Theoretical Study of Tetrahydrofuran-Stabilized Al13 Superatom

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A Theoretical Study of Tetrahydrofuran-Stabilized Al Superatom Cluster Jing Chen, Zhixun Luo, and Jiannian Yao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02958 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 23, 2016

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The Journal of Physical Chemistry

A Theoretical Study of Tetrahydrofuran-Stabilized Al13 Superatom Cluster Jing Chen1,2, Zhixun Luo*,1 and Jiannian Yao*,1,2 1. State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China 2. Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China E-mail: [email protected]; [email protected]; Tel: +86-010-62553453

Abstract We present here an in-depth study upon the interaction between a neutral cluster Al13 and a typical ligand tetrahydrofuran (THF) via density functional theory (DFT) calculation. It is found that electron delocalization over the framework of Al13 and THF facilitates ligand-to-Al13 charge transfer leading to enhanced stability for the superhalogen cluster Al13. Further study on the stabilization of Al13(THF)n cluster complexes with n = 1 – 8 reveals that the adsorption of more THF ligands gradually enhances the total binding energy and the total electronic charge transfer from the ligand to Al13. The bonding nature and stabilization of Al13(THF)n cluster are then demonstrated by rationalizing the interactions between superatomic and molecular orbitals of Al13 and THF respectively.

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1.

Introduction As the most abundant metal in the Earth’s crust, aluminum is remarkable for its

low density and ability to resist corrosion due to surface passivation, serving as vital component materials in aerospace industry, transportation, and building industry. Nanoscale aluminum structures have been found being able to expand the metal’s performance and applications in such fields as superconducting spintronics,1-6 plasmonics,7-10 optics,11-13 and so forth. Obtaining stable clusters of aluminum is critical to tune its performance at the atomic-precise level. For this purpose, stable metalloid Al clusters have been successfully synthesized by solution methods with protective ligands such as C5Me5,14-16 N(Me3Si)2,17-18 and tetrahydrofuran,19-20 etc.21 The introduction of such ligands can tune the valence electron count of Al clusters preventing the etching of the metallic cores,16, 22-23 hence profiting to the stability of monolayer-protected clusters (MPCs)24. Besides the electronic shell interpretation,25-27 effective overlaps of cluster and molecular orbitals considerably contribute to the MPCs stabilization,28 as also proposed by a recent interesting study on the Al13 cluster and ligand N-ethyl-2-pyrrolidone.29 Further insights into the electronic shell structures of Al clusters and the cluster-ligand interactions will provide clues for the synthesis of ligand-protected Al clusters. Extensive investigations have established that MPCs on a basis of superatom clusters bear reasonable stabilities and unique properties.30-36 The discovery of superatoms is retrospect to nearly thirty years ago when Al13- was found inert in 2


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reacting with oxygen in the gas phase.37 The 13-atom Al cluster that possesses superatomic property has a close-packed icosahedral geometry and a high electron affinity up to 3.6 eV.25, 38-40 Researchers expect to synthesize such stable clusters for materials by the assumption of supplying one additional electron to the open-shell Al13 simply,29,

37, 41

but challenge remains. Recently, we have attained an insight

regarding to the factors that determine if a ligand activates or passivates a superatom cluster.42 It has been concluded that, the stability of small metal clusters is maximized when: i) the cluster bears a closed electron shell and possesses a large HOMO-LUMO gap;43 and ii) the charge density on the edge atoms of the cluster is evenly distributed thus preventing the presence of active sites and providing matched orbitals for the ligand-cluster interactions.38 In view of this, it is vital to seek for a proper ligand that not only contributes to the electronic shell but also bears reasonable interactions matching the cluster and molecular orbitals. Tetrahydrofuran (THF), a small organic molecule and typical solvent for chemical synthesis,19-20, 44 causes our attention in this study. We systematically studied the interactions between Al13 cluster and THF molecules and the stability of Al13(THF)n where n equals to 1 to 8 by density functional theory (DFT) calculations. Overlaps between certain molecular orbitals of THF and superatomic orbitals of Al13 were found essential in stabilizing the cluster. In addition, the delocalization of electrons and the THF-to-Al13 charge transfer through the Al-O bonds do important contributions. We also discussed the effect of adsorption sites and the number of ligands on the stability of an Al cluster. 3



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2.

Calculation methods All calculations are performed using the Gaussian 09 package. Local

lowest-energy structures are optimized with charge of zero and low spin multiplicity of 2. To ensure ground states, we have checked and compared with high-spin structures (with spin multiplicity = 4) for the Al13(THF)n complexes with n = 1-8. We used the unrestricted PBE0/6-311G(d) level of theory considering likely weak interactions at the adsorption of multiple THF molecules, which was previously found appropriate for such MPCs.

29, 45-49

Vibrational activities were checked on all

optimized structures to ensure no imaginary frequencies for the minima. The total binding energies were calculated by subtracting the total energy of Al13 and n THFs from Al13(THF)n (n = 1 – 8). To estimate the amount of charge transfer between ligands and Al13 cluster in Al13(THF)n, natural bond orbital (NBO) analysis was performed. The charge distribution on the Al13 moiety is represented by ∆Q. The NBO orbitals were mapped via VMD50 and Multiwfn software51. The negative values of ∆Q indicate an increase of electronic charge on Al13 cluster, that is, a charge transfer from THF to Al13 on the adsorption of THF. In this work, we only considered the oxygen atom binding Al13(THF)n compounds. All the initial structures are configured with the oxygen atom of THF pointed toward an Al atom of the Al13 cluster.

3.

Results and discussion The optimized structures and fully displayed molecular orbitals (MOs) of the

superatomic Al13 are given in Figure 1. The removal of one electron from the magic 4


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40e cluster (Al13-) leads to a slight distortion of the icosahedral structure and minor changes of electron occupation (A comparison is provided in Figure S1, ESI). The neutral Al13 cluster is recognized to have an electron configuration as (1S)2(1P)6(1D)10(2S)2(1F)14(2P)5 according to the near-free electron gas (NFEG) model.

25, 29, 52-54

However, there is a small academic debate regarding the occupied

molecular orbitals when taking into consideration of the energy level crossing. Watanabe et al.29 assigned the single occupied molecular orbital (SOMO) of neutral Al13 as one of 1F superatomic orbitals,29 but according to our calculation, the SOMO of Al13 is 2PZ orbitals due to the complex energy-level crossing between the subshells 2P and 1F. As demonstrated by the energy diagram46 and shapes of superatomic orbitals, the 2P subshell splits into three orbitals (2Px, 2Py and 2Pz), and 1F subshell orbitals split into two groups: three orbitals above the 2Px/2Py and four orbitals below 2Px/2Py (Figure 1). This clarification reinforces the established superhalogen characteristics of the Al13 cluster by having a 2P SOMO.55 After an in-depth cognition on the Al13 geometry and electronic structure, we then emphasized on the structure stability of Al13 ligated by different number of THF molecules. Because the three isomers (3, 1, 12) of ground-state Al13(THF)1 share equivalent structure and stability (Figure S2, ESI), we thereafter considered only the multiple-molecules THFs-ligated Al13 complexes having the first THF adsorbing on the site Al(3). Figure 2 gives the optimized ground-state structures of Al13(THF)n compounds for n = 1 – 8 on the basis of isomer 3. A comparison of their total energies with that at high spin states is summarized in Table S1 (Supporting Information). For 5



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Al13(THF)n (n ≥ 2), the most stable structures also exhibit common characteristics by having at least two THF molecules at the meta-position to each other. Regarding to geometry evolution, the bond length of Al-O bond at No. 3 atom becomes shorter for Al13(THF)2-5 than that in Al13(THF)1 (Figure 2a-e); but when the number of THF molecules is beyond five, the bond lengths of all Al-O bonds become longer than 2.000 Å (Figure 2f-h). This evolution indicates that the addition of THF molecules at meta-sites of No. 3 atom cooperatively enhances the strength of the Al(3)-O bond, but the adsorption at ortho-site (e.g., No. 1 atom) leads to a longer Al-O bond probably owing to the steric repulsion between THF molecules at ortho-position. Other than the Al(3)-O bond, the strength of other Al-O bonds decreases with the increasing number of ligands. To show the evolution of the geometry of Al13 core, we checked the bond length of Al(3)-Al(1) in all Al13(THF)n and found it becomes longer in cases of more THF molecules (for n = 1-4, Figure 2a-d); nevertheless, the framework of Al13 maintains in all Al13(THF)n compounds. Other isomers of Al13(THF)n (n = 1-5, respectively) are given in Supporting Information (Figure S2-S9). To demonstrate how the THF adsorption influences the stabilization of Al13, we plotted the curves of total binding energy (BE) and quantity of charge transfer (∆Q) against the number of THF molecules (Figure 3). In these curves, we used the ratio of each value (e.g., BE or ∆Q) to that having the largest absolute value in the series to demonstrate the different variation tendency. When the number of ligands increases, both the values of BE and ∆Q arise but at a decreasing rate, indicating that each ligand contributes positively to the stabilization but the interactions between an 6


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individual ligand and the Al cluster keep weakening (Figure 3a). In the curves of the difference of binding energy ∆(BE) and difference of charge transfer ∆(∆Q) against the number of ligands, the addition of a THF molecule on Al13(THF)n leads to a smaller ∆(BE) and ∆(∆Q) than that in Al13(THF)n-1, indicating weakened interaction between THF and Al cluster in the Al13(THF)n (Figure 3b). The finding of weakened interaction between individual THF and Al cluster but enhanced stabilization effect suggests that THF molecule is a proper protective ligand for Al13 cluster. The energies of SOMOs of Al13(THF)n also arise with the adsorption of more ligands gradually (Figure 3a). It is worth noting in Figure 3B that, when two and six THF molecules are adsorbed on the Al cluster, the differences of energy of SOMO, ∆(SOMO), are larger than that of their formers (n = 1 and 5, respectively). The irregularity at n = 2 and 6 of ∆(SOMO), and also irregular behavior at n=8 and 7 for ∆(∆Q) and ∆(BE) likely relate to the specific symmetry of these THF-ligated Al13 clusters, synergy effects due to particular active sites occupation by THF, and the relating steric repulsion between THF molecules.29, 42 Moreover, an area between curves of BE and ∆Q against number of ligands appears (the area between blue and yellow curves), and the curve of BE is always above that of charge transfer (∆Q), indicating that charge transfer is not the only factor that contributes to the cluster stabilization (Figure 3a). An area between ∆(BE) and ∆(∆Q) exists as well, further indicating the difference in change rate between the stability of system and charge transfer (Figure 3b). The original values for binding energy and charge transfer are given in Table 1. The negative value of ∆Q indicates 7



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that total charge transfer is from THF ligand to the Al core. More details for binding energy and charge transfer in other stable isomers are summarized in Table S2-S6 (Supporting Information). Moreover, charge distribution by natural population analysis (NPA) on THF moieties shows that each THF ligand in Al13(THF)n (n = 1-8) donates electrons to Al core (NBO charges distributed on each ligand for n = 1-4 are shown in Figure S2-4 and S6). Figure 4 shows the IR spectra of a free THF molecule and the most stable species of Al13(THF)n (n = 1-8). The C-H swing (at ~1065 cm-1) and THF-ring breathing (~890 cm-1) modes that are absent in IR spectrum of a free THF molecule show up in the IR spectra of Al13(THF)n (Figure 4a-4i). For Al13(THF)1, the appearance of (O) Al-Al stretching modes at ~447 cm-1 demonstrates the chemisorption nature of THF on Al13 (through Al-O bonds). When the number of THF molecules increases, the vibrational modes corresponding to THF-ring breathing (~890 cm-1) show up as broadened peaks, because THF molecules located in difference positions bring coupling vibrational relaxation. The vibrational modes corresponding to Al-Al(3)(O) show blue shift from 447 to 486 cm-1 (green arrows) as the number of ligands increases from one to five, indicating that the adding of another THF molecule can further strengthen the Al-Al(3)(O) bonds (Figure 4b-4f). However, when the number of ligands increases to six, the Al-Al stretching mode at the highest wavenumber (that involves simultaneously stretching of several Al-Al bonds) shifts to lower wavenumber (~440 cm-1) again (Figure 4e-4i), probably owing to the appearance of THF at the para-position (of each other). Moreover, the (O) Al-Al 8


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stretching mode splits to several peaks as the number of THFs increases, indicating that the adsorption of THF at different sites (Al atom) modifies the Al-Al(O) bonds to a different degree. The above analysis on binding energy, geometric symmetry, and infrared activity has revealed a charge transfer from THF to Al13 and also indicates decent interactions within the Al13(THF)n complexes. To further illustrate the interactions between the ligand THF and Al core, we plotted the SOMOs of the most stable Al13(THF)n complexes (n = 1-8) (Figure 5). First, the shape of SOMO of Al13(THF)n (n = 1-2) is very alike with that of Al13 cluster (Figure 5a-b and Figure 1, right). When the number gradually increases (n = 3-5), the SOMO shapes of THF-Al clusters differ from that of Al13 (Figure 5c-e), indicating that the adsorption of THF brings interference to the electronic properties/configuration. However, when the number of ligands increases (n = 6-8), the shapes of SOMOs of become similar with that of bare Al13 again (Figure 5f-h, Figure S3c, and Figure 1, right). In complexes Al13(THF)n (n = 6-8), two ligands are adsorbed at nearly opposite (or para) positions (No. 1 and No. 6 atoms) of each other, leading to minor modification of SOMOs of the Al core. Nevertheless, all of these SOMOs are dominantly donated by the superatomic orbitals (SOMOs) of Al core, as illustrated by the shapes of MOs where electron clouds distribute majorly on Al core (Figure 5), indicating that the interaction between SOMOs of Al cluster and MOs of THF is rather weak, and their overlaps barely contribute to the lower of system energy. These weak interactions can be explained by the energy diagram of frontier 9



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molecular orbitals (FMOs) of the two species, as shown in Figure 6A. Both the gap between LUMO of THF and SOMO of Al cluster (as large as 7.15 eV) and that between HOMO of THF and LUMO of Al cluster (2.69 eV) are too large to promote the effective electron transfer. In comparison, the gap between HOMO of THF and SOMO of Al cluster is 1.12 eV, which enables the chemical reaction through FMOs but still not enough for effective overlapping that stabilizes the cluster (Figure 6A). To locate the superatomic orbitals contributing to the stabilization of THF-ligated Al13 cluster, we searched the MOs having electron clouds distributed on the frameworks of both Al core and THF ligands.29 For Al13(THF)1, the MOs are constructed by the HOMO – 1 (MO 19) and HOMO – 9 (MO 11) of THF and the 1Px (MO 67) and 1S (MO 66) of Al13 (Figure 6B). Moreover, the stabilization energy derived from 1P superatomic orbital is larger than that from 1S (Figure 6B), which is quite different from EP-bonding Al13 cluster where the stabilization energies dominantly derive from 1S or 1D orbitals.29 Similarly, by checking the energy diagram and features of MOs, we recognized that the stability of Al13(THF)2 also originates from overlaps between superatomic orbitals (1S, 1P) of Al13 and MOs (19, 11) of THF (Figure 7, and Figure S10-11). In 3-12 and 3-1, five MOs (90, 97, 98, 100, and 101) have electrons distributed over frameworks of both Al13 and THF ligands, but in 3-11 only four MOs (90, 97, 100, and 101) show obvious electron delocalization. This difference is in coincidence with the low stability of 3-11 as compared with 3-12 and 3-1. In addition, the MOs providing effective overlaps in three isomers of Al13(THF)2 show different patterns, 10


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indicating that effective overlaps between superatomic orbitals of Al13 and MOs of THF molecules are also influenced by the aforementioned steric repulsion among ligands. It is reasonably inferred that the delocalization of electrons caused by overlaps between superatomic orbitals of Al core and MOs of ligands probably has facilitated the charge transfer between the Al core and THF ligands. To clarify the charge transfer, we have plotted the natural bond orbital (NBO) isosurfaces of THF-ligated Al13 clusters. Typically, the donor-acceptor overlaps are through Al-O bond in the Al13(THF)1-2 as illustrated in Figure 8. The donor-acceptor overlap is between anti-bonding lone pair of Al atom, LP*(Al), and the lone pair of O atom, LP(O). For the Al13(THF)1, the isosurfaces of donor and acceptor orbitals of overlap LP(O)LP*(Al) (~25.8 kcal/mol) are of the same signs (e.g., both positive), we tentatively define this type of donor-acceptor overlap as bonding-type (Figure 8a). Correspondingly, we define the donor-acceptor overlap where donor and acceptor orbitals have different signs as antibonding-type. To show the effect of adsorption sites, we analyzed the donor-acceptor interactions in three isomers of Al13(THF)2, where two THF molecules are at the ortho-, meta-, and para- position of each other respectively (Figure 8b-d and Figure S3, ESI). In isomer 3-1, two bonding-type donor-acceptor overlaps LP(O)LP*(Al) (27.6, and 27.8 kcal/mol) are observed (Figure 8b). In 3-12, one LP(O)LP*(Al) is of bonding-type (28.3 kcal/mol); but in the other LP(O)LP*(Al) interaction (23.3 kcal/mol), positive isosurface of LP(O) is overlapped with negative part of LP*(Al) (i.e., anti-bonding type) (Figure 8c). In 3-11, 11



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both two LP(O)LP* (Al) overlaps are of anti-bonding type (18.7 and 17.6 kcal/mol) (Figure 8d). Analogy to bonding and anti-bonding molecular orbitals, the bonding-type donor-acceptor overlap is probably more favorable for the charge transfer between moieties than the anti-bonding overlap. Briefly, the total donor-acceptor overlaps in 3-1 are more efficient than that in 3-12 and 3-11, which is consistent with the order of degree of charge transfer found in the calculation (Table S3, ESI). Interestingly, the appearance of two THF molecules at para-position of each other generally leads to low efficient charge transfer and very alike shapes of SOMOs, regardless of the number of ligands (e.g., structures 3-1, 3-1-11, 3-1-2-11, 3-12-6-10-1, 3-12-6-10-8-1, 3-12-6-10-8-4, and 3-12-6-10-1-8-4-9) (Figure S3f, Figure S5, Figure S7, Figure S8e, Figure S9g-h, and Table S3-S7). These structures are corresponding to the relative unstable isomers in each kind of Al13(THF)n, indicating that ligands-to-metal charge transfer is vital to the stabilization of the Al13 cluster. It is worth noting that, ligands-to-metal charge transfer within these Al13(THF)n could not simply induce a singlet HOMO towards 40e-magic Al13- core. Figure 9 presents the spin density isosurfaces of Al13(THF)n (n = 1-8) where, however, similar shapes pertaining to SOMO of Al13 were addressed (for the comparison of Al13 vs Al13- see Figure S1, Supporting Information), indicating that the electron(s) donated from THF ligands is likely not accommodated to the SOMO of Al13.29

4. Conclusions Using DFT calculations, we have studied the stabilization of Al13 cluster ligated by 12


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THF molecules. The stabilization originates from the overlaps between superatomic orbitals (1S and 1P) of Al13 and MOs of THF, which leads to the delocalization of electrons and thus electronic charge transfer from THFs to Al13. In addition, the steric repulsion and relative orientation of charge transfer influence the efficiency of stabilization. The bond length, binding energy, and charge transfer analysis show that when the number of ligands is beyond two, interactions between an individual THF molecule and Al13 are weakened by the addition of another ligand; but every ligand contributes positively to the stabilization.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx/xxxx (1) The structure and HOMO of neutral and anionic Al13 cluster (Figure S1); (2) comparison of energies of high spin and low spin state structures of Al13(THF)1-8 (Table S1); (3) the structures and IR spectra of Al13(THF)1 (Figure S2); (4) binding energy (BE) and charge distributed on Al13 cluster in Al13(THF)n (n = 1, 3, 4, 5-8) (Table S2-7); (5) all typical stable structures, charge distribution, and SOMOs of Al13(THF)2-8 (Figure S3-9); (5) isosurfaces of MOs of Al13(THF)2 (3-1 and 3-11) (Figure S10-11); (6) output coordinate for most stable structures of Al13(THF)1-8.

Acknowledgements This work is supported by Young Professionals Program in Institute of Chemistry, 13



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Chinese Academy of Sciences (Y3297B1261), and the National Basic Research Program of China (973) (Granted No. 2013CB933503). In addition, we thank the national Thousand Youth Talents Program and financial support from CAS project with Grant No. Y31M0112C1 and Y5294512C1.

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30. Malola, S.; Lehtovaara, L.; Enkovaara, J.; Häkkinen, H., Birth of the Localized Surface Plasmon Resonance in Monolayer-Protected Gold Nanoclusters. ACS Nano 2013, 7, 10263-10270. 31. Natarajan, G.; Mathew, A.; Negishi, Y.; Whetten, R. L.; Pradeep, T., A Unified Framework for Understanding the Structure and Modifications of Atomically Precise Monolayer Protected Gold Clusters. J. Phys. Chem. C 2015, 119, 27768-27785. 32. Woodward, W. H.; Blake, M. M.; Luo, Z. X.; Weiss, P. S.; Castleman, A. W. Jr., Soft-Landing Deposition of Al17- on a Hydroxyl-Terminated Self-Assembled Monolayer. J. Phys. Chem. C 2011, 115, 5373-5377. 33. Long, C. G.; Gilbertson, J. D.; Vijayaraghavan, G.; Stevenson, K. J.; Pursell, C. J.; Chandler, B. D., Kinetic Evaluation of Highly Active Supported Gold Catalysts Prepared from Monolayer-Protected Clusters: An Experimental Michaelis-Menten Approach for Determining the Oxygen Binding Constant during CO Oxidation Catalysis. J. Am. Chem. Soc. 2008, 130, 10103-10115. 34. Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D., Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Angstrom Resolution. Science 2007, 318, 430-433. 35. Hickey, N.; Larochette, P. A.; Gentilini, C.; Sordelli, L.; Olivi, L.; Polizzi, S.; Montini, T.; Fornasiero, P.; Pasquato, L.; Graziani, M., Monolayer Protected Gold Nanoparticles on Ceria for an Efficient CO Oxidation Catalyst. Chem. Mater. 2007, 19, 650-651. 36. Pohjolainen, E.; Hakkinen, H.; Clayborne, A., The Role of the Anchor Atom in 18


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the Ligand of the Monolayer-Protected Au-25(XR)18- Nanocluster. J. Phys. Chem. C 2015, 119, 9587-9594. 37. Luo, Z. X.; Castleman, A. W., Jr., Special and General Superatoms. Acc. Chem. Res. 2014, 47, 2931-2940. 38. Neumaier, M.; Olzmann, M.; Kiran, B.; Bowen, K. H.; Eichhorn, B.; Stokes, S. T.; Buonaugurio, A.; Burgert, R.; Schnöckel, H., The Reaction Rates of O2 with Closed-Shell and Open-Shell Alx– and Gax– Clusters under Single-Collision Conditions: Experimental and Theoretical Investigations toward a Generally Valid Model for the Hindered Reactions of O2 with Metal Atom Clusters. J. Am. Chem. Soc. 2014, 136, 3607-3616. 39. Burgert, R.; Schnöckel, H.; Grubisic, A.; Li, X.; Stokes, S. T.; Bowen, K. H.; Ganteför, G. F.; Kiran, B.; Jena, P., Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2. Science 2008, 319, 438-442. 40. Reber, A. C.; Khanna, S. N.; Roach, P. J.; Woodward, W. H.; Castleman, A. W., Spin Accommodation and Reactivity of Aluminum Based Clusters with O2. J. Am. Chem. Soc. 2007, 129, 16098-16101. 41. Luo, Z. X.; Smith, J. C.; Berkdemir, C.; Castleman, A. W., Jr., Gas-Phase Reactivity of Aluminum Cluster Anions with Ethanethiol: Carbon-Sulfur Bond Activation. Chem. Phys. Lett. 2013, 590, 63-68. 42. Luo, Z.; Reber, A. C.; Jia, M.; Blades, W. H.; Khanna, S. N.; Castleman, A. W., Jr., What Determines If a Ligand Activates or Passivates a Superatom Cluster? Chem. Sci. 2016, 7, 3067-3074. 19



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43. Luo, Z.; Grover, C. J.; Reber, A. C.; Khanna, S. N.; Castleman, A. W., Jr., Probing the Magic Numbers of Aluminum–Magnesium Cluster Anions and Their Reactivity toward Oxygen. J. Am. Chem. Soc. 2013, 135, 4307-4313. 44. Duan, T.; Baum, E.; Burgert, R.; Schnöckel, H., The Polyhedral Gallium Subhalide [Ga24Br22]⋅10 THF: The First Step on the Path to a New Modification of Gallium? Angew. Chem. Int. Ed. 2004, 43, 3190-3192. 45. Aguado, A.; López, J. M., Structures and Stabilities of Aln+, Aln, and Aln−(N=13– 34) Clusters. J. Chem. Phys. 2009, 130, 064704. 46. Smith, Q. A.; Gordon, M. S., Electron Affinity of Al13: A Correlated Electronic Structure Study. J. Phys. Chem. A 2011, 115, 899-903. 47. Huong, V. T. T.; Tai, T. B.; Nguyen, M. T., A Theoretical Study on Charge Transport of Dithiolene Nickel Complexes. Phys. Chem. Chem. Phys. 2016, 18, 6259-6267. 48. Adamo, C.; Barone, V., Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158-6170. 49. Fernando, A.; Weerawardene, K. L. D. M.; Karimova, N. V.; Aikens, C. M., Quantum Mechanical Studies of Large Metal, Metal Oxide, and Metal Chalcogenide Nanoparticles and Clusters. Chem. Rev. 2015, 115, 6112-6216. 50. Humphrey, W., Dalke, A. and Schulten, K., VMD-Visual Molecular Dynamics. J. Molec. Graphics 1996, 14, 33-38. 51. Lu, T.; Chen, F., Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580-592. 20


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52. Abreu, M. B.; Powell, C.; Reber, A. C.; Khanna, S. N., Ligand-Induced Active Sites: Reactivity of Iodine-Protected Aluminum Superatoms with Methanol. J. Am. Chem. Soc. 2012, 134, 20507-20512. 53. Han, Y. K.; Jung, J., Does The "Superatom" Exist in Halogenated Aluminum Clusters? J. Am. Chem. Soc. 2008, 130, 2-3. 54. Zhao, J. Y.; Zhao, F. Q.; Gao, H. X.; Ju, X. H., DFT Studies of the Adsorption and Dissociation of H2O on the Al13 Cluster: Origins of This Reactivity and the Mechanism for H2 Release. J. Mol. Model. 2013, 19, 1789-1799. 55. Bergeron, D. E.; Roach, P. J.; Castleman, A. W., Jr.; Jones, N.; Khanna, S. N., Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts. Science 2005, 307, 231-235.

Figures and Tables

21



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Figure 1. Energy diagram and 1S – 2P superatomic orbitals (MOs) of the superatom cluster Al13, spin up (left) and spin down (right).

22


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Figure 2. Optimized structures of the most stable isomers of Al13(THF)n complexes, n = 1 – 8 for (a)-(h), respectively. Black numbers represent the bond lengths (in Å). White numbers indicate the No. of atom. Red, green and white atoms refer to O, C, and H, respectively.

23



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Figure 3. The variation tendency of binding energy (BE, blue), natural charge on Al13 moieties (∆Q, yellow), and energy of single occupied molecular orbitals (SOMO, grey) (a) and the difference of BE, ∆Q and SOMO (b) of the most stable Al13(THF)n against the number (n) of THF molecule (n = 0 – 8). In (b), the difference of binding energy ∆(BE), difference of charge transfer ∆(∆Q), and difference of SOMO energy ∆(SOMO) are defined by the difference of BE, ∆Q, SOMO of Al13(THF)n with respect to that of Al13(THF)n-1, respectively. In Figure 3, all these values are given by the ratio of each value (e.g., BE or ∆Q) to that having the largest absolute value within its series.

24


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Figure 4. IR spectra of (a)THF and the most stable isomers (b) 3, (c) 3-12, (d) 3-12-6, (e) 3-12-6-10, (f) 3-12-6-10-8, (g) 3-12-6-10-8-1, (h) 3-12-6-10-8-1-4, and (i) 3-12-6-10-8-1-4-9 of Al13(THF)n (n = 1 – 8).

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Table 1. Binding energies (BE) and charge transfer in Al13(THF)n (n = 1 – 8) structures

Al13(TH

Al13(T

Al13(TH

Al13(TH

Al13(TH

Al13(TH

Al13(TH

Al13(TH

F)1

HF)2

F)3

F)4

F)5

F)6

F)7

F)8

BE (eV)

-1.07

-1.64

-2.14

-2.50

-2.77

-2.93

-3.04

-3.22

∆(BE)(eV)

1.07

0.57

0.50

0.36

0.27

0.16

0.11

0.18

∆Q (e)

-0.149

-0.271

-0.388

-0.493

-0.592

-0.666

-0.749

-0.770

∆(∆Q)

0.149

0.122

0.117

0.105

0.099

0.074

0.083

0.021

Note: The values of ∆(BE) and ∆(∆Q) refer to the difference of binding energy and difference of charge transfer in Al13(THF)n with respect to that in Al13(THF)n-1, respectively. Negative values in ∆Q indicate that electronic charge is transferred from THF to Al13.

Figure 5. SOMOs of Al13(THF)n complexes, n = 1-8 for (a)-(h), respectively. Red, 26


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green, and white atoms refer to O, C, and H, respectively.

Figure 6. (A) Energy diagram of frontier molecular orbitals of THF and Al13. The solid (blue) and dash (red) lines represent HOMO and LUMOs, respectively. The numbers (in eV, black) noted the gap between HOMOs and LUMOs. (B) Energy diagram of Al13, Al13(THF)1 and THF, respectively. Molecular orbitals (MOs) constructed by superatomic orbitals (1S, 1P) of Al13 in Al13(THF)1. The atoms in red, green, and white refer to O, C, and H, respectively. MO 78 is from 1S of Al13, MOs 82 and 85 from 1Px of Al13. Because the shapes of alpha and beta orbitals of MO 78, 82, and 85 are very alike, we give only the alpha-spin orbitals.

27



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Figure 7. MOs constructed by 1S (a-b) and 1P (c-j) superatomic orbitals of Al13 in Al13(THF)2 (3-12). Red, green, and white atoms refer to O, C, and H, respectively. (a) – (b) MO 90, (c) – (d) MO 97, (e) – (f) MO 98, (g) – (h) MO 100, (i) – (j) MO 101.

Figure 8. Natural bond orbital (NBO) donor-acceptor overlaps in (a) Al13(THF)1 (3) and (b) – (d) Al13(THF)2: (b) 3-11, (c) 3-12, and (d) 3-1 complexes. Red, green, and white atoms refer to O, C, and H, respectively. Positive and negative isosurfaces of donor are represented by green and blue, positive and negative isosurfaces of acceptor are yellow and purple, respectively. 28


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Figure 9. Spin density isosurfaces of Al13(THF)n. (a)-(h) is for n = 1-8, respectively.

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Table of Content Graphic

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Figure 1. Energy diagram and 1S – 2P superatomic orbitals (MOs) of the superatom cluster Al13 spin up (left) and spin down (right). 367x305mm (192 x 192 DPI)



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Figure 2. Optimized structures of the most stable isomers of Al13(THF)n complexes, n = 1 – 8 for (a)-(h), respectively. Black numbers represent the bond lengths (in Å). White numbers indicate the No. of atom. Red, green and white atoms refer to O, C, and H, respectively. 284x512mm (192 x 192 DPI)


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Figure 3. The variation tendency of binding energy (BE, blue), natural charge on Al13 moieties (∆Q, yellow), and energy of single occupied molecular orbitals (SOMO, grey) (a) and the difference of BE, ∆Q and SOMO (b) of the most stable Al13(THF)n against the number (n) of THF molecule (n = 0 – 8). In (b), the difference of binding energy ∆(BE), difference of charge transfer ∆(∆Q), and difference of SOMO energy ∆(SOMO) are defined by the difference of BE, ∆Q, SOMO of Al13(THF)n with respect to that of Al13(THF)n-1, respectively. In Figure 3, all these values are given by the ratio of each value (e.g., BE or ∆Q) to that having the largest absolute value within its series. 213x257mm (96 x 96 DPI)



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Figure 4. IR spectra of (a)THF and the most stable isomers (b) 3, (c) 3-12, (d) 3-12-6, (e) 3-12-6-10, (f) 3-12-6-10-8, (g) 3-12-6-10-8-1, (h) 3-12-6-10-8-1-4, and (i) 3-12-6-10-8-1-4-9 of Al13(THF)n (n = 1 – 8). 174x256mm (192 x 192 DPI)


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Figure 5. SOMOs of Al13(THF)n complexes, n = 1-8 for (a)-(h), respectively. Red, green, and white atoms refer to O, C, and H, respectively. 246x445mm (192 x 192 DPI)



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Figure 6. (A) Energy diagram of frontier molecular orbitals of THF and Al13. The solid (blue) and dash (red) lines represent HOMO and LUMOs, respectively. The numbers (in eV, black) noted the gap between HOMOs and LUMOs. (B) Energy diagram of Al13, Al13(THF)1 and THF, respectively. Molecular orbitals (MOs) constructed by superatomic orbitals (1S, 1P) of Al13 in Al13(THF)1. The atoms in red, green, and white refer to O, C, and H, respectively. MO 78 is from 1S of Al13, MOs 82 and 85 from 1Px of Al13. Because the shapes of alpha and beta orbitals of MO 78, 82, and 85 are very alike, we give only the alpha-spin orbitals. 341x186mm (192 x 192 DPI)


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Figure 7. MOs constructed by 1S (a-b) and 1P (c-j) superatomic orbitals of Al13 in Al13(THF)2 (3-12). Red, green, and white atoms refer to O, C, and H, respectively. (a) – (b) MO 90, (c) – (d) MO 97, (e) – (f) MO 98, (g) – (h) MO 100, (i) – (j) MO 101. 393x617mm (192 x 192 DPI)



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Figure 8. Natural bond orbital (NBO) donor-acceptor overlaps in (a) Al13(THF)1 (3) and (b) – (d) Al13(THF)2: (b) 3-11, (c) 3-12, and (d) 3-1 complexes. Red, green, and white atoms refer to O, C, and H, respectively. Positive and negative isosurfaces of donor are represented by green and blue, positive and negative isosurfaces of acceptor are yellow and purple, respectively. 223x157mm (192 x 192 DPI)


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Figure 9. Spin density isosurfaces of Al13(THF)n. (a)-(h) is for n = 1-8, respectively. 306x536mm (192 x 192 DPI)


Theoretical Study of Tetrahydrofuran-Stabilized Al13 Superatom

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