Al12Cu Superatom as Stable Building Block of Ionic Salts - The

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Al Cu Superatom as Stable Building Block of Ionic Salts J. Ulises Reveles, Tunna Baruah, and Rajendra R. Zope J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512261v • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Al12Cu Superatom as Stable Building Block of Ionic Salts J.U. Reveles,1,2,* Tunna Baruah,2† and Rajendra R. Zope2‡ 1

Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA 2

Department of Physics, University of Texas El Paso, El Paso, TX 79958 USA Email: *[email protected],†[email protected], ‡[email protected]

ABSTRACT

The neutral copper aluminum cluster complex Al12CuM3 [M = K, K3O, and K(2,2,2-crypt)] has been investigated at the PBE96 level of theory. It is found that Al12Cu could be considered a superatom mimic of a phosphorus atom. It shows large electron affinity and is able to receive three electrons from elements or compounds with low ionization energies like K, K3O or K(2,2,2-crypt) to become a stable electronic closed shell with a large gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap). Theoretical calculations confirm that, similar to the phosphorus atom in PK3, the superatom Al12Cu cluster could form ionic salts as shown from stable dimers, trimers and tetramers of the Al12Cu{K(2,2,2crypt)}3 complex. Based on the maintenance of its integrity in these assemblies it could be predicted that Al12CuM3 holds great potential as a building block for the development of future nanostructured materials. Further, the choice of the K(2,2,2-crypt) molecules to stabilize the salt opens a route to experimentally generate cluster base ionic salts and we expect that our work motivates experimental investigations of these assemblies.

Keywords: Cluster assemblies, closed shell clusters, chemical stability, density functional theory

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1. Introduction The search for stable clusters is one of the most active fields in experimental and theoretical materials science today aimed to develop nanostructured materials with tailored properties.1-8 It has been found that collective phenomena exhibited in bulk materials break down for very small sizes. For example, small clusters of Rh, a non-magnetic material are magnetic,9 noble elements like gold and platinum turn active catalysts when reduced at the nanoscale,10-11 and highly reactive elements like aluminum, which is readily oxidized by oxygen, is unreactive for selected anionic cluster sizes.1,6 As the properties of the clusters depend on its size, geometry, composition and charge state, one can envision designing novel materials made of stable clusters with selected catalytic, magnetic, and structural properties. Given the vast possibilities of sizes, composition and charge states, developing an understanding of the factors that govern the structural and electronic stability is a critical step towards a rational search of stable species. Earlier experimental studies of gas phase sodium (Nan) clusters by Knight and co-workers12 showed noticeable peaks in the mass spectra at sizes of 2, 8, 18, 20, 40, and 58 atoms. The abundance of clusters of particular sizes indicated that these clusters must be more stable than others. To obtain an understanding of this stability, further investigations of the electronic properties were undertaken and the ionization energy, electron affinity, polarizability and fragmentation patterns of these species were determined. It was observed a drop in ionization energies at sizes corresponding to electronic count one above the stable species as well as low values of electronic affinities at the stable sizes.13 Further, experiments on the dissociation of clusters showed that the stable sizes were the favored fragmentation products and that more energy was required to fragment these sizes.14-15 In simple metals, a confined free electron model (CFE) is a good representation of the electronic spectrum and Knight and Ekardt12,16 proposed such a model to explain the experimentally observed stability of the alkali clusters. In this model, the nuclei together with the innermost electrons form a positively-charged background that acts over the valence electrons of the system resulting in the arrangement of the cluster’s valence electrons into 1S2, 1P6, 1D10, 2S2, 1F14, 2P6 . . . sub-shells similarly to the 1s2, 2s2, 2p6, 3s2, 3p6, ... sub-shells found in individual atoms. Since each alkali atom contributes with one electron to the valence pool, the stable sizes at 2, 8, 18, 20, 40... all correspond to electron counts that result in filled electronic shells. At larger cluster sizes the extended cluster orbitals tend to condense into highly degenerated shells 2 ACS Paragon Plus Environment

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and geometrical effects begin to contribute to the stability.17-19 For example, experiments on sodium clusters containing up to several hundred atoms showed that the principal peaks in the mass spectrum could be reconciled as due to geometric effects.17 These principles that rule electronic and geometrical stability have been successfully applied to other metallic clusters. Experimental observations by Castleman and co-workers1 on the reactivity of aluminum cluster anions with oxygen found that while most clusters were etched away by oxygen, clusters containing 13 and 23 atoms were relatively intact. Subsequent theoretical studies by Khanna and Jena20 and Gong and Kumar21 showed that both Al13- and Al23clusters correspond to closed shell systems. For example, Al13- is a stable chemical cluster that would neither give nor receive electrons and has a large HOMO-LUMO gap of 1.87 eV. The cluster also showed to be energetically stable as it would take almost 4.65 eV to remove an Al atom from Al13- as opposed to 3.02 and 2.08 eV required to remove an atom from Al12− and Al14−, respectively.20,22 Further, the electronic structure of Al13- showed 40 valence electrons (3 valence electrons per Al atom) that can be arranged according to the CFE model in an 1S2, 1P6, 1D10, 2S2, 1F14and 2P6 closed shell configuration. Subsequent theoretical23 and experimental studies24 indicated that Al13 has a large electron affinity of 3.57 eV and having 39 valence electrons is only one electron short of a 40 electron closed shell. These results raised the question whether the neutral Al13 and anionic Al13- clusters may be considered as mimics of halogen and noble gas elements respectively, and led to the proposition that selected clusters resembling the electronic and chemical behavior of different atoms could be regarded as its superatom mimics forming a third dimension to the periodic table. Over the past few years, superatoms resembling alkaline earth,25 multiple valence,26 superalkali,27 transition metals,28-30 and halogen,6,31 atoms have been proposed. Superatom complexes have also been investigated towards the goal of building nanostructure materials with tailored properties. Khanna and Jena proposed that Al13- being analogous to Cl- could form an ionic species similar to K+Cl-.32 Their results showed that Al13-K+ is a stable ionic species with an intact Al13- anionic moiety and that Al13K has a large HOMOLUMO gap of 1.72 eV being consistent with an ionic salt. Calculations by Liu et al.33 however showed that the alkali cations are not large enough to form a stable lattice. Further experimental and theoretical studies have reported other Al13M (M = Li, Na, and Cs) species,34-37 and Zheng et al.38 successfully produced Al13K in the gas phase. All these systems are, in fact, stepping stones 3 ACS Paragon Plus Environment


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towards the formation of ionic cluster assemblies composed of aluminum based superatoms and appropriate cations. However, one has to realize that strong metal-metal interactions between clusters favor interatomic bonding rather than the stabilization of separate units. For instance, Seo and Corbett showed that small aluminum clusters tend to collapse with an increased number of Al–Al contacts39 and Ashman et al. studied the formation of ionic compounds of Al7C- with alkali metals founding hardly any distortion in the original cluster unit, nevertheless, dimers and tetramers of the Al7CLi species formed intermolecular Al-Al bonds.40 On the other hand, Reber et al.41 reported Al13M3O as a strongly bound molecule that can be used to form stable superatom assemblies (Al13M3O)n (M = Na and K, n = 1–5) with Al13- and M3O+ as the superatom building blocks, and Qiong31 showed that the halogen superatom Al3C maintains its integrity in ionic complexes with K atoms: [(Al3C)(KCAl3)n] (n = 1–5). These reports are certainly encouraging, they however are also indicative of the challenges in designing superatom assemblies. Both Reber et al. and Qiong found local minima resulting from a unique orientation of the cationic and anionic units in their cluster assemblies. In experiments, however, it is likely that random orientations of the clusters will favor configurations in which the metalmetal intermolecular interactions will collapse the building units towards the more stable global minimum. The aim of this paper is to explore if truly stable ionic superatomic assemblies are indeed possible. Motivated by the work of Kumar and Yoshiyuki that reported a high spin icosahedral ground state for Al12Cu with a magnetic moment of 3µB42 we investigate if Al12Cu, being able to accept three electrons from elements and compounds with a low ionization energy to form ionic (Al12Cu)3-(M+)3 species may a suitable candidate to form stable nanostructures. In particular, following questions are addressed. Is the large negative charge in the anionic (Al12Cu)3superatom able to overcome the intermolecular attractions in the ionic salts preserving its identity? and, how large should an ideal cationic M species needs to be to help sustaining the assembly through steric hindrance? Our study shows that neither K or K3O cations can avoid collapse of the ionic species in the cluster assemblies, and that it is only the bulkier K(crypt-222) cation that allows formation of chemically and energetically stable ionic salts [Al12Cu{K(crypt222)}3]n (n=1-4) of the Al12Cu cluster superatom in a similar way as phosphorus does in PK3.


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2. Computational Method The theoretical calculations are carried out within density functional formalism with description of exchange-correlation effects at the generalized gradient approximation level by means Perdew, Burke, and Ernzerhof (PBE96) functional.43 The electronic orbitals and eigenstates are determined by using a linear combination of Gaussian atomic type orbital molecular orbital (LCGTO) approach in the deMon2k software.44-45 The calculation of fourcenter electron repulsion integrals are avoided using a variational fitting of the Coulomb potential46 and the exchange correlation energy and potential are calculated via a numerical integration of the fitted density47 using the GEN-A2 auxiliary function set.48 All electrons of the Al, Cu, K and O atoms are treated explicitly using the double zeta valence plus polarization (DZVP) basis sets.49 To speed up the calculations of the larger [Al12Cu{K(crypt-222)}3]n (n=1-4) systems, which range from 202 to 808 atoms, relativistic effective core potentials with 1, 4, 5, 6, 3 and 19 valence electrons for K, C, N, O, Al and Cu were employed respectively with their corresponding basis sets.50 The geometry optimization used a quasi-Newton method in internal delocalized coordinates and all possible spin configurations were investigated to determine the most stable states.51-52 To partial charges were determined using a natural bond orbital (NBO) analysis.53 To further eliminate any uncertainty associated with the choice of basis set, the numerical procedure, and the use of pseudo potentials, we carried out supplementary all-electron calculations using the Naval Research Laboratory Molecular Orbital Library (NRLMOL) set of codes developed by Pederson and co-workers.54-56 For these calculations the same PBE96 generalized gradient approximation used in deMon2k was employed.43 We found excellent agreement in the optimized geometries and energy order of the investigated spin states for the cluster calculations between deMon2k and NRLMOL calculations. 3. Results and Discussion 3.1 The Al12Cu superatom As noted earlier, several studies have explored Al superatoms as potential building blocks of ionic materials but strong intermolecular interactions between these have limited its potential to build truly stable cluster assemblies. In this work we study the Al12Cu neutral species with a goal to design a stable superatom with a large negative charge since such a system can result in ionic 5 ACS Paragon Plus Environment


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compounds with increased stability. Kumar and Yoshiyuki reported the finding of a high spin icosahedral ground state for Al12Cu with a magnetic moment of 3µB in accordance with the Hund’s rule of maximum spin at half-filling of a P shell forming an open shell superatom.42 Several possible geometries and spin states were investigated in our study and it was found that the most stable structure for Al12Cu did correspond to the Cu atom inside a slightly distorted icosahedral cage with D3d symmetry and Al-Al and Al-Cu bonds of 2.79 Å and 2.65 Å respectively as shown in Figure 1; and presenting a quartet spin state (having three unpaired electrons) in agreement with the previous report.42 An analysis of the electronic structure of Al12Cu accounts for a total of 37 delocalized valence electrons (3 electrons for each Al and 1 electron for the Cu atom) arranged in 1S2, 1P6, 1D10, 2S2, 1F14 and 2P3 sub-shells according to the CFE model as shown by the one electron energy levels and molecular orbital isosurfaces in Figure 2. Note that the filled 3d states of Cu are located between the 1P6and 1D10 states. Notably, the highest occupied energy levels correspond to triple-degenerate 2P3 states with 3 electrons short to complete the shell, similar to a phosphorus atom with a [Ne] 4s23p3 configuration. Moreover, our calculated electron affinity of 2.92 eV, in good agreement with the experimental value of 3.24±0.03 eV reported by Pal et al.,57 indicated a larger tendency of Al12Cu to receive electrons. Pal et al. also employed a Basin-hopping global optimization method and found a slightly distorted icosahedral with D3d symmetry to be ground state geometry of the anionic Al12Cu-. The gas phase anionic Al12Cu- has also been produced as reported in an experimental and theoretical study by Roach et al.58 The authors, however, noted that the cluster was oxygenetched and did not show signs of chemical stability, presenting a small HOMO-LUMO gap of only 0.2 eV. The rationale for this behavior is rooted in its electronic structure. One extra electron is not enough to complete the cluster’s 2P subshell to generate a stable species. The 2P subshell of Al12Cu cluster can however be completed by the addition of 3 electrons as discussed below.

3.2 Al12CuKn clusters and its assemblies With the goal to obtain a closed shell cluster, we studied the interaction of K atoms (an electron donor with low ionization energy) with the Al12Cu cluster. We investigated the charge transfer by calculating NBO charges and the energy gained with successive additions of K atoms to Al12Cu, incremental binding energy (IBE) in Al12CuKn by using the equation: 6 ACS Paragon Plus Environment

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IBE = E[Al12CuKn] - E[K] - E[Al12CuKn-1], with n = 1 - 5. Here, E[X] represents the total energy of the gas phase species. Note that according to this definition a positive IBE corresponds to a bound structure and a larger value of IBE is indicative of an energetically more stable structure. Addition of one K atom resulted in a large energy gain of 2.0 eV to form Al12CuK in which the K sits on top of a hole of a slightly distorted Al12Cu cage (Fig. 1). The one electron energy levels indicate that one electron was transferred to the 2P subshell of Al12CuK that now presents a triplet state with still two unpaired electrons and a small HOMO-LUMO gap of 0.24 eV (Fig. 3). Our population analysis support this conclusion with -0.97 and 0.97 electronic charges for the Al12Cu and K fragments. Further addition of K atoms also showed large energy gains for the second and third K atoms of 1.93 and 1.85 eV respectively and a drop in the IBE after 3 K atoms to only around 0.7 eV (Fig. 4a). The optimized geometries and electronic charges for these species are presented in Fig. 1. Moreover, the calculations also indicate a progressive increase in the HOMO-LUMO gap (and thus in the chemical stability) with each addition of K atoms as the 2P subshell gets filled with the charge given by the K atoms and reaching a maximum gap of 1.44 eV for the compact closed shell Al12CuK3 cluster (Fig. 1 and 3). We confirm the ionic nature of this species via our population analysis that found -2.88 and 0.96 electronic charges in Al12Cu and each K atom respectively. Figure 4a graphically presents the calculated IBE and HOMO-LUMO gap for the Al12CuKn clusters attesting for both the energetic and chemical stability of the Al12CuK3 species that presents maxima compared to neighboring sizes. Our next step in the investigation of ionic cluster assemblies was to study the structural and electronic properties of dimers of the ionic Al12CuK3 cluster. Several initial orientations of the (Al12CuK3)-(Al12CuK3) dimer were prepared and full geometry optimizations were performed. The initial geometries showed signs of structural stability and we did actually found a stable dimer in which the Al12CuK3 units remain separated and exhibited a large HOMO-LUMO gap of 1.36 eV as shown in Fig. 5a. However, due to the metal-metal interactions between the Al12Cu units another more stable complex was also found with an Al-Al bonding between adjacent cores at a short distance of 2.90 Å. This structure resulted more stable by 1.21 eV and had a smaller HOMO-LUMO gap of only 0.64 eV which is indicative of less chemical stability (Fig. 5a). This result is also consistent with a report by Ashman et al.22 that studied the mobility 7 ACS Paragon Plus Environment


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of alkali atoms on the Al13 surface. The authors found that the barrier for the motion of a K atom from one hollow site to another is less than 0.13 eV indicating that it may be possible for K atoms to be mobile at ordinary temperatures and hence not being a good protecting cation for the aluminum core.

3.3 Al12Cu(K3O)n clusters and its assemblies We then changed our strategy by turning efforts to the use of molecular counterions rather than atomic ions. These ions have two main advantages. First, they present low ionization energies like single alkali atoms assuring a good charge transfer when forming a molecular salt. Second, and the more important advantage is that the bulkier nature of these molecular counterions increase the steric hindrance which helps in avoiding the agglomeration of separate clusters into a bulk material. Motivated by the work of Reber et al.41 who reported Al13M3O as a strongly bound molecule that retains its integrity in ionic complexes of the type (Al13M3O)n (M = Na and K, n = 1–5) with separated Al13- and M3O units; we first studied the binding of the super alkali K3O molecule with our Al12Cu superatom aimed to form a stable closed shell species like the one found for Al12CuK3. We begin by investigating the charge transfer and the energy gained with successive additions of K3O molecules (incremental binding energy, IBE) in forming Al12Cu(K3O)n by using the equation: IBE = E[Al12Cu(K3O)n] - E[K3O] - E[Al12Cu(K3O)n-1], with n = 1 - 5 . Addition of the first K3O resulted in a large energy gain of 6.19 eV to form Al12CuK3O in which the O atom strongly binds to an Al atom at a short distance of 1.77 Å. This distance is within the range of 1.7–2.0 Å reported for the Al-O bond in AlnO2 (n=1-5) clusters by Sun et al.59 Surprisingly, Al12CuK3O presented a closed shell configuration (Fig. 1) with a HOMO-LUMO gap of 0.7 eV. This result is intriguing given that according to our population analysis only 1.43 electrons are transferred from the K3O superalkali cluster into Al12Cu which is in need of three electrons to complete its 2P subshell as mentioned above. The analysis of the electronic structure sheds light into this apparent contradiction. According to the one electron energy levels shown in Fig. 6a a set of occupied energy levels of K3O lie above the unoccupied 2P levels of Al12Cu. The bonding combinations of these levels give rise to an Al12CuK3O complex that exhibits a polar 8 ACS Paragon Plus Environment

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covalent interaction between the two units. The polar nature of the bonding is attested by the calculated charge transfer and the covalent character by the observed bonding between Al and O. Furthermore, the stronger interaction in this case becomes evident from the large energy gain of 6.16 eV for the addition of K3O to Al12Cu. This energy is three times larger than the energy gain (1.99 eV) for the addition of a single K to Al12Cu. Addition of another K3O to Al12CuK3O also resulted in a large energy gain of 4.61 eV. The population analysis of the optimized Al12Cu(K3O)2 cluster indicates an increase in the charge transfer towards the Al12Cu core which now acquires a negative -2.49 electronic charge (Fig. 1). The energy levels for this cluster reveals a 2P5 configuration for the highest occupied sub-shell in a doublet state with still one unpaired electron and a smaller HOMO-LUMO gap of 0.33 eV. Finally, addition of a third K3O resulted in an energy gain of 4.46 eV accompanied by additional charge transfer such that the Al12Cu core now acquires a negative -3.38 charge in the closed shell Al12Cu(K3O)3 species with a HOMO-LUMO gap of 0.67 eV. Further addition of K3O resulted in smaller energy gains and species with smaller HOMO-LUMO gaps as shown in Fig. 4b. These results suggested that the stable closed shell Al12Cu(K3O)3 cluster can be a promising candidate as a building block of ionic assemblies. In order to explore this, we prepared several initial orientations of the Al12Cu(K3O)3 --- Al12Cu(K3O)3 dimer which were then fully optimized. Notably, we again found the formation of a stable complex with separated Al12Cu(K3O)3 units exhibiting a HOMOLUMO gap of only 0.47 eV as shown in Fig. 5b. However, as in the case of the (Al12CuK3)2 dimer, strong metal-metal interactions between the Al12Cu units conducted to another more stable complex with the presence of Al-Al bonding between adjacent cores at a distance of 2.69 Å. This structure resulted more stable by 0.76 eV and had a smaller HOMO-LUMO gap of only 0.15 eV in a triplet state configuration (Fig. 5b). Interestingly, our results disagree with the report of Reber et al.41 wherein the lowest energy dimer of (K3OAl13)2 was found to be a bent structure with oxygen occupying on-top positions at the Al13 surface and with an energy barrier of more than 0.14 eV towards the formation of the Al26-K6O3 species. To investigate the cause of discrepancy, we also calculated the Al13K3O -- Al13K3O dimers starting from several initial configurations and found that it is only when the species are disposed such that the K3O on one Al13K3O is oriented towards the Al13 side of the second Al13K3O that we were able to reproduce the results of Ref. 41. All other possible configurations of the dimers led to formation of more stable Al26-K6O3 species without 9 ACS Paragon Plus Environment


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any energy barrier. In actual experiments, it is likely that random orientations of the clusters will favor configurations in which the metal-metal intermolecular interactions will collapse the building units towards the more stable global minimum with the loss of the individual cluster identities and properties. Similarly, another study by Qiong L.31 showed that the halogen superatom Al3C maintains its integrity in ionic complexes when bonding with K atoms in the form: [(Al3C)(KCAl3)n] (n = 1–5). We again calculated these clusters and found that it is only in the structural configuration given by the author that the species retain its identity, and that other possibilities resulted in collapsing of the aluminum clusters into more stable configurations. These results suggest that an even larger molecular cation is needed to prohibit fusion of aluminum clusters in the formation of ionic solids.

3.4 Al12Cu(Crypt)n clusters and its assemblies The results in previous sections indicate that the K and K3O ions cannot prevent coalescence of the Al12CuM3 (M = K and K3O) cluster due to their smaller sizes, and we therefore choose the bulkier K encapsulated K(2.2.2-cryptand), from now on called Crypt, as the electron donor for our Al12Cu superatom with the hope that its larger size could stabilize the ionic assemblies. Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations.60 These molecules are three-dimensional analogues of crown ethers but are more selective, complex the guest ions more strongly and exhibit low ionization energies. We begin by studying the binding of the Crypt molecules with our Al12Cu superatom with a goal to obtain a stable closed species like the ones found for Al12CuK3 and Al12Cu(K3O)3. We investigated the charge transfer and the energy gained with successive additions of Crypt molecules (IBE) in forming Al12Cu(Crypt)n in a similar way as we did above for the K and K3O electron donors. Addition of the first Crypt resulted in a large energy gain of 3.19 eV to form the Al12CuCrypt complex in which the K atom of the Crypt sits on top of an Al atom of the Al12Cu cage at a distance of 4.56 Å (Fig. 1). The optimized Al12CuCrypt has multiplicity of 3 and a small HOMO-LUMO gap of 0.14 eV. The state with spin multiplicity of 1 is energetically degenerate with this state. Fig. 6b presents the one electron energy levels for Al12Cu, Crypt and Al12Cu(Crypt)n, n=1-3. It can be seen that a single occupied energy level from the Crypt lies 10 ACS Paragon Plus Environment

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above the 2P unoccupied states of Al12Cu and that is this Crypt state that transfers one electron to form a 2P4 configuration in the triplet state of Al12CuCrypt. The population analysis results are consistent with this observation and show 1.04 and -1.04 electronic charges for the Crypt and Al12Cu fragments respectively. The calculated energy gain of 3.19 eV for the addition of Crypt to Al12Cu is more than 1 eV larger than the energy gain for the addition of a single K atom to Al12Cu of only 1.99 eV. Addition of second Crypt to Al12Cu(Crypt) resulted in an energy gain of 2.63 eV and the resultant structure of Al12Cu(Crypt)2 has the K atom of the second Crypt sitting on top of an Al atom of the Al12Cu cage. The one electron energy levels of Al12Cu(Crypt)2 in Fig. 6b indicate that an additional electron from the second Crypt has been transferred to fill another empty 2P level of Al12Cu such that the Al12Cu(Crypt)2 complex now presents a doublet spin state with only one unpaired electron in a 2P5 electronic configuration and with a small HOMO-LUMO gap of 0.19 eV. The population analysis in this case shows that the Al12Cu core acquires a negative charge of -2.03 electrons from the Crypt molecules and each Crypt molecule has a positive electronic charge of nearly 1 electron (Fig. 1). Finally, addition of the third Crypt resulted in an energy gain of 2.2 eV were the Al12Cu core has a -2.98 charge in the closed shell Al12Cu(Crypt)3 species that presented a rather large HOMO-LUMO gap of 1.38 eV. Addition of a fourth Crypt resulted in a smaller energy gain of 1.1 eV and the formation of an Al12Cu(Crypt)4 less stable species with a smaller HOMO-LUMO gap of 0.27 eV. Further addition of a Crypt resulted in a non-bonded species showing that the maximum number of Crypt molecules that can bind the Al12Cu core is four. These results are collected in Fig. 4c. We then directed our efforts to explore the potential of the closed shell Al12Cu(Crypt)3 species as a resilient building block of ionic assemblies. We prepared several initial orientations of the Al12Cu(Crypt)3 --- Al12Cu(Crypt)3 dimer which were then fully optimized. In this case and for the first time in our studies we found that the Al12Cu(Crypt)3 units were able to retain its identity towards the formation of the {Al12Cu(Crypt)3}2 dimer as shown Fig. 7, and with a binding energy of 1.41 eV. Notably, the assembly is also chemically stable presenting a large HOMO-LUMO gap of 1.09 eV. Further additions of Al12Cu(Crypt)3 units to form trimers and tetramers were investigated by approaching the additional Crypt units from the possible orientations and fully optimizing the geometries. We found that as in the case of the dimer, both trimer and tetramer resulted in stable assemblies with energy gains of 0.26 eV and 0.96 eV for 11 ACS Paragon Plus Environment


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the addition of Al12Cu(Crypt)3 units to the dimer and trimer respectively. The dimer, trimer and tetramers also had relatively large HOMO-LUMO gaps of 1.09, 0.98, and 1.00 eV respectively (Table 1). Most importantly for the formation of stable ionic salts is that the Al12Cu(Crypt)3 cluster retained its identity in all the optimized geometries as shown in Fig. 7. Based in these results we confirm that, similar to the phosphorus atom in PK3, the superatom Al12Cu cluster could indeed form truly ionic salts of the Al12Cu(Crypt)3 complex and that assemblies of the Al12CuM3 type holds great potential as building blocks for the development of novel nanostructured materials with tailored properties.

Conclusions Our theoretical study demonstrates that Al12Cu exhibits electronic and chemical behavior similar to a phosphorus atom and can be considered its superatom mimic. Al12Cu being a 37 valence electron system corresponds to an open shell system three electrons short to complete its superatomic 2P subshell. It forms ionic compounds with K, K3O or Crypt with a high energetic and chemical stability. Our results show that although both, K and K3O, provide the required number of electrons, the size of these ions is not large enough to keep strong metal-metal intermolecular interactions from collapsing the cluster during the formation of cluster assemblies. Notably, the larger Crypt ion is found to be the ideal cation to donate the needed electrons and to prevent the Al12Cu core from coalescing. Our studies confirm that, similar to the phosphorus atom in PK3, the superatom Al12Cu cluster could then form truly stable ionic salts as shown from dimers, trimers, and tetramers of the Al12Cu(Crypt)3 complex calculated from several random possible initial orientations. The integrity of Al12Cu(Crypt)3 unit in the cluster assemblies show that the Al12Cu(Crypt)3 clusters holds a great potential as a building block for the development of novel nanostructured materials. We believe that the understanding of the interplay between the electronic and geometrical structure and the chemical stability of the investigated systems has the potential to provide a descriptor (or template) toward the computational discovery and design of families of novel nanostructured materials with potential applications. The choice of the Crypt molecules to stabilize the salt opens a route to experimentally generate cluster base ionic salts and we hope that new ionic assemblies may be developed by extending our findings and that this work motivate experimental investigations that confirm our results. 12 ACS Paragon Plus Environment

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Acknowledgements This work was partially supported by the DOE Basic Energy Science under award DESC0006818 (R.R.Z.), DE-SC0002168 (T.B.) and the NSF PREM program between UCSB and UTEP (DMR-1205302) (T.B.), and the University of Texas at El Paso (JUR). Authors thank the Texas Advanced Computing Center (TACC) from the National Science Foundation (NSF) (Grant No. TG-DMR090071) and NERSC for the computational time.

TOC



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Table 1. Gap between the highest occupied and lowest unoccupied molecular orbitals (HOMOLUMO gap) for the {Al12CuCryptx}y (x = 1-4, y = 1-4) complex. Molecular spin multiplicity is given by the superscripts.

HOMO-LUMO gap (eV) 1

Al12Cu(Crypt)

0.14

2

Al12Cu(Crypt)2

0.19

1

Al12Cu(Crypt)3

1.38

2

Al12Cu(Crypt)4

0.27

1

{Al12Cu(Crypt)3}2

1.09

1

{Al12Cu(Crypt)3}3

1.18

1

{Al12Cu(Crypt)3}4

1.13


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Figure Captions Figure 1. Optimized geometries for Al12Cu and Al12CuXn (X = K, K3O and Crypt, n = 1-3). Molecular spin multiplicity is given by the superscripts. The electronic charges are marked next to the Al12Cu and X fragments. Light blue, yellow, brown, grey, red, dark blue, and white circles represent Al, Cu, K, C, O, N and H atoms respectively. Figure 2. One electron energy levels and molecular orbital isosurfaces for Al12Cu. The main and angular quantum numbers for the delocalized superorbitals and the 3d Cu localized orbitals are also given. The 2P superorbitals are marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. Figure 3. One electron energy levels near to the HOMO level for the Al12Cu, K, Al12CuK, Al12CuK2 and Al12CuK3 species. Molecular spin multiplicity is given by the superscripts. Main and angular quantum numbers are given for selected levels showing the progressive filling of the 2P superorbitals marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. Figure 4. HOMO-LUMO gap (HL gap) and incremental binding energies (IBE) for the a) Al12CuKn, b) Al12Cu(K3O)n and c) Al12Cu(Crypt)n n=1-5 clusters. Figure 5. Optimized geometries for dimers of the a) Al12CuK3 and b) Al12Cu(K3O)3 clusters. Molecular spin multiplicity is given by the superscripts. The relative energies (Erel) and HOMOLUMO gaps (HL gap) are also given. Figure 6. One electron energy levels for the a) Al12Cu, K3O, Al12Cu(K3O)n, and b) Al12Cu, Crypt, Al12Cu(Crypt)n, n=1-3 clusters. Molecular spin multiplicity is given by the superscripts. Main and angular quantum numbers are given for selected levels showing the progressive filling of the 2P superorbitals marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. Figure 7. Optimized geometries for the ionic solids formed by the successive addition of Al12Cu(Crypt)3 units. Molecular spin multiplicity is given by the superscripts.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Figure 6.



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Figure 7.


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Optimized geometries for Al12Cu and Al12CuXn (X = K, K3O and Crypt, n = 1-3). Molecular spin multiplicity is given by the superscripts. The electronic charges are marked next to the Al12Cu and X fragments. Light blue, yellow, brown, grey, red, dark blue, and white circles represent Al, Cu, K, C, O, N and H atoms respectively. 192x185mm (300 x 300 DPI)

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One electron energy levels and molecular orbital isosurfaces for Al12Cu. The main and angular quantum numbers for the delocalized superorbitals and the 3d Cu localized orbitals are also given. The 2P superorbitals are marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. 280x282mm (300 x 300 DPI)


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One electron energy levels near to the HOMO level for the Al12Cu, K, Al12CuK, Al12CuK2 and Al12CuK3 species. Molecular spin multiplicity is given by the superscripts. Main and angular quantum numbers are given for selected levels showing the progressive filling of the 2P superorbitals marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. 228x160mm (300 x 300 DPI)



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HOMO-LUMO gap (HL gap) and incremental binding energies (IBE) for the a) Al12CuKn, b) Al12Cu(K3O)n and c) Al12Cu(Crypt)n n=1-5 clusters. 121x271mm (300 x 300 DPI)


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Optimized geometries for dimers of the a) Al12CuK3 and b) Al12Cu(K3O)3 clusters. Molecular spin multiplicity is given by the superscripts. The relative energies (Erel) and HOMO-LUMO gaps (HL gap) are also given. 196x169mm (300 x 300 DPI)



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One electron energy levels for the a) Al12Cu, K3O, Al12Cu(K3O)n, and b) Al12Cu, Crypt, Al12Cu(Crypt)n, n=1-3 clusters. Molecular spin multiplicity is given by the superscripts. Main and angular quantum numbers are given for selected levels showing the progressive filling of the 2P superorbitals marked in a blue rectangle. Solid black lines and dotted red lines represent respectively occupied and unoccupied states. 189x275mm (300 x 300 DPI)


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Optimized geometries for the ionic solids formed by the successive addition of Al12Cu(Crypt)3 units. Molecular spin multiplicity is given by the superscripts. 200x126mm (300 x 300 DPI)



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Al12Cu Superatom as Stable Building Block of Ionic Salts - The

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