Superatomic Aromaticity in Cyclic Superatomic Molecules: Ligand-Protected Penta-Icosahedral [M@Au11]5 (M = Au, Pt) Clusters.

Pure or doped gold icosahedra, which can be generally viewed as superatoms, are promising candidates for cluster-assembled structures. As the first large-scale ring-like gold cluster, the report of [Au60Se2(Ph3P)10(SeR)15]+ has arisen much interest, where its Au60 core is composed of five vertex-sharing gold icosahedra in a cyclic way. From electronic characters, this Au60 core is a 40e cyclic penta-superatom network formed by five 8e closed-shell superatoms (S2P6). When more valence electrons are introduced into the penta-superatom network by atomic doping, global delocalized bonds are induced in its bonding framework. In the 42e Au60 core of the [Au60Se2Cl15]- cluster, two extra electrons occupy one delocalized π-bonding orbital formed by super D orbitals of five superatoms, resulting in superatomic π aromaticity. In the 46e [Pt@Au11]5 core of [(Pt@Au11)5Ga2Cl15] cluster, three delocalized super-π bonds are formed, which are organized in the similar way as the aromatic C5H5- molecule. The unveiling of superatomic aromaticity promotes our understanding of the stability of cyclic superatom assemblies and extends the family of superatomic bonding patterns.

The nature of stability and adsorption interactions of binary Au-Li clusters with bridge adsorption structures.

Earlier findings have confirmed that CO molecules have propensities to adsorb on low-coordinated gold atoms (top sites) of Au-based clusters, which can be treated by the Blyholder model wherein the σ donation and π-back donation take place. Here, the structural features and stability of (AuLi)n (n = 1-9) clusters were first analyzed using the GA-DFT method. The new adsorption modes, vibration frequencies and electronic interactions for Au-Li clusters with CO were investigated in detail. More excitingly, we found that CO prefers to adsorb on the bridge sites of the Au-Li clusters rather than on the top sites, which are much lower in energies than the top adsorptions, and the C-O stretching frequencies are also red-shifted. AIMD simulations show that the adsorption structures still have good thermal stability at 500 K. The density of states reveals that the electronic structures of Au-Li clusters have excellent stability for the bridge adsorptions of CO molecules. The ETS-NOCV analysis and NPA charges show that the direction of charge flow is from Au-Li clusters → CO. Our study provides an idea to elucidate the new adsorption mechanism on Au-Li clusters and the connection between the geometries and reaction properties.

Unraveling the flexible aromaticity of C13H9+/0/-: a 2D superatomic-molecule theory.

Phenalenyl (C13H9) is the smallest triangular unit of a graphene nanosheet, and has been experimentally verified to be stable in radical (C13H9˙), cationic (C13H9+), and anionic (C13H9-) states. All these three species feature high symmetry and stability as well as delocalized π electrons, a visible sign of aromaticity, but their aromatic origin remains a challenge. This work reports new chemical insights into the π electrons of C13H9+/0/- and deciphers their aromaticity using a recently emerged two-dimensional (2D) superatomic-molecule theory. 12π-C13H9+, 13π-C13H9˙, and 14π-C13H9- are seen as triangular 2D superatomic molecules ◊O3, ◊O3-, and ◊O32-, respectively, where ◊O denotes a 2D benzenoid superatom bearing 4 π electrons. Visualized superatomic Lewis structures show that each ◊O can dynamically adjust its π electrons to satisfy the superatomic sextet rule of benzene via superatomic lone pairs and covalent bonds. C13H9+/0/- are representatives of adaptive aromaticity in the 2D superatomic-molecule system, exhibiting flexible π electronic structures to achieve shell-closure. Moreover, we specially adopt a progressive methodology to study the evolution of 2D periodic materials, by applying this theory to the similar family of C6H3N7, C18H6N22 and graphitic carbon nitride (g-C3N4) crystals, and meanwhile accounting for the special stability of g-C3N4. This work enriches 2D superatomic bonding chemistry and provides a useful strategy to design new 2D functional nanostructured materials.

Compressive Symbol: A New Way to Evaluate the High-Pressure Behaviors of Energetic Tetrazole Materials.

Energetic materials may transit to different phases or decompose directly under compression. Their reactivity in the explosions can be evaluated by their high-pressure induced behaviors, including polymorphism or phase transition. Here, we applied DFT methods to understand high-pressure behaviors of four typical tetrazole derivate crystals, including 5-aminotetrazole (ATZ), 1,5-aminotetrazole (DAT), 5-hydrazinotetrazole (HTZ), and 5-azidotetrazole (ADT), under the gradually increased pressure from ambient pressure to 200 GPa. In response to the extreme-high pressures, the performances are dominated by compressibility of crystals, reflected by compressive symbols on the basis of the molecular orientation in crystals. The crystal with weak compressibility (large symbol) generally dissociates, triggered by cleavage of weak bonds. However, the crystal with low compressive symbol is generally corresponding to a pressure-induced structural transformation or phase transition.

Planar σ-Aromaticity in Ga-Doped Au Clusters.

Recently, the first example of Au-Ga clusters is synthesized and characterized, which can be described by the jellium model as a superatom with 8 valence electrons that come from the joint contribution of Au and Ga atoms, opening a whole new field for further research. Here, the structure features and stability of one Ga-doped Au cluster with magic number electrons (6 and 8) are analyzed in detail. Moreover, the valence electron fillings and chemical bonding of them are also further explored. It is found that Au3Ga and Au5Ga clusters present planar configurations, and they have higher stability than that of neighbor clusters. The AIMD simulations show that these two clusters still have a good thermal stability at 500 K. The molecular orbital analyses show that the Au3Ga and Au5Ga have three and one typical delocalization orbital throughout the whole planar spaces, respectively, following the planar σ-aromaticity rule. The ELF and LOL analyses are further performed, and the results are consistent with the molecular orbital analyses. The NICSzz-scan curves confirm that the Au3Ga is more aromatic than the Au5Ga, and the reason is that the former has more delocalized electrons than the latter. Our work opens up aromaticity studies in the Au-Ga clusters.

Unraveling the Aromatic Rule of Cyclic Superatomic Molecules in π-Conjugated Compounds.

The aromaticity of π-conjugated compounds has long been a confusing issue. Based on a recently emerged two-dimensional (2D) superatomic-molecule theory, a unified rule was built to decipher the aromaticity of cyclic superatomic molecules of π-conjugated compounds from the chemical bonding perspective. Herein, a series of planar [n]helicenes and [n]circulenes, composed of benzene, thiophene, or furfuran, are systemically studied and seen as superatomic molecules ◊On-2◊F2 or ◊On, where superatoms ◊F and ◊O denote π-conjugated units with 5 and 4 π electrons, respectively. The ascertained superatomic Lewis structures intuitively display aromaticity with each basic unit meeting the superatomic sextet rule of benzene, similar to classical valence bond theory, which is favored by the synthesized complex π-conjugated compounds comprising different numbers and kinds of subrings. The evolutionary trend of ring currents and chemical bonding suggests a local ribbon-like aromaticity in these π-conjugated compounds. Moreover, nonplanar helical π-conjugated compounds have the potential to evolve into spring-like periodic materials with excellent physical properties.

Enhanced Stability of Rhombic Dodecahedron Nb15- with Well-Organized Superatomic States.

Well-resolved Nbn- clusters are produced and reacted with ethene and propene via a downstream flow tube reactor. Interestingly, the Nbn- clusters readily react with ethene and propene to form dehydrogenation products; however, Nb15- shows up in the mass spectra with prominent mass abundance indicating its inertness to react with olefins. For this cluster, we conduct photoelectron velocity map imaging (VMI) experiments and verify the stability of Nb15- within a highly symmetrical rhombic dodecahedron structure. Theoretical studies show that the stability of the Nb15- cluster is correlated with its superatomic nature pertaining to both geometric and electronic shell closures. Notably, the superatomic 1s orbital is dominated by the 5s electron of the central Nb atom, while the other superatomic orbitals are contributed by s-d hybridization, especially a remarkable contribution of s-dz2 hybridization. Apart from the closed shells, the highly symmetric geometry of Nb15- is associated with a regular polyhedral structure directed by all rhombus facets, embodying a magic number for body-centered dodecahedra, indicative of enhanced stability as a double magic cluster free of olefin adsorption.

Theoretical study of the saturation and nature of the hydrogen bonds to gold.

Traditional hydrogen bonds are well-known to exhibit directionality and saturation. By contrast, gold involved hydrogen bonds (GHBs) have been extensively studied but remain lack of in-depth understanding towards the intrinsic nature and saturation property. This work exemplifies three series of complexes: [L-Au-L]-⋯(HF)n (L = H, CH3, (CH3)3; n = 1-8) containing GHBs to dig into the intrinsic nature with the aid of multiple theoretical analysis methods, finding that the formation of GHB is highly subject to orbital interactions along with steric hindrance. Moreover, the saturation level of GHBs largely depends on the ligand attached to the gold center, since different ligands typically possess varying electron-giving ability and steric volume. This work confirms the coexistence of as many as 6 GHBs for one Au atom and thoroughly studies the saturation level of GHBs, which will provide new insights into GHBs and facilitate future synthesis of more complicated gold complexes.

The Covalent AuI-AuI Bond in (AuF)n (n = 2∼4): A Perspective to Understand the Closed-Shell AuI···AuI Interaction.

The nature of closed-shell AuI···AuI attraction is still a conundrum in theoretical chemistry. However, for Au2F2 with a zigzag conformation, the d10-d10 closed-shell interaction between the AuF monomers is demonstrated as a coordinate covalent bond. Chemical bonding analysis reveals that the strong AuI···AuI attraction is caused by the participation of the extraordinary active 5d orbital of Au. Based on our study, one of the 5d orbitals of the Au atom is activated to hybridize with its 6s and 6p orbitals to form hybridized dsp2 orbitals, where each Au atom is both an electron donor (Lewis base) and acceptor (Lewis Acid) in dimerization. Actually, the closed-shell AuI···AuI interaction in the zigzag conformation of Au2X2 (X = F, Cl, Br, I, or NH2) is covalent. Our results provide a rather simple but clear-cut example, where mysterious AuI···AuI attractions can be possibly explained by the covalent bond theory.

Adsorption properties of pyramidal superatomic molecules based on the structural framework of the Au20 cluster.

The pyramidal Au20 cluster is a highly inert and stable superatomic molecule, but it is not suitable as a potential catalyst for covalent bond activations, e.g., CO oxidation reaction. Herein, the adsorption and electronic properties of CO molecules on various pyramidal clusters based on the structural framework of Au20 are investigated using density functional theory. According to the SVB model, we constructed isoelectronic superatomic molecules with different pyramid configurations by replacing the vertex atoms of the Au20 using metal M atoms (M = Li, Be, Ni, Cu, and Zn group atoms). After the CO molecules are adsorbed on the vertex atoms of these metal clusters, we analyzed the CO adsorption energies, C-O bond stretching frequencies, and electronic properties of the adsorption structures. It was found that the adsorption of CO molecules results in minimal changes in the parent geometries of the pyramidal clusters, and most adsorption structures are consistent with the geometry of CO adsorption at the vertex site of the Au20 cluster. There are significant red shifts when CO molecules are adsorbed on the Ni/Pd/Pt atoms of the clusters, and their CO adsorption energies were also greater. The molecular orbitals and density of states reveal that there are overlaps between the frontier orbitals of the clusters and CO, and the electronic structure of NiAu19- is not sensitive to CO. The ETS-NOCV analysis shows that the increase in the density of the bonding area caused by the orbital interactions between the fragments is higher than the decrease in the density of the bonding area caused by Pauli repulsion, presenting that the direction of charge flow in the deformation density is from CO → clusters. From energy decomposition analysis (EDA) and NPA charge, we find a predominant covalent nature of the contributions in CO⋯M interactions (σ-donation). Our study indicates that the SVB model provides a new direction to expand the superatomic catalysts from the superatom clusters, which also provides inference for the extension of the single atom catalysis.

FeP2 monolayer: isoelectronic analogue of MoS2 with excellent electronic and optical properties.

Two-dimensional semiconductors with suitable indirect band gaps, excellent light absorption capacity, and oxidation resistance are particularly suitable for material applications. Here based on first-principle calculations, we report that the FeP2 monolayer, which is isoelectronic with MoS2, has novel electronic properties and an ultra-low diffusion energy barrier of K on the surface, indicating its potential as an anode material of K-ion batteries. The calculated phonon dispersion curves, molecular dynamics, and elastic constants showed that it has high structural stability and oxidation resistance. The monolayer was a semiconductor with an indirect band gap of 0.68 eV. In addition, the FeP2 monolayer had obvious light absorption in the infrared, visible, and ultraviolet regions, which can be widely used in optoelectronic devices. Bonding analysis showed that there were multicenter bonds inside every hexagonal ring. As the anode material of K-ion batteries, the FeP2 monolayer had a capacity of 456.84 mA h g-1, low diffusion energy barrier, and open-circuit voltage. All these characteristics suggest that the FeP2 monolayer is a potential anode material for K-ion batteries, which needs to be further verified by experiments.

New insight into the electronic structure of SiF4: synergistic back-donation and the eighteen-electron rule.

SiF4 demonstrated high thermal stability in dry air or vacuum, and a Si-F bond length of 1.554 Å is close to the second period element C-C bond length (1.54 Å) of C2H6. To determine which factors confer this property of SiF4, here we conduct a comparative study of a series of molecules SiHnF4-n (n = 0, 1, 2, 3), SiX4 (X = Cl, Br, I), CF4 and TiF4 in terms of bond length and energy, molecular orbitals, and adaptive natural density partitioning (AdNDP) analysis. The AdNDP analysis shows that there are five 5c-2c bonds in SiF4, here named synergistic back-donation (SBD) bonds. These SBD bonds together with the Si-F σ bonds and the eighteen-electron rule are demonstrated as the main factors contributing to the short Si-F bond length and the high thermal stability of SiF4 in dry air or vacuum. Moreover, the SBD bonds exist widely in other isoelectronic species of SiF4 such as ClO4-, SO42-, PO43- and XeO4.

The phase stability of predicted pentazole derivate compounds under high pressure.

Pentazole as one typical extreme explosive has been applied in the synthesis of a metal pentazole complex under extremely-high pressure and low temperature. In order to evaluate the stabilities and detonation performances of pentazole complexes in possible applications, we predict four pentazole derivate molecules and crystals (dipentazole, octaazapentalene, azidopentazole and tripentazolamine) based on DFT and Monte-Carlo methods, wherein both crystalline octaazapentalene and tripentazolamine display remarkable dynamic stabilities and excellent denotation properties. To understand the relationship between the structures and gradually increased pressure, all the predicted crystal structures are studied under gradually increased pressure from ambient pressure to 200 Gpa. In response to extremely high pressures, the stability of the energetic crystals is dominated by molecular compressibility under limit states, where the bond cleavage results in structural dissociation under high pressure. However, low molecular planarization energy generally corresponds to a pressure-induced phase transition of pentazolate crystals.

Exoergic pathways triggered by O/H radicals in different metallic carbohydrazide perchlorates (M2+ = Mn2+, Fe2+, Co2+, Ni2+, Zn2+ and Cd2+).

Metallic carbohydrazide perchlorates (M[(N2H3)2C = O](ClO4)2, M2+ = Mn2+, Fe2+, Co2+, Ni2+, Zn2+ and Cd2+, simplified as MCPs) are a series of energetic primary explosives, among which ZnCP and CdCP are already applied in civilian/military fields. The six MCPs possess similar structures but demonstrate different energetic performances in their decomposition, which are obviously determined by their different central metals. Here, we apply DFT and Car-Parrinello molecular dynamics (CPMD) to understand the electronic structures and decomposition pathways of the MCPs. Based on the results, the crystal MCPs with larger electronic band gaps show lower impact sensitivity. However, the friction sensitivity of MCPs is dominated by the strength of their intermolecular O⋯H interactions. In the CPMD simulations, we obtained a different conclusion from the traditional viewpoint, where the decomposition is spontaneous from the cleavage of M-N bonds. Indeed, there are two stages in the decomposition of the MCPs, based on our calculations: (I) nonspontaneous 3-step departure of the CHZ groups and (II) spontaneous exoergic decomposition pathways of the CHZ groups triggered by the transfer of O/H radicals. Our study provides a systematic study of the MCP family, which also affords a new route for understanding the relationship between the energetic properties and electronic structures of energetic metal complexes.

Theoretical study on L-H+-L with identical donors: Short strong hydrogen bond or not?

Short strong hydrogen bonds (SSHBs) play a crucial role in many chemical processes. Recently, as the representative of SSHBs, [F-H-F]- was experimentally observed. [F-H-F]- has a symmetric structure, which can be described as a H+ acid shared by two terminal F- donors (F--H+-F-). To explore whether two identical donors are bound to result in SSHBs, we performed theoretical studies on a series of compounds (L-H+-L) with two identical electron donors (L corresponds to donors containing group 14, 15, 16, and 17 elements). The results show that identical donors do not definitely lead to SSHBs. Instead, typical hydrogen bonds also exist. Both electronegativity and basicity contribute to the patterns of hydrogen bonds, where more electronegative and weaker donors benefit to SSHBs. In addition, it was found that zero-point energies also respond to the hydrogen bonding systems. This systemic work is expected to provide more insights into SSHBs.

Benchmark studies on the large errors of calculated binding energies in metallophilic interactions.

Aurophilicity is a d10-d10 closed-shell interaction, which is repulsively calculated by the Hartree-Fork (HF) method, whereas binding energies (Eb) are largely overestimated under the second-order Møller-Plesset (MP2) method, compared to the coupled cluster singles and doubles with perturbative triples [CCSD(T)] method. The unusual energy errors between different wave functional methods were also verified in other closed-shell metallophilic systems and even were taken as a label of metallophilic interaction. Here, we performed a benchmark study on a collection of structures with weak interactions, sp-sp bonds, sp-d bonds, and d-d bonds, to investigate the influence factor of the errors of HF and MP2 methods. It was found that the large energy errors of HF and MP2 methods were not specified for closed-shell interactions, and the errors could also be very large for many covalent bonds, which was strongly related to the azimuthal quantum number of interaction orbitals. Compared to the CCSD(T) method, the MP2 method weakens the s-s covalent interactions slightly, strengthens the p-p covalent interactions slightly, and overestimates the d-d covalent interactions largely (can be -170 kcal/mol for the Re-Re quadruple bond). This benchmark study suggests that the special energy errors in metallophilicity may result from the participation of d orbitals. Benchmark studies on various density functional methods were also given for calculating binding energies of d-d bonds.

Prediction of an Al4C4 superatom organic framework (SOF) material based on the superatom network model.

Metal organic framework (MOF) materials have attracted significant attention due to their wide potential applications, but it is still a challenge to design MOFs with advanced properties by exploring novel metal nodes. In this study, a kind of superatom organic framework (SOF) material is proposed based on the superatom network (SAN) model. Tetrahedron Al4 superatom unit is used as nodes in the MOF structure, and linear -CC- ligands are chosen as linkers. Localized chemical bonding analysis and nucleus-independent chemical shift (NICS) scan confirm that the Al4 core keeps the superatom electronic shell in the SOF structure. Further calculations demonstrate that this Al4C4 crystal has high dynamic and thermal stabilities, with an indirect semiconductor band gap of 2.57 eV. Analysis of its optical properties indicates its potential applications as an optoelectronic device. This novel kind of SOF material has both porous framework as traditional MOFs and superatomic character in its nodes, indicating its unique potential properties. Our work would provide a new way for designing functional MOF materials.

Superatomic and adsorption properties of Ni atom doped Au clusters.

Due to their strong relativistic effects, Au clusters exhibit many unusual geometric structures. Among them, Au7-, Au8 and Au9+ have 18 valence electrons satisfying the magic numbers in the jellium model, respectively, but these three non-spherical clusters are not superatoms. In general, a single dopant atom can drastically change the structural and electronic properties of Au clusters. Here, we searched structures of NiAu7-, NiAu8 and NiAu9+ clusters using the genetic algorithm program (GA) combined with density functional theory (DFT). It was found that the alloy clusters are all 3D spherical structures. The molecular orbitals and density of states analysis indicate that they have completely filled superatomic shells (1S21P6), in which the electronic structure of the Ni atom is d10. Then, the electrostatic potential surfaces of the alloy clusters are analyzed, and the calculated results show that the NiAu8 superatom has remarkable σ-holes with positive potential regions. Moreover, these electron-deficient regions can be considered as interaction sites with some electron donors. After a Lewis base CO gas molecule is adsorbed on the Au-based superatom, we found that the C-O bond distance becomes slightly elongated and its stretching frequency presents a significant red-shift. This is due to the fact that 5d electrons of the Au atom of the NiAu8 transfer towards the anti-bond π orbitals of the CO molecule. Hence, this is an effective strategy for finding new types of superatoms and potential catalysts for covalent bond activation.

Revisiting the π-Back-Donation in the NHC-B≡B-NHC Molecule.

As the first thermal stable molecule with a B≡B bond, the diboryne complex protected by N-heterocyclic carbene ligands (NHC-B≡B-NHC) has attracted much interest. Researchers point out that π-back-donation highly stabilizes the B≡B bond besides σ-donation, both of which are induced by NHC ligands. In this work, details of the π-back-donation are revisited by using DFT calculations. There are two delocalized π* orbitals in NHC, and the symmetry of one π* orbital is highly adaptive to the π orbitals in B≡B bond, whereas the other cannot be involved in the π-back-donation. In staggered configuration, two orthogonal π orbitals of B≡B interact with this π* orbital in each NHC ligand, respectively, to form π-back-donations in both sides. This interaction has proven to be more intensive than π-conjunction, resulting in the lower energy of the staggered isomer compared with the eclipsed one containing greater π-conjunction. Moreover, intensity of the π-back-donation can be enhanced by reducing the energy levels of the matched π* orbitals in ligands, which gives references for the design of stable diborynes.

CPMD investigation of α-RDX and ε-CL-20: the transition of deflagration to detonation depending on the self-produced radicals.

1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) are the classic high energy nitramine compounds. Herein, we performed simulations of the overall decomposition pathways of condensed α-RDX and ε-CL-20 by applying the Car-Parrinello molecular dynamics (CPMD) method. Both of them appear to have similar distinct initial decomposition pathways, which are the bond cleavages of N-NO2 bonds. Interestingly, we find that the continuous explosion is nonspontaneous without the participation of self-produced hydrogen radicals of RDX or oxygen radicals of CL-20. Increased radicals are produced gradually with increasing temperature, which activates further entropy-increased steps, resulting in the uncontrollable transition of deflagration to detonation with the formation of NOx, COx and HCN. Herein, we provide a detailed and systematic description of the decomposition for unit-cell α-RDX and ε-CL-20 under increased temperature, which can be summarized as C3H6O6N6 (RDX) → NO + HNO + H2 + CO2 + HCHO + HNCN + N2O and C6H6O12N12 (CL-20) → NO + HONO + 5HCN + CO2 + N2O + 3NO2.