40 while keeping the cage intact; chemical bonding analyses and electron localization plot for the C2v Na@B40; molecular orbital analyses and simulated FIR spectra of the lowest-energy MB40 (M = Li, K, Ba, and Tl). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (86-25-83595330) or [email protected].

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We are grateful to Dr. H. Bai, Prof. H. J. Zhai, and Prof. Y. Gao for useful discussions. WF acknowledges the Natural Science Foundation of China (No. 11474150) and the computing support by the High Performance Computing Center of Nanjing University with the IBM Blade cluster system. The computation is performed at University of Nebraska Holland Computing Center. REFERENCES (1) Szwacki, N. G.; Sadrzadeh, A.; Yakobson, B. I. B80 Fullerene: An Ab Initio Prediction of Geometry, Stability, and Electronic Structure. Phys. Rev. Lett. 2007, 98, 166804; erratum, 2008， 100, 159901. (2) Szwacki, N. G. Boron Fullerenes: A First- Principles Study. Nano. Res. Lett. 2008, 3, 49-54. (3) De, S.; Willand, A.; Amsler, M.; Pochet, P.; Genovese, L.; Goedecker, S. Energy Landscape of Fullerene Materials: A Comparison of Boron to Boron Nitride and Carbon. Phys. Rev. Lett. 2011, 106, 225502. (4) Sadrzadeh, A.; Pupysheva, O. V.; Singh, A. K.; Yakobson, B. I. The Boron Buckyball and Its Precursors: An Electronic Structure Study. J. Phys. Chem. A 2008, 112, 13679-13683. (5) Zhao, J.; Wang, L.; Li, F.; Chen, Z. B80 and Other Medium-Sized Boron Clusters: Core-Shell Structures, Not Hollow Cages. J. Phys. Chem. A 2008, 114, 9969-9972. (6) Li, F.; Jin, P.; Jiang,D.; Wang, L.; Zhang, S. B.; Zhao, J.; Chen, Z. B80 and B101-103 Clusters: Remarkable Stability of the Core-Shell Structures Established by Validated Density Functions. J. Chem. Phys. 2012, 136, 074302.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Zope, R. R. The α-Boron Cages with Four-Member Rings. Europhys. Lett. 2009, 85, 68005. (8) Zope, R. R.; Baruah, T.; Lau, K. C.; Liu, A. Y.; Pederson, M. R.; Dunlap, B. I. Boron Fullerenes: From B80 to Hole Doped Boron Sheets. Phys. Rev. B 2009, 79, 161403(R). (9) Sheng, X.; Yan, Q.; Zheng, Q.; Su, G. Boron Fullerenes B32+8k with Four-Membered Rings and B32 Solid Phases: Geometrical Structures and Electronic Properties. Phys. Chem. Chem. Phys. 2011, 11, 9696-9702. (10) Polad, S.; Ozay, M. A New Hole Density as a Stability Measure for Boron Fullerenes. Phys. Chem. Chem. Phys. 2013, 15, 19819-19824. (11) Quarles, K. D.; Kah, C. B.; Gunasinghe, R. N.; Musin, R. N.; Wang, X. Filled Pentagons and Electron Counting Rule for Boron Fullerenes. J. Chem. Theory Comput. 2011, 7, 2017-2020. (12) Cheng, L. B14: An All-Boron Fullerene. J. Chem. Phys. 2012, 136, 104301. (13) Lu, H.; Li, S. Three-Chain B6n+14 Cages as Possible Precursors for the Syntheses of Boron Fullerenes. J. Chem. Phys. 2013, 139, 224307. (14) Lv, J.; Wang, Y.; Zhu, L.; Ma, Y. B38: An All-Boron Fullerene Analogue. Nanoscale 2014, 6, 11692-11696; Tai, T. B.;

Nguyen, M. T. Comment on “B38: An All-Boron Fullerene

Analogue” by J. Lv, Y. Wang, L. Zhu, and Y. Ma, Nanoscale, 2014, 6, 11692. Nanoscale 2015, 7, 3316. (15) Pham, H. T.; Duong, L. V.; Tam, N. M.; Pham-Ho, M. P.; Nguyen, M. T. The Boron Conundrum: Bonding in the Bowl B30 and B36, Fullerene B40 and Triple Ring B42 Clusters. Chem. Phys. Lett. 2014, 608, 295-302.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

(16) Zhai, H. J., Zhao, Y. F.; Li, W. L.; Chen, Q.; Bai,H.; Hu, H. S.; Piazza, Z. A.; Tian, W. J.； Lu, H. G.; Wu, Y. B.; Mu, Y. W.; Wei, G. F.; Liu, Z. P.; Li, J.; Li, S. D.; Wang, L. S. Observation of an All-Boron Fullerene. Nat. Chem. 2014, 6, 727-731. (17) Chen, Q.; Li, W. L.; Zhao,Y. F.; Zhang, S. Y.; Hu, H. S.; Bai, H.; Li, H. R.; Tian, W. J.; Lu,H. G.; Zhai, H. J.; Li, S. D.; Li, J.; Wang, L. S. Experimental and Theoretical Evidence of an Axially Chiral Borospherene. ACS Nano 2015, 9, 754-760. (18) He, R. X.; Zeng, X. C. Electronic Structures and Electronic Spectra of All-Boron Fullerene B40. Chem. Commun. 2015, 51, 3185-3188. (19) Gao, G.; Ma, F.; Jiao, Y.; Sun, Q.; Jiao, Y.; Waclawik, E.; Du, A. Modelling CO2 Adsorption and Separation on Experimentally-Realized B40 Fullerene. Computat. Mater. Sci. 2015, 108, 38-41. (20) Tang, H.; Ismail-Beigi, S. Novel Precursors for Boron Nanotubes: The Competition of TwoCenter and Three-center Bonding in Boron Sheets. Phys. Rev. Lett. 2007, 99, 115501. (21) Dong, H.; Hou, T.; Lee, S.; Li, Y. New Ti-Decorated B40 Fullerene as a Promising Hydrogen Storage Material. Sci. Rep. 2015, 5, 09952. (22) Bai, H.; Chen, Q.; Zhai, H. J.; Li, S. D. Endohedral and Exohedral Metalloborospherenes: M@B40 (M = Ca, Sr) and M&B40 (M = Be, Mg). Angew. Chem. Int. Ed. 2015, 54, 941-945. (23) Jin,P.; Hou, Q.; Tang,C.; Chen, Z. Computational Investigation on the Endohedral Borofullerenes M@B40 (M = Sc, Y, La). Theor.Chem. Acc. 2015, 134, 13. (24) Slater, J. C. Atomic Radii in Crystals. J. Chem. Phys. 1964, 41, 3199.


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Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Wales, D. J.; Scheraga, H. A. Global Optimization of Clusters, Crystals, and Biomolecules, Science 1999, 285, 1368-1372. (26) Yoo, S.; Zeng, X. C. Motif Transition in Growth Patterns of Small to Medium-Sized Silicon Clusters. Angew. Chem. Int. Ed. 2005, 44, 1491-1494. (27) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508. (28) Delley, B. Hardness Conserving Semilocal Pseudopotentials. Phys. Rev. B 2002, 66, 155125. (29) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (30) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: the PBE0 Model. J. Chem. Phys. 1999, 110, 6158-6165. (31) Krishnan,R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650-654. (32) Tao,J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401. (33) Feller, D. The Role of Data bases in Support of Computational Chemistry Calculations. J. Comput. Chem. 1996, 17, 1571-1586. (34) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. Basis Set Exchange: A Community Database for Computational Sciences. J.


19


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Page 20 of 21

Chem. Inf. Model 2007, 47, 1045-1052. (35) Gaussian 09, Revision D.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian, Inc., Wallingford CT, 2013. (36) Vandevondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing T.; Hutter, J. QUICKSTEP: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach.Comput. Phys. Commun. 2005, 167, 103-128. (37) Lippert, G.; Hutter, J. R.; Parrinello, M. A Hybrid Gaussian and Plane Wave Density Functional Scheme. Mol. Phys. 1997, 92, 477-488. (38) Holz, M.; Mao, X.-A.; Seiferling, D.; Sacco, A. Experimental Study of Dynamic Isotope Effects in Molecular Liquids: Detection of Translation-Rotation Coupling. J. Chem. Phys. 1996, 104, 669. (39) Hartwigsen, C.; Goedecker, S.; Hutter, J. Relativistic Separable Dual-Space Gaussian Pseudopotentials from H to Rn. Phys. Rev. B 1998, 58, 3641. (40) VandeVondele, J.; Hutter, J. Gaussian Basis Sets for Accurate Calculations on Molecular Systems in Gas and Condensed Phases. J. Chem. Phys. 2007, 127, 114105. (41) Zubarev, D. Y.; Boldyrev, A. I. Developing Paradigms of Chemical Bonding: Adaptive Natural Density Partitioning. Phys. Chem. Chem. Phys. 2008, 10, 5207-5217. (42) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Structures of Neutral Au7, Au19, and Au20 Clusters in the Gas Phase. Science 2008, 321, 674-676.


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Stability of Metal-Encapsulating Boron Fullerene B40 - The Journal of

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