Hydrogen Storage on Volleyballene: The Prediction of the

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C: Physical Processes in Nanomaterials and Nanostructures 20

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Hydrogen Storage on Volleyballene: The Prediction of the Sc C H Cluster Alfredo Tlahuice-Flores J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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The Journal of Physical Chemistry

Hydrogen Storage on Volleyballene: The Prediction of the Sc20C60H70 Cluster Alfredo Tlahuice-Flores* Universidad Autónoma de Nuevo León, CICFIM-Facultad de Ciencias Físico-Matemáticas, San Nicolás de los Garza, NL 66450, México.

ABSTRACT

This study is devoted to the hydrogenation of the volleyballene (Sc20C60) compound. It is determined that up to 70 H atoms can be adsorbed on the volleyballene structure, and its hydrogenated structure (Sc20C60H70) holds a 1.1 eV HOMO-LUMO gap value. The new structure is comprised by scandium atoms forming part of the framework, which avoid their clustering and enhance the H2 uptake. Worthy of note is a calculated H2 uptake capacity of 4.20 wt %, with a calculated -0.11 eV/H2 adsorption energy value. Moreover, calculated thermodynamic properties as enthalpy (negative sign) and entropy (positive sign) assure that the hydrogenation reaction of volleyballene can be obtained at ambient temperature. The calculated IR and Raman spectra do not feature imaginary frequencies attesting the experimental convenience of the predicted Sc20C60H70 cluster.

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1. INTRODUCTION The initial strategy to obtain new materials based on fullerenes (C60, C70 and so on) was to insert metal atoms into their cavities.1 Unfortunately, a low production yield of the fullerene-metal hybrid materials was obtained with a limitation on the number of metal atoms able to be incorporated (one or two). To date the experimental incorporation of scandium carbide (two Sc atoms) inside C84 fullerene was obtained by Wang et al. in 2001,2 and one amply used method to generate carbide endohedral metallofullerenes is to burn a graphite rod which is filled with metal alloys. Recently, it has been explored to deposit metal atoms on fullerenes cages with the goal to obtain new hydrogen storage materials,3,6 due to the well known capacity of metal atoms to incorporate electrons coming from H atoms or H2 molecules into their empty d orbitals. In 2016 the structure of volleyballene (hereafter named as V-ball) was proposed by Jin Wang et al.7 and Tlahuice-Flores8 carried out vibrational and absorption spectra calculations to help into its identification. Volleyballene structure is comprised by scandium atoms that are incorporated into its framework, and therefore its hydrogenation capacity is maintained due to the lack of clustering efects.9-10 Noteworthy is that the formula of Volleyballene (Sc20C60) resemble the well known C80 fullerene, but the displayed bonding closely resembled one of the early hypotheses of Alvarez et al. in 1991 for the LaC82, La2C80 series,11 where two carbon atoms are substituted by one Transition Metal.12 The importance of this study is based on the necessity of clean electricity that can be provided by hydrogen (fuel cells), hence the search of a new material able to storage hydrogen represents an opportunity area. Nowadays, the prevailing issue is to storage the hydrogen which requires a safe medium, and in consequence, new materials need to keep a good interaction with hydrogen


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atoms to accomplish this task.13-14 Following this line of thought, several computational studies have tested metal based hydrides,13 carbon nanotubes,14-15 and metal-organic frameworks (MOF)16-17 as hydrogen adsorbents, because they are porous, they feature high gravimetric capacities, high surface area, but unfortunately most of them are far from the ideal range of interaction energies (Indeed, MOFs have the lowest energy values). Among all the reported materials, transition metal and organic compounds present a high gravimetric hydrogen uptake, for example, Durgun et al. in 2006 predicted a titanium-ethylene complex with an average binding energy of 0.45 eV/H2,18 which has been determined experimentally to hold a H2 uptake of 12 wt%.19 However, those compounds suffered of clustering effects resulting in a reduction of their H2 uptake capacity.20 In this work, is reported a structural and vibrational study of a new Sc20C60H70 compound that represents a heavy hydrogenated structure, where vibrational characterization (IR and Raman spectra calculations) assures its experimental feasibility.14,20,21 Prior to the search of preferred sites to adsorb H2 in the V-ball, it is important to keep in mind that Sc atoms are part of two different sets. One set is comprised by twelve Sc atoms forming one icosahedron-like structure, while eight Sc atoms are part of one Sc8 cube. The correctness of the distinction between both type of Sc atoms is evident from the calculated frontier molecular orbitals, previously.8 It was stated that the major contribution to HOMO level is due to Sc atoms linked to pentagonal rings, but with an important contribution from carbon atoms as well. Guided by the 18-electron rule, H2 molecules were added up to the volleyballene and specifically to the transition metal atoms.3,6 This rule when is applied to C5 rings linking to a Sc atoms, predicts a maximum of five H2 molecules bonded to every Sc atom. The electron accounting is as follows: Scandium atoms hold three valence electrons that can be added to five electrons


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coming from the linked pentagonal rings and the remaining ten electrons, needed to fulfil a d orbital, are obtained from the adsorbed hydrogen atoms. In the present work, after a systematic search for a heavy hydrogenated structure (hereafter named as HV-ball), it was found that only up to two H2 molecules and one H atom are linked to 12 of the available scandium atom and the obtained structure is free of imaginary frequencies. Worthy of note is the huge requirements in memory (1 TB of RAM) and in disk storage (> 100 GB) for the calculation of the IR and Raman spectra of the HV-ball comprised by 150 atoms. 2. METODOLOGY This study is based on DFT (ORCA package) calculations,22 by considering a gas phase reaction, and a vibrational (IR and Raman spectra) characterization. The calculations were carried out with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional.23 For C, and H atoms, the def2-TZVPP Ahlrichs basis set and the Coulomb-fitting auxiliary basis def2/J were used, which is defined due to the usage of the RI approximation.24 Effective Core Potentials were defined for the valence electrons of scandium atoms.25-26 The energy and gradient convergence criterion were selected as 1x10-6 and 3x10-5, respectively. The election of mentioned parameters reduced the computation time without lack of the quality in the obtained results. In Figure S1 is included a comparison of calculated IR and Raman spectra of V-ball by using ORCA and Gaussian03 (Ref. 8) computational packages. Noteworthy is the good agreement with calculations included in the Ref. 8.

3. RESULTS AND DISCUSSION Prior to the study of HV-ball, is important to deliver an explanation about the free of hydrogen structure. Volleyballene or V-ball is comprised by 20 scandium atoms, distributed as one


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distorted icosahedron (Sc12) and one cube (Sc8). Scandium atoms comprising the cube are located in the center of eight of 20 triangular faces of the distorted icosahedron. Carbon atoms are forming C5 rings which are located at the center of twelve triangular faces (green planes in Figure 1) of the Sc12 icosahedron. Regarding the bonding featured by HV-ball, is important to describe the initial structure (before the optimization stage) because it offers a simple view. The initial HV-ball is comprised by six H-C5-C5-H units containing two Sc atoms linked to five H atoms each one, respectively. Thus, the six Sc2C10H12 motifs are bound together and with the underlying Sc8 cube through Sc-C bonds (Top left of the Figure 1). The optimized structure contains rotated C5 rings and the tilting angle with respect to triangular faces spans a range of 22 to 57 degrees. The full set of calculated tilt angle values is included in the Table S1. Clearly the optimized structure is not anymore a Thstructure and instead it can be described as a distorted icosahedral-like structure (bottom right in Figure 1). In order to highlight the role of scandium atoms in the stability of the HV-ball, a detailed description is provided. It can be detected four types of the Sc-C bonds based on the manner that Sc atoms are forming bonds. a) Six Sc atoms are located more internally into the framework and they are connected exclusively to C5 rings (Figure S2) which means that they do not adsorb hydrogen atoms. They can be considered as part of the inner Sc8 cube of the initial structure. b) Each Sc2C10H12 motif, contains Sc atoms linking two C atoms and up to two H2 molecules and one H atom.


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c) The connection between neighbouring Sc2C10H12 motifs is through Sc-C bonds. A further bond length analysis of the HV-ball revealed the presence of H-H (0.78 Å), and C-H bonds (1.1 Å) in the lower range of distances. Sc-Sc bonds displayed by the V-ball were enlarged (> 3.2 Å) after its hydrogenation, in such manner that the structure can be thought as comprised by organic chains linked by metal atoms. In the HV-ball, C-C (1.4-1.5 Å range), Sc-Sc, and mainly Sc-C bonds suffer of distortion spanning and amply bond lengths range (Fig. 2 lower panel). Calculated Sc-C bonds (2.12-2.44 Å) are in agreement with the experimental value of 2.26 Å measured in the Sc2C2@C84 cluster.2 Scandium and hydrogen atoms, feature two set of bond lengths: twelve Sc-H bonds are circa 1.9 Å in length, and 46 Sc-H bonds are included in the 2.1-2.3 Å range. This means that scandium atoms form short bonds (hydride type bond) with atomic hydrogen and slightly large bonds with H2 molecules. Mentioned difference in Sc-H bonds might conduce to different vibrational frequencies (confirmed after the vibrational calculations). The adsorption energy of hydrogen on HV-ball, in gas phase, is estimated by using the following formula:

=

− ( + 35 ) 35

The estimation of the adsorption energy can be done by using total energy or Gibbs free energy values. By considering Gibbs free energy, it was calculated a -0.11 eV/ H2 value. The corrected value by including Gibbs free energy provide us with the hydrogenation value at different temperatures. The adsorption value by considering only total energy increases to -0.3255 eV/ H2


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which is comparable to the value reported at a recent paper by Guo et al.27 The calculated value then, might assure the correct adsorption/desorption of H2 molecules. The stability of the Sc20C60H70 cluster is attested by its displayed 1.1 eV HOMO-LUMO gap value which is close related to the reported 1.47 eV value of parent V-ball.8 On the other hand the calculated enthalpy (negative sign) and entropy (positive sign) values associated with the hydrogenation of V-ball attest the experimental convenience of the HV-ball at room temperature. In order to elucidate the ionic or covalent character of Sc-H, Sc-C, and C-C bonds comprising the HV-ball structure, the electron localization function (ELF) was used.28 ELF values vary among zero and one. In the case of lone electron pairs, inner shells of one atom, or covalent bonds, the value is close to one, meaning that electrons are greatly localized. In contrast, metallic bonds hold an intermediate ELF value (circa 0.5). In this manuscript, ELF contour plots were processed by means of the Multiwfn program by Tian Lu and Feiwu Chen.29 The analysis of the charge distribution on the HV-ball is based on the atomic dipole moment corrected Hirshfeld Population.29,30 The election is dictated by the known capacity to solve issues related to the poor dipole moment reproducibility of Hirshfeld charge. ADCH charges values are more reliable and it can be extracted from ORCA calculations easily. In general, Sc atoms hold positive charges in the range from 0.20 to 0.89 a. u. independently if they are linked to H or C atoms attesting their donator character. Carbon atoms participating in the Sc-C bonds feature negative charges in the range from -0.02 to -0.39 a. u., but eleven hold positive charges (0.02-0.15 a. u.) and correspond with C-C atoms forming bridges between C5 rings. H atoms hold negative charges included in the range from -0.001 to -0.24 a. u. (Sc-H bonds are hydride type), while positive charges span the range from 0.003 to 0.17 a. u. (C-H


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bonds). In figure 3 is provided the charge distribution along the HV-ball structure and is evident that the variation on the charge state is determined by the local chemical ambient of participating atoms. The presence of Sc-H or Sc-H2 bonds in the structure of HV-ball is due to the capacity of Sc atoms to donate electrons. The interaction occurs due to the Kubas mechanism consisting in donation of H2 bonding molecular orbital toward empty d orbitals of Sc with the back donation of charge from antibonding molecular orbital of H atom.31,32 In the case of H2 molecules, the incorporation of electrons into the antibonding σ* orbitals coming from the Sc atoms might result in an enlargement of the H-H bond with respect to the isolated H2 molecule. Taking into account that Sc atoms are linked to C atoms and transfer charge to them, the transfer charge toward H2 molecules or H atoms is reduced. It is important to note that the Sc-H bond implies that H atoms sustain a negative charge and therefore, the bond type can be considered as ionic. The Sc20C60H70 cluster is shown in figures 4-6, and its charge distribution and bonding is done by means of ADCH and ELF plots. The charge and bonding is studied by considering three cutting planes with the thought of simplicity. The first cutting plane (Figure 4a) was selected to study the shortest Sc-C bond with a bond length of 2.12 Å (Figure 4b). The implied charge values (bottom of Figure 4) are the extremes in the ADCH scale, with Sc and C atoms holding a 0.89 and -0.43 values, respectively. The contour lines surround a large positive charge on Sc atom and a high negative charge on the C atom, in agreement with the picture of the ionic bond. The C atom holding a -0.43 value is linked to another C atom holding a 0.064 charge value and located at the C5-C5 bridge position with a separation distance of 1.43 Å. This result is in agreement with the general view of the charge distribution given in figure 3. This view can be strengthening by


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comparing other region of the contour plot where one large C-C bond (circa 1.46 Å) is located. C atoms sustain charges of -0.033 and -0.21 a. u., respectively. The C atom with large negative charge is linked to one Sc atom (not present in the cutting plane). The dotted contour lines show that a negative charge is surrounding both C atoms. In fact, both C atoms are attached to one inner Sc atom (with a 0.44 a. u.) featuring Sc-C bonds of 2.42 and 2.44 Å, respectively. The proximity of C and Sc atoms produces a major charge transfer and a reduced bond length. ELF plot (Figure 4c) shows a large value in the region between the C atoms, which means that the covalent interaction is dominant. The two Sc-C bonds in the considered cutting plane are different, and the ELF value is large between Sc-C atoms with a small bond length; this might imply that the covalent character is enhanced. The second cutting plane is basal to one C5 ring (Figures 5a and 5b) and it confirms the covalent interaction in the ring. Large ELF values correspond with red regions located among C atoms (Figure 5c). In the same way, the C-H bonds display covalent character and ELF values are large. Moreover, the cutting plane contains one C atom of the near C5 ring. The bottom left region of the ELF plot has a Sc-C bond, displaying a blue intense color which indicates that no electrons are localized between Sc and C atoms (ionic bond). The ionic character is confirmed by the analysis of the ADCH plot (Figure 5d). with C atoms holding a negative charge in the C5 ring and the contour lines which are not enclosed the Sc atoms, indicating the separation of charge in the Sc-C bond. The third cutting plane (Figures 6a, 6b) is pertinent because it contains Sc-H and H-H bonds. The ELF plot values characterizes the Sc-H bond as ionic, being this bond shorter than the Sc-H2 one. Interestingly, it is observed that the charge in the H2 molecule is polarized by the Sc positive


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charge, the positive charge seems to be localized mainly in one of H atom, which indicate that HH distance is increased with respect to the isolated H2 molecule. A further analysis of the vibrational properties of the Sc20C60H70 structure (HV-ball) was carried out (Figure 7). It holds a C1 point group and its IR and Raman spectra display an intense peak, located at 1382/1391 cm-1 (within the A1 irreducible representation) which is attributed to the stretching of Sc-H bonds. This value is in agreement with the Sc-H stretching mode calculated (1386.9 cm-1) by Cho et al. for anionic acetylene-Sc complex (H-Sc-CCH-).21 In contrast, parent V-ball structure displays a more intense IR peak located at 478.06 cm-1 attributed to the stretching of Sc-C bonds among the Sc8 and C10 units.8 The optimization of the neutral Th-volleyballene (or V-ball) was carried out by means of ORCA methodology and it is important to mention that its Raman spectrum is less intense with respect to the HV-ball. Indeed, HV-ball features an intense profile and the presence of a peak located at 1391 cm-1. Furthermore, the IR spectrum of HV-ball features and intense profile with a peak located at 1384 cm-1. In the SI is shown a comparison among V-ball and HV-ball spectra. In Figure 8 are provided IR and Raman active modes displayed by HV-ball and their 2D representation. Calculated normal modes can be listed as follow: The Bending of C-H bonds are located circa 1081 cm-1. The breathing of pentagonal rings was found at 1085 cm-1, and C-C stretching in C5 rings spans a range from 1088 to 1242 cm-1. Intense bending movements of C-H bonds combined with the bending of H2 molecules and bending of Sc-H bonds are located at a range starting at 1030 cm-1. Bending modes of pentagonal rings linked to H atoms are found in the range from 1283 to 1330 cm-1, approximately. The stretching modes of C-C carbons linking two pentagonal rings are observed at 1347-1395 cm-1. Peaks located in the range from 3589 to 4000 cm-1 correspond with stretching movements of apical H atoms, and they feature more


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intense peaks in the Raman spectrum. It is important to note that the calculated vibrational frequency of one isolated H2 molecule is located at 4332.44 cm-1. This mean that the adsorption of H2 molecule shifts its stretching vibration by more than 300 cm-1. A more intense Raman active vibrations are attribute to stretching movements between C and H atoms and they are located at 2906- 3075 cm-1. On the low frequency range is found the stretching vibration mode of H2 molecule moving toward linked Sc atoms (410-500 cm-1) combined with the bending of the Sc-H bond. Finally, the calculated IR spectrum shows intense peaks circa 500 cm-1 associated with Sc-H bending modes. All mentioned normal modes displayed by the Sc20C60H70 cluster (or HV-ball) and its assignment represent a huge effort dedicated to the prediction and vibrational characterization of a new material able to adsorb hydrogen. 4. CONCLUSIONS This work addresses the prediction of the Sc20C60H70 cluster that is a stable compound including Sc atoms in its framework and therefore it represents a new material able to adsorb hydrogen with the expected application in fuel cells. Ideally, the structure might be comprised by six Sc2C10H12 motifs plus an inner Sc8 cube, but the number of H atoms is reduced by two in order to maintain the neutrality of the structure. The hydrogenation produces a structural distortion reducing the symmetry to C1, and as expecting, the calculation time is incremented significantly. Based on the charge distribution study and the ELF analysis, the Sc20C60H70 cluster is comprised by C-C covalent bonds, and Sc-C and Sc-H bonds displaying an ionic/covalent character. It was calculated after a vibrational characterization (IR and Raman spectra), thermodynamics properties (enthalpy and entropy), which let us conclude that the hydrogenation of volleyballene


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might occur at room temperature. Calculated adsorption energy values for hydrogen on V-ball can produce a correct hydrogen adsorption/desorption process. Moreover, along this study the preferred sites to adsorb hydrogen and the bonding/distortion displayed by Sc20C60H70 cluster were determined. It was determined that the IR spectrum of the Sc20C60H70 cluster features an intense peak circa 1382 cm-1 attributed to the stretching of Sc-H bonds. Finally, the calculated vibrations and their assignment can be used by experimentalists as a guide to detect the Sc20C60H70 cluster.


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FIGURES


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Figure 2. Bond lengths displayed by the Volleyballene (Lower panel), and the Sc20C60H70 cluster (Upper panel). The induced distortion after the hydrogenation is evident in C-C, Sc-C, and Sc-Sc bonds which can be attributed to the angle formed by the plane passing through pentagonal rings and the triangular faces of volleyballene. The Sc20C60H70 cluster features large Sc-Sc bonds and only six are circa 3.2-3.3 Å. The Hydrogenation occurs preferentially in the Sc atoms but also in pentagonal rings (twelve C-H bonds). Sc-H bonds (green color) are distributed as two groups, corresponding to short bonds between Sc and H atoms, while large bonds are found between H2 molecules and Sc atoms. Sc-C and Sc-H bonds are found circa 2.12 Å in the Sc20C60H70 cluster. Interestingly, Sc-C bonds are short in the Sc20C60H70 cluster allowing a strong interaction. Dashed lines are used to indicate bond distortions after hydrogenation reaction.

Figure 3. Charge distribution on HV-ball. It is considered the atomic dipole moment corrected Hirshfeld Population. ADCH charges are given in the y-axis, while x-axis contains atoms indexes. The three curves represent charge distribution on Sc, C and H atoms. It is evident that Sc atoms transfer major charge toward C atoms. C atoms located at bridge positions among C5 rings holds positive charge values. In contrast, H atoms feature negative charge in Sc-H bonds (from -0.15 to 0.24 a. u.) and positive charge when they are forming C-H bonds (from 0.05 to 0.17 a. u.). H atoms of H2 molecules holds small positive charges (from 0.00 to 0.12 a. u.).


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Figure 4. Charge distribution and bonding displayed by Sc-C and C-C bonds comprising the HV-ball. (a) The HV-ball is intersected by a cutting plane (gray color) and all atoms located behind the plane are shaded. (b) In the cutting plane are located one C-C and three Sc-C bonds; On the bottom left region, a Sc atom is linked to two C atoms (2.42 and 2.44 Å, respectively); in the right region, the shortest Sc-C bond (2.12 Å) is located, and the C atom at the same time is forming a C-C bond. (c) The ELF plot attests the covalent character of the C-C bond located at the bottom left region. The Sc-C bond located at the bottom left region is slightly covalent (orange region between Sc and C atoms). Moreover, the shortest Sc-C bond has an enhanced covalent character (large intermediate region with a large ELF value). (d) The ADCH charge distribution plot shows that positive charge is sustained by Sc atoms, while negative charge is located at C atoms attesting the donator role of Sc atoms. Contour lines are surrounding the C-C bonds indicating the same shared negative charge and in case of Sc-C bonds the metal is releasing charge to C atoms. Clearly, on the shortest Sc-C bond, Sc atom is donating a major charge toward the C atom.

Figure 5. Charge distribution and bonding displayed by the HV-ball. (a) The orientation of HV-ball is chosen to facilitate the view of the cut plane (gray color). A deeper observation revealed shaded atoms that are located behind the plane. (b) The cutting plane contains a complete C5 ring and one C atom of neighbouring C5 ring, but also Sc and H atoms are contained. It is possible to study in this image, Sc-C, and C-C bonds. (c) The bottom left region of the ELF plot has a Sc-C bond, displaying a blue intense color which indicates that no electrons are localized between Sc and C atoms (ionic bond). In contrast in the ring, the red regions indicate that C atoms are forming covalent bonds. (d) ADCH plot confirms that C atoms are sharing negative charge in the C5 ring and the contour lines are not surrounded the Sc atoms, indicating the separation of charge in the Sc-C bonds. ACS Paragon Plus Environment

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Figure 6. Charge distribution and bonding displayed by Sc-H and H-H bonds. (a) The cutting plane is indicated in gray color. (b) The cutting plane contains a Sc atom linked to both one H2 molecule and one H atom. (c) The ELF plot characterizes the Sc-H bond as ionic, this bond is shorter than the Sc-H2 one. (d) It is interesting to observe that the charge in the H2 molecule is polarized by linking the Sc atom, the positive charge seems to be localized in one of both H atoms, which might indicate that H-H distance is increased with respect to the isolated H2 molecule.


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Figure 7. Calculated IR (lower panel), and Raman (upper panel) spectra of the Sc20C60H70 cluster. The calculated intensities are represented as colored sticks and under the black curves. Both spectra feature an intense peak centred -1 at 1382/1391 cm attributed to the stretching of Sc-H bonds. IR spectrum has intense vibrations in the low -1 frequencies range (400-750 cm ), while Raman spectrum features more intense peaks at high frequencies range -1 -1 (3000-4000 cm ). A gaussian broadening of 20 cm was used. A comparison with the parent volleyballene is shown in the Figure S3.


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Figure 8. Various distinctive normal modes of the Sc20C60H70 cluster are shown. Displacement vectors are represented as arrows and their length indicates the magnitude of the movement. As expected the low frequency range is dominated by bending movements of hydrogen atoms mainly. Stretching Sc-H modes are located around -1 1350 cm being the more intense peaks in both Raman and IR spectra.


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ASSOCIATED CONTENT Supporting Information. Table of tilt angles between pentagonal rings and triangular faces in the Sc20C60H70 cluster (or HV-ball) ; Comparison of vibrational spectra for V-ball(Sc20C60); Comparison of IR and Raman spectra of V-ball against HV-ball. AUTHOR INFORMATION Corresponding Author Alfredo Tlahuice-Flores. e-mail: [email protected] Universidad Autónoma de Nuevo León, CICFIM-Facultad de Ciencias Físico-Matemáticas, San Nicolás de los Garza, NL 66450, México ACKNOWLEDGMENT The author thankfully acknowledges the computer resources, technical expertise and support provided by the Laboratorio Nacional de Supercómputo del Sureste de México, CONACyT network of national laboratories. ABBREVIATIONS HV-ball, hydrogenated volleyballene (or the Sc20C60H70 cluster); V-ball, Volleyballene (Sc20C60 cluster. REFERENCES

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Kroto, H. W.; Heath, J. R.; O’Brien, Curl, R. F.; Smalley, R. E. Nature, 1985, 318, 162-163. Wang, C. R.; Kai, T.; Tomiyama, T.; Yoshida, T.; Kobayashi, Y.; Nishibori, E.; Takata, M.; Sakata, M.;Shinohara, H. A Scandium Carbide Endohedral Metallofullerene: (Sc2C2)@C84. Angew. Chem.-Int. Ed. 2001, 40, 397. Zhao, Y. F.; Kim, Y. H.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Hydrogen Storage in Novel Organometallic Buckyballs. Phys Rev Lett, 2005, 94, 155504-1–155504-4.


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Weck, P. F.; Dhilip Kumar, T. J. Computational study of hydrogen storage in organometallic compounds. J. Chem. Phys. 2007, 126, 094703. Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of hydrogen in single-walled carbon nanotubes. Nature, 1997, 386, 377–379. Iñiguez, J.; Zhou, W.; Yildirim, T. Vibrational properties of TiHn complexes adsorbed on carbon nanostructures. Chem. Phys. Lett., 2007, 444, 140–144. Wang, J.; Ma, H. M.; Liu, Y. Sc20C60: a volleyballene. Nanoscale, 2016, 8, 1144111444. Tlahuice-Flores, A. New Insight into the structure of the C60Sc20 clsuter: bonding, vibrational and optical properties. Phys. Chem. Chem. Phys. 2016, 18, 12434-12437. Tlahuice-Flores, A. et al. On the structure and normal modes of hydrogenated TiFullerene compounds. J. Nanopart. Res. 2012, 14, 1065- 1075. Sun, Q.; Wang, Q.; Jena, P.; Kawazoe, Y. Clustering on Ti on a C60 surface and Its Effect on Hydrogen Storage. J. Am Chem. Soc. 2005, 127, 14582-14583. Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Pure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. Fullerenes with metals inside. J. Phys. Chem. 1991, 95, 7564-7568. Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. S.; Whetten, R. L. La2C80: A soluble Dimetoallofullerene. J. Phys. Chem, 1991, 95, 10561-10563. Alapati, S. V.; Jhonson, S. K.; Sholl, D. S. Identification of Destabilized Metal Hydrides for Hydrogen Storage Using First Principles Calculations. J. Phys, Chem B. 2006, 110, 8769-8776. Shiraishi, M.; Takenobu, T.; Ata, M. Gas solid interactions in the hydrogen singlewalled carbon nanotube system. Chem. Phys. Lett. 2003, 367, 633-636. Oku, T.; Kuno, M.; Narita, I. Hydrogen storage in boron nitride nanomaterials studied by TG/DTA and cluster calculation. J. Phys Chem Solid, 2004, 65, 549-552. Suh, M. P.; Park, H. J.; Prasad, T. K., Lim, D. W. Hydrogen storage in Metal-Organic Frameworks. Chemical reviews, 2011, 112(2), 782-835. Han, S. S.; Mendoza-Cortés, J. L.; Goddard, W. A. Recent advances on simulation and theory of hydrogen storage in metal-organic frameworks and covalent organic frameworks. Chem. Soc. Rev. 2009, 38, 1460-1476. Durgun, E.; Ciraci, S.; Zhou, W.; Yildirim, T. Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage media. Phys Rev Lett, 2006, 97, 226102-1-4. Phillips, A. B.; Shrivaram, B. S. High capacity hydrogen absorption in transitionmetal ethylene complexes: consequences of nanoclustering. Nanotechnol, 2009, 20, 204020-3. Ma, L. J.; Jia, J.; Wu, H. S.; Ren, Y. Ti-η2-(C2H2) and HCC-TiH as High capacity hydrogen storage media. Int. J. Hydrogen Energy. 2013, 38, 16185-92. Cho, H.-G.; Andrews, L. Infrared Spectra of M-η2-C2H2, HM-C≡CM- Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Acetylene. J. Phys. Chem. A. 2012, 116, 10917-10926. Neese, F. The ORCA program system. WIREs Comput Mol Sci, 2012, 2, 73-78. Perdew, P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.


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Schaefer, A.; Horn, H.; Ahlrichs, R. Fully optimized contracted Gaussian basis set for atoms Li to Kr. J. Chem. Phys.,1992, 97, 2571. Weigend, F.; Ahlrichs, R. Balanced basis set of split valence, triple zeta and cuadruple zeta valence quality for H to Rn: Design and assessment of accuracity. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305. Dolg, M.; Stoll, H.; Preuss, H. Energy-adjusted abinitio pseudopotential for the rare earth elements J. Chem. Phys. 1989, 90, 1730-1734. Guo, C.; Wang, C. Remarkable hydrogen storage on Sc2B42+ cluster: A computational Study. Vacuum. 2018, 149, 134-139. Becke, A. D. Edgecombe, K. E. A simpre measure of electron localization in atomic and molecular systems. J. Chem. Phys. 1990, 92, 5397-5403. Lu, T.; Chen, F. Multiwfn: A Mutifunctional Wavefunction Analizer. J. Com. Chem. 2012, 33, 580-592. Lu, T.; Chen, F. Atomic Dipole Moment Corrected Hirshfeld Population Method. J. Theor. Comput. Chem. 2012, 11, 163-183. Kubas, G. J. Fundamental of H2 Binding and Reactivity on Transistion Metals Underlying Hydrogenase Function and H2 Production and Storage. Chem. Rev, 2007, 107, 4152-4205. Kubas, G. J. Metal Dihydrogen and σ-bond Complexes: Structure, Theory and Reactivity. Klumer Academic/Plenum, New York, 2001.

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Figure 1. Structure of the Sc20C60H70 cluster, anti-clockwise from top right: six Sc2C10H12 structures are located on each faces of an internal Sc8 cube displayed in green color. The top left structure corresponds with the Th-symmetry hydrogenated Volleyballene before the optimization stage. After the optimization stage the structure has two less hydrogen atoms suffering of a strong distortion. The obtained Sc20C60H70 cluster holds a C1 symmetry and it can be described as a distorted icosahedron (bottom right representation), where eight Sc atoms (red balls) are located at the centre of triangular faces (blue planes) while C5 rings (grey balls) are located over each of the twelve faces of the Sc20 icosahedron (green planes). It is evident the distinctive angles comprised between pentagonal and triangular planes after the hydrogenation (ranging from 22 to 57 degrees). 74x70mm (300 x 300 DPI)



Bond lengths displayed by the Volleyballene (Lower panel), and the Sc20C60H70 cluster (Upper panel). The induced distortion after the hydrogenation is evident in C-C, Sc-C, and Sc-Sc bonds which can be attributed to the angle formed by the plane passing through pentagonal rings and the triangular faces of volleyballene. The Sc20C60H70 cluster features large Sc-Sc bonds and only six are circa 3.2-3.3. The Hydrogenation occurs preferentially in the Sc atoms but also in pentagonal rings (twelve C-H bonds). Sc-H bonds (green color) are distributed as two groups, corresponding to short bonds between Sc and H atoms, while large bonds are found between H2 molecules and Sc atoms. Sc-C and Sc-H bonds are found circa 2.12 in the Sc20C60H70 cluster. Interestingly, Sc-C bonds are short in the Sc20C60H70 cluster allowing a strong interaction. Dashed lines are used to indicate bond distortions after hydrogenation reaction. 89x101mm (300 x 300 DPI)


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Charge distribution on HV-ball. It is considered the atomic dipole moment corrected Hirshfeld Population. ADCH charges are given in the y-axis, while x-axis contains atoms indexes. The three curves represent charge distribution on Sc, C and H atoms. It is evident that Sc atoms transfer major charge toward C atoms. C atoms located at bridge positions among C5 rings holds positive charge values. In contrast, H atoms feature negative charge in Sc-H bonds (from -0.15 to -0.24 a. u.) and positive charge when they are forming C-H bonds (from 0.05 to 0.17 a. u.). H atoms of H2 molecules holds small positive charges (from 0.00 to 0.12 a. u.). 59x44mm (300 x 300 DPI)



Figure 4. Charge distribution and bonding displayed by Sc-C and C-C bonds comprising the HV-ball. (a) The HV-ball is intersected by a cutting plane (gray color) and all atoms located behind the plane are shaded. (b) In the cutting plane are located one C-C and three Sc-C bonds; On the bottom left region, a Sc atom is linked to two C atoms (2.42 and 2.44 , respectively); in the right region, the shortest Sc-C bond (2.12 ) is located, and the C atom at the same time is forming a C-C bond. (c) The ELF plot attests the covalent character of the C-C bond located at the bottom left region. The Sc-C bond located at the bottom left region is slightly covalent (orange region between Sc and C atoms). Moreover, the shortest Sc-C bond has an enhanced covalent character (large intermediate region with a large ELF value). (d) The ADCH charge distribution plot shows that positive charge is sustained by Sc atoms, while negative charge is located at C atoms attesting the donator role of Sc atoms. Contour lines are surrounding the C-C bonds indicating the same shared negative charge and in case of Sc-C bonds the metal is releasing charge to C atoms. Clearly, on the shortest Sc-C bond, Sc atom is donating a major charge toward the C atom. 80x77mm (300 x 300 DPI)


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Figure 5. Charge distribution and bonding displayed by the HV-ball. (a) The orientation of HV-ball is chosen to facilitate the view of the cut plane (gray color). A deeper observation revealed shaded atoms that are located behind the plane. (b) The cutting plane contains a complete C5 ring and one C atom of neighbouring C5 ring, but also Sc and H atoms are contained. It is possible to study in this image, Sc-C, and C-C bonds. (c) The bottom left region of the ELF plot has a Sc-C bond, displaying a blue intense color which indicates that no electrons are localized between Sc and C atoms (ionic bond). In contrast in the ring, the red regions indicate that C atoms are forming covalent bonds. (d) ADCH plot confirms that C atoms are sharing negative charge in the C5 ring and the contour lines are not surrounded the Sc atoms, indicating the separation of charge in the Sc-C bonds. 80x77mm (300 x 300 DPI)



Figure 6. Charge distribution and bonding displayed by Sc-H and H-H bonds. (a) The cutting plane is indicated in gray color. (b) The cutting plane contains a Sc atom linked to both one H2 molecule and one H atom. (c) The ELF plot characterizes the Sc-H bond as ionic, this bond is shorter than the Sc-H2 one. (d) It is interesting to observe that the charge in the H2 molecule is polarized by linking the Sc atom, the positive charge seems to be localized in one of both H atoms, which might indicate that H-H distance is increased with respect to the isolated H2 molecule. 80x77mm (300 x 300 DPI)


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Figure 7. Calculated IR (lower panel), and Raman (upper panel) spectra of the Sc20C60H70 cluster. The calculated intensities are represented as colored sticks and under the black curves. Both spectra feature an intense peak centred at 1382/1391 cm-1 attributed to the stretching of Sc-H bonds. IR spectrum has intense vibrations in the low frequencies range (400-750 cm-1), while Raman spectrum features more intense peaks at high frequencies range (3000-4000 cm-1). A gaussian broadening of 20 cm-1 was used. A comparison with the parent volleyballene is shown in the Figure S3. 109x151mm (300 x 300 DPI)



Figure 8. Various distinctive normal modes of the Sc20C60H70 cluster are shown. Displacement vectors are represented as arrows and their length indicates the magnitude of the movement. As expected the low frequency range is dominated by bending movements of hydrogen atoms mainly. Stretching Sc-H modes are located around 1350 cm-1 being the more intense peaks in both Raman and IR spectra. 169x361mm (300 x 300 DPI)


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Hydrogen Storage on Volleyballene: The Prediction of the

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