Boron Silicon B2Si3q and B3Si2p Clusters: The Smallest Aromatic Ribbons.

The small binary boron silicon clusters B2Si3q with q going from -2 to +2 and B3Si2p with p varying from -3 to +1 were reinvestigated using quantum chemical methods. The thermodynamic stability of these smallest ribbon structures is governed by both Hückel and ribbon models for aromaticity. The more negative the cluster charge, the more ribbon character is shown. In contrast, the more positive the charge state, the more pronounced the Hückel character becomes. The ribbon aromaticity character can also be classified into ribbon aromatic, semiaromatic, antiaromatic, and triplet aromatic when the electron configuration of a ribbon structure is described as [...π2(n+1)σ2n], [...π2n+1σ2n], [...π2nσ2n], and [...π2n+1σ2n-1], respectively. Geometry optimizations of the B2Si3 lowest-energy structure by some density functional theory (DFT) functionals result in a nonplanar shape because it possesses an antiaromatic ribbon character. However, its π aromaticity assigned by the Hückel rule is stronger in such a way that several other DFT and coupled-cluster theory CCSD(T) calculations show that B2Si3 is indeed stable in a planar form (Cs). A new global equilibrium structure for the anion B2Si32-, which is a ribbon semiaromatic species, was identified. Some benchmark tests were also carried out to evaluate the performance of popular methods for the treatment of binary B-Si clusters. At odds with some previous studies, we found that with reference to the high accuracy CCSD(T)/CBS method, the hybrid TPSSh functional is reliable for a structure search, whereas the hybrid B3LYP functional is more suitable for simulations of some experimental spectroscopic results.

The binary boron lithium clusters B12Lin with n = 1-14: in search for hydrogen storage materials.

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B12Lin with n = 1-14, were investigated using density functional theory with the TPSSh functional and the 6-311+G(d) basis set. Energetic parameters including relative energies, average binding energies and second-order energies of the entire series were predicted using the coupled-cluster theory (U)CCSD(T) in conjunction with the cc-pVTZ basis set. Several lowest-lying isomers were determined for each size B12Lin whose energies differ from each other by <3 kcal mol-1, except for n = 1, 2 and 4 (≤5 kcal mol-1), and particularly n = 8 (∼13 kcal mol-1). Electronic structure and chemical bonding in some specific sizes such as B12Li4, B12Li8 and B12Li14 were analyzed in detail. We established the electron shells of some magic clusters such as the B12Li4 cone for which we proposed a mixed cone-disk electron shell model. Thanks to both the phenomenological shell and Clemenger-Nilsson models, B12Li8 which contains a specific set of shells of 44 valence electrons is a high stability species. The arrangement of Li atoms around a fullerene B12 framework shows that the mixed B12Li8 emerges as the most suitable of this cluster series to adsorb molecular hydrogen. Up to 32 H2 molecules can strongly be attached to the B12Li8 cluster which is thus predicted to be a realistic candidate for hydrogen storage material with gravimetric density reaching up to a theoritical limit of 26 wt%. Attachment of the fifth H2 molecule to each Li atom of B12Li8 results in weaker average bonds but can give rise to a total of 40 H2 molecules, corresponding to 30 wt% of hydrogen.

Comment on 'Structural characterization, reactivity and vibrational properties of silver clusters: a new global minimum for Ag16' by P. L. Rodríguez-Kessler, A. R. Rodríguez-Domínguez, D. MacLeod Carey and A. Muñoz-Castro, Phys. Chem. Chem. Phys., 2020, 22, 27255, DOI: D0CP04018E.

A recent paper by Rodríguez-Kessler et al., Phys. Chem. Chem. Phys., 2020, 22, 27255-27262, reported not only results of quantum chemical computations (using the PW91 density functional) on Ag16 clusters as emphasized in the article's title, but also on the Ag15 size. These authors confirmed previous results obtained by McKee and Samokhvalov (J. Phys. Chem. A, 2017, 121, 5018-5028 using the M06 density functional) that the most stable isomer of Ag15 is a C2v structure. We wish to point out that two low symmetry isomers of Ag15 that have a similar energy content are even lower in energy than their reported C2v global minimum. The relative energies between low-lying Ag15 isomers were again found to be method-dependent, and within the expected accuracy of DFT and CCSD(T) methods they could be considered as energetically degenerate, and likely coexist in a molecular beam. The new lower-energy Ag15 isomers appear to fit more consistently within the structural evolution of small silver clusters.

The binary aluminum scandium clusters Al x Sc y with x + y = 13: when is the icosahedron retained?

Geometrical and electronic structures of the 13-atom clusters Al x Sc y with x + y = 13, as well as their thermodynamic stabilities were investigated using DFT calculations. Both anionic and neutral isomers of Al x Sc y were found to retain an icosahedral shape of both Al13 and Sc13 systems in which an Al atom occupies the endohedral central position of the icosahedral cage, irrespective of the number of Al atoms present. Such a phenomenon occurs to maximize the number of stronger Al-Al and Sc-Al bonds instead of the weaker Sc-Sc bonds. NBO analyses were applied to examine their electron configurations and rationalize the large number of open shells and thereby high multiplicities of the mixed clusters having more than three Sc atoms. The SOMOs are the molecular orbitals belonged to the irreducible representations of the symmetry point group of the clusters studied, rather than to the cluster electron shells. Evaluation of the average binding energies showed that the thermodynamic stability of Al x Sc y clusters is insignificantly altered as the number y goes from 0 to 7 and then steadily decreases when y attains the 7-13 range. Increase of the Sc atom number also reduces the electron affinities of the binary Al x Sc y clusters, and thus they gradually lose the superhalogen characteristics with respect to the pure Al13.

The teetotum cluster Li2FeB14 and its possible use for constructing boron nanowires.

Systematic density functional theory (DFT) calculations using the TPSSh functional and the def2-TZVP basis set were carried out to identify the global energy minimum structure of the Li2FeB14 cluster. Keeping the double ring tubular shape of FeB14, capping of two Li atoms leads to a teetotum form at a low spin state, in which the Fe atom is endohedrally covered by two B7 strings, and both Li atoms are attached to Fe along the C7 axis at both sides. Calculated results show that strong electrostatic interactions between 2Li+ and Fe2- arising from Li electron transfer upon doping particularly provide a key driving force for stabilizing this charge-transfer structure. The bonding pattern of the teetotum can be understood from the hollow cylinder model (HCM). TD-DFT calculations demonstrate that this cluster can also be regarded as a useful material for transparent optoelectronic devices. Furthermore, the Li2FeB14 superatom can be used as a building block for making boron-based nanowires with metallic character. Replacement of Li atoms by Mg atoms was also found to lead to nanowires.

Elucidating the binding mechanism of thione-containing mercaptopurine and thioguanine drugs to small gold clusters.

Density functional theory methods were employed to clarify the adsorption/desorption behaviors of the thione-containing mercaptopurine and thioguanine drugs on the gold surface using both small Au6 and Au8 clusters as model reactants. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated using the Perdew-Burke-Ernzerhof (PBE) and LC-BLYP functionals along with correlation-consistent basis sets, namely cc-pVDZ-PP for gold and cc-pVTZ for non-metals. Computed results show that the drug molecules tend to anchor on the gold cluster at the S atom with binding energies around -34 to -40 kcal/mol (in vacuum) and - 28 to -32 kcal/mol (in aqueous solution). As compared to Au8 , Au6 undergoes a shorter recovery time and a larger change of energy gap that could be converted to an electrical signal for selective detection of the drugs. Furthermore, interactions between the drugs and gold clusters are reversible processes and a drug release mechanism was also proposed. Accordingly, the drugs are able to separate from the gold surface due to either a slight change of pH in tumor cells or the presence of cysteine residues in protein matrices.

A theoretical approach to the role of different types of electrons in planar elongated boron clusters.

We analyze the thermodynamic stability of some small elongated boron clusters and confirm the relationship between their planarity and their inherent electron configuration […σ2(n+1)π12(n+1)π22n]. Delocalized σ electrons in an elongated bare boron cluster and 2c-2e C-H bonds in a corresponding elongated hydrocarbon play a vital role in maintaining their planar structure. Through the eigenstates derived from a model of a particle moving in a rectangle, the rectangle model, our study suggests that the larger planar elongated boron clusters are not thermodynamically stable. A partition of the electron densities which is consistent with the electron count, points out that the dianionic, neutral and dicationic B102-/0/2+ clusters are doubly σ and π aromatic, singly π aromatic, and doubly σ and π antiaromatic, respectively.

Effects of single and double nickel doping on boron clusters: stabilization of tubular structures in BnNim, n = 2-22, m = 1, 2.

A systematic investigation on structure, relative stabilities, dissociation behavior and bonding of the singly and doubly Ni doped boron clusters BnNim with n = 2-22 and m = 1-2, was carried out using density functional theory (TPSSh functional) calculations. Calculated results indicate that for n < 14, BnNim structures are generally formed by capping Ni atom(s) on the edge or the surface of the pure boron Bn frameworks. From n = 14, the Ni dopants exert stronger effects in such a way that the most stable isomers BnNim adopt the shape of the related double ring tubular boron structures. With n ≥ 20, the Bn double ring appears to possess a large enough volume to entirely enclose the Ni2 dimer. The B14Ni and B22Ni2 turn out to be remarkable species with enhanced thermodynamic stability with larger average binding energies along with surprising geometric structures. Their higher thermodynamic stability can be understood in terms of the MO energy levels predicted by a hollow cylinder model, and other electronic properties. The (2 0 2)-orbital derived from the model of particle in a hollow cylinder appears to play a key role in the stabilization of the boron double ring.

Structure, stability, absorption spectra and aromaticity of the singly and doubly silicon doped aluminum clusters Al n Si m 0/+ with n = 3-16 and m = 1, 2.

Structures of the binary Al n Si m clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Al n cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al4Si2, Al12Si and Al11Si2 + as "magic" clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al13 -, Al12Si, and Al11Si2 + that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the "magic" 40 shell electrons.

The electronic structure and stability of germanium tubes Ge30H12 and Ge33H12.

The geometries of non-tetrahedral and ultrastable silicon and germanium nanocrystals X18H12 and X19H12 (X = Si, Ge) have recently been predicted for the development of cluster-based nanomaterials for energy and microengineering purposes. To further explore the possibility of larger Ge clusters, we investigated in this work the molecular and electronic structure of the germanium tube Ge30H12, composed of six parallel, planar hexagons using DFT calculations. Insertion of Ge atoms at the center of three inner hexagons of Ge30H12 leads to a Ge33H12 tube, which is also an energy minimum structure. The electronic structure and molecular orbital shapes of these tubes can be predicted by the wavefunctions of a particle on a hollow cylinder model and a cylinder model. Different aromaticity indices including PDI, Iring, ING, MCI, and INB, as well as the electron localization function (ELF) were calculated to evaluate the electron delocalization and the aromaticity of the Ge tubes considered.

Electronic Structure and Thermochemical Parameters of the Silicon-Doped Boron Clusters BnSi, with n = 8-14, and Their Anions.

We performed a systematic investigation on silicon-doped boron clusters BnSi (n = 8-14) in both neutral and anionic states using quantum chemical methods. Thermochemical properties of the lowest-lying isomers of BnSi(0/-) clusters such as total atomization energies, heats of formation at 0 and 298 K, average binding energies, dissociation energies, etc. were evaluated by using the composite G4 method. The growth pattern for BnSi(0/-) with n = 8-14 is established as follows: (i) BnSi(0/-) clusters tend to be constructed by substituting B atom by Si-atom or adding one Si-impurity into the parent Bn clusters with n to be even number, and (ii) Si favors an external position of the Bn frameworks. Our theoretical results reveal that B8Si, B9Si(-), B10Si and B13Si(-) are systems with enhanced stability due to having high average binding energies, second-order difference in energies and dissociation energies. Especially, by analyzing the MOs, ELF, and ring current maps, the enhanced stability of B8Si can be rationalized in terms of a triple aromaticity.

Mn2@Si15: the smallest triple ring tubular silicon cluster.

The smallest triple ring tubular silicon cluster Mn2@Si15 is reported for the first time. Theoretical structural identification shows that the Mn2@Si15 tubular structure whose triple ring is composed by three five-membered Si rings in anti-prism motif, is stable in high symmetry (D5h) and singlet ground state ((1)A1'). The dimer Mn2 is placed inside the tubular along the C5 axis, and the Mn dopant form single Si-Mn bonds with Si skeleton, whereas the Mn-Mn is characterized as a triple bond. The effect of Mn2 on the stability of the Si15 triple ring structure arises from strong orbital overlap of Mn2 with Si15.

Fullerene-like boron clusters stabilized by an endohedrally doped iron atom: B(n)Fe with n = 14, 16, 18 and 20.

Stabilized fullerene and tubular forms can be produced in boron clusters Bn in small sizes from n∼ 14 to 20 upon doping by transition metal atoms. B14Fe and B16Fe are stable tubes whereas B18Fe and B20Fe are stable fullerenes. Their formation and stability suggest the use of dopants to induce different growth paths leading to larger cages, fullerenes and tubes of boron.

A particle on a hollow cylinder: the triple ring tubular cluster B27(+).

We determined the geometries and chemical bonding phenomena of the B27 system in its dicationic, cationic, neutral, anionic and dianionic states using DFT computations. In both cationic and neutral states, the triple ring tubular forms correspond to the lowest-energy isomers, especially in B27(+). The cation B27(+) represents the first stable hollow cylinder having a triple ring among the pure boron clusters. In the anionic and dianionic states, the quasi-planar structures are favoured due to a charge effect. In the triple ring tube B27(+), strong diatropic responses to external magnetic field occur in both radial and tangential types of electrons, and thus confer it a characteristic tubular aromaticity. The presence of a consistent aromatic character contributes to its high thermodynamic stability. The shapes of calculated MOs of B27(+) TR can be predicted by the eigenstates of a simple model of a particle on a hollow cylinder. The number of electrons in a hollow cylinder should attain a number of (4N + 2M) with M = 0 and 1 for both radial and tangential electrons, depending on the number of non-degenerate MOs occupied, in order to properly fulfill the closed electron shells. In the case of B27(+), M = 0 for radial electrons and M = 1 for tangential electrons.

A disk-aromatic bowl cluster B30: toward formation of boron buckyballs.

The B30 boron cluster has a bowl rather than a double-ring or a triple-ring tubular structure. This bowl isomer exhibits disk-aromaticity similar to that found for B20(2-) and B19(-) clusters. We confirmed that the concept of disk-aromaticity can be applied to both planar and non-planar systems.