Search Results (14)

We studied the boron-based composite cluster B₈Al₃⁺ doped with Al atoms. The global minimum structure of the B₈Al₃⁺ cluster is a three-layer structure, consisting of three parts: an Al₂ unit, a B₈ ring and an isolated Al atom. Charge calculations analysis shows that the cluster can be expressed as [Al]⁺[B₈]²⁻[Al₂]²⁺, has 6π/6σ double aromaticity and follows the (4n+2) Hückel rule. Born–Oppenheimer molecular dynamics (BOMD) simulation shows that the B₈Al₃⁺ cluster has dynamic fluxionality properties. Remarkably, at the single-point coupled cluster singles, doubles and triples (CCSD(T)) level, the energy barrier for intramolecular rotation is merely 0.19 kcal mol⁻¹. [B₈]²⁻ molecular wheels have magical 6π/6σ double aromaticity properties, providing a continuous cloud of delocalized electrons, which is a key factor in the dynamic fluxionality of the cluster. The B₈Al₃⁺ cluster provides a new example of dynamic structural fluxionality in molecular systems. Full article

The aromatic boron cluster B₈^2– (D_7h) has similar π bonding to C₆H₆, which is named “borozene”. The B₈^2– ligand has been observed to stabilize monovalent Ln(+I) in C_7v-LnB₈⁻ (Ln = La, Pr, Tb, Tm, and Yb) borozene complexes. Low-valency actinide complexes have been reported more rarely, and B₈^2– may be one of the potential ligands. Here, we report a theoretical study on a series of actinide metal-doping octa-boron clusters AnB₈ (An = Pa, U, Np, and Pu). It was found that each species has both half-sandwich and chair-like structures. Except for PaB₈, the half-sandwich structures of UB₈, NpB₈, and PuB₈ are more energetically stable than the chair-like structures, and the half-sandwich clusters of AnB₈ are found to be actinide(II) borozene complexes with the M^II[B₈²⁻] type. For each of the half-sandwich clusters, the B₈²⁻ ligand has σ and π double aromaticity. Various bonding analyses of AnB₈ confirm the covalent interactions between the doped actinide metals and the octa-boron clusters, which further stabilize the complexes and determine the relative stability of AnB₈. As expected, these complexes show high bond dissociation energies, especially PaB₈ with stronger Pa-B covalent bonds. These results demonstrate that the B₈²⁻ doubly aromatic ligand is able to stabilize divalent actinides. Full article

Planar tetracoordinate silicon, germanium, tin, and lead (ptSi/Ge/Sn/Pb) species are scarce and exotic. Here, we report a series of penta-atomic ptSi/Ge/Sn/Pb XB₂Bi₂ (X = Si, Ge, Sn, Pb) clusters with 20 valence electrons (VEs). Ternary XB₂Bi₂ (X = Si, Ge, Sn, Pb) clusters possess beautiful fan-shaped structures, with a Bi–B–B–Bi chain surrounding the central X core. The unbiased density functional theory (DFT) searches and high-level CCSD(T) calculations reveal that these ptSi/Ge/Sn/Pb species are the global minima on their potential energy surfaces. Born–Oppenheimer molecular dynamics (BOMD) simulations indicate that XB₂Bi₂ (X = Si, Ge, Sn, Pb) clusters are robust. Bonding analyses indicate that 20 VEs are perfect for the ptX XB₂Bi₂ (X = Si, Ge, Sn, Pb): two lone pairs of Bi atoms; one 5c–2e π, and three σ bonds (two Bi–X 2c–2e and one B–X–B 3c–2e bonds) between the ligands and X atom; three 2c–2e σ bonds and one delocalized 4c–2e π bond between the ligands. The ptSi/Ge/Sn/Pb XB₂Bi₂ (X = Si, Ge, Sn, Pb) clusters possess 2π/2σ double aromaticity, according to the (4n + 2) Hückel rule. Full article

We present an investigation on the structures and chemical bonding of two Bi-doped boron clusters BiB_n⁻ (n = 4, 5) using photoelectron spectroscopy and theoretical calculations. The electron affinities of BiB₄ and BiB₅ are measured to be 2.22(2) eV and 2.61(2) eV, respectively. Well-resolved photoelectron spectra are obtained and used to compare with theoretical calculations to verify the structures of BiB₄⁻ and BiB₅⁻. Both clusters adopt planar structures with the Bi atom bonded to the periphery of the planar B_n moiety. Chemical bonding analyses reveal that the B_n moiety maintains σ and π double-aromaticity. The Bi atom is found to induce relatively small structural changes to the B_n moiety, very different from transition metal-doped boron clusters. Full article

As one of the important probes of chemical bonding, planar tetracoordinate carbon (ptC) compounds have been receiving much attention. Compared with ptC clusters, the heavier planar tetracoordinate silicon, germanium, tin, lead (ptSi/Ge/Sn/Pb) systems are scarcer and more exotic. The 18-valence-electron (ve)-counting is one important guide, though not the only rule, for the design of planar tetra-, penta-coordinate carbon and silicon clusters. The 18ve ptSi/Ge system is very scarce and needs to be expanded. Based on the isoelectronic principle and bonding similarity between the Al atom and the BeH unit, inspired by the previously reported ptSi global minimum (GM) SiAl₄²⁻, a series of ternary 18 ve XBe4H5⁻ (X = Si, Ge, Sn, Pb) clusters were predicted with the ptSi/Ge/Sn/Pb centers. Extensive density functional theory (DFT) global minimum searches and high-level CCSD(T) calculations performed herein indicated that these ptSi/Ge/Sn/Pb XBe₄H₅⁻ (X = Si, Ge, Sn, Pb) clusters were all true GMs on their potential energy surfaces. These GMs of XBe₄H₅⁻ (X = Si, Ge, Sn, Pb) species possessed the beautiful fan-shaped structures: XBe₄ unit can be stabilized by three peripheries bridging H and two terminal H atoms. It should be noted that XBe₄H₅⁻ (X = Si, Ge, Sn, Pb) were the first ternary 18 ve ptSi/Ge/Sn/Pb species. The natural bond orbital (NBO), canonical molecular orbitals (CMOs) and adaptive natural densitpartitioning (AdNDP) analyses indicated that 18ve are ideal for these ptX clusters: delocalized one π and three σ bonds for the XBe₄ core, three Be-H-Be 3c-2e and two Be-H σ bonds for the periphery. Additionally, 2π plus 6σ double aromaticity was found to be crucial for the stability of the ptX XBe₄H₅⁻ (X = Si, Ge, Sn, Pb) clusters. The simulated photoelectron spectra of XBe₄H₅⁻ (X = Si, Ge, Sn, Pb) clusters will provide theoretical basis for further experimental characterization. Full article

Inverse sandwich clusters composed of a monocyclic boron ring and two capping transition metal atoms are interesting alloy cluster systems, yet their chemical bonding nature has not been sufficiently elucidated to date. We report herein on the theoretical prediction of a new example of boron-based inverse sandwich alloy clusters, V₂B₇⁻, through computational global-minimum structure searches and quantum chemical calculations. This alloy cluster has a heptatomic boron ring as well as a perpendicular V₂ dimer unit that penetrates through the ring. Chemical bonding analysis suggests that the inverse sandwich cluster is governed by globally delocalized 6π and 6σ frameworks, that is, double 6π/6σ aromaticity following the (4n + 2) Hückel rule. The skeleton B−B σ bonding in the cluster is shown not to be strictly Lewis-type two-center two-electron (2c-2e) σ bonds. Rather, these are quasi-Lewis-type, roof-like 4c-2e V−B₂−V σ bonds, which amount to seven in total and cover the whole surface of inverse sandwich in a truly three-dimensional manner. Theoretical evidence is revealed for a 2c-2e Lewis σ single bond within the V₂ dimer. Direct metal–metal bonding is scarce in inverse sandwich alloy clusters. The present inverse sandwich alloy cluster also offers a new type of electronic transmutation in physical chemistry, which helps establish an intriguing chemical analogy between inverse sandwich clusters and planar hypercoordinate molecular wheels. Full article

Planar tetracoordinate carbon (ptC) species are scarce and exotic. Introducing four peripheral Te/Po auxiliary atoms is an effective strategy to flatten the tetrahedral structure of CAl₄ (T_d, ¹A₁). Neutral CAl₄X₄ (X = Te, Po) clusters possess quadrangular star structures containing perfect ptC centers. Unbiased density functional theory (DFT) searches and high-level CCSD(T) calculations suggest that these ptC species are the global minima on the potential energy surfaces. Bonding analyses indicate that 40 valence-electron (VE) is ideal for the ptC CAl₄X₄ (X = Te, Po): one delocalized π and three σ bonds for the CAl₄ core; four lone pairs (LPs) of four X atoms, eight localized Al–X σ bonds, and four delocalized Al–X–Al π bonds for the periphery. Thus, the ptC CAl₄X₄ (X = Te, Po) clusters possess the stable eight electron structures and 2π + 6σ double aromaticity. Born–Oppenheimer molecular dynamics (BOMD) simulations indicate that neutral ptC CAl₄X₄ (X = Te, Po) clusters are robust. Full article

Doping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters, owing to the small electronegativity of the former elements. We report on the computational design of a three-layered sandwich cluster, Na₅B₇, on the basis of global-minimum (GM) searches and electronic structure calculations. It is shown that the Na₅B₇ cluster can be described as a charge-transfer complex: [Na₄]²⁺[B₇]³⁻[Na]⁺. In this sandwich cluster, the [B₇]³⁻ core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6π/6σ double aromaticity underlies the stability of the [B₇]³⁻ molecular wheel, following the (4n + 2) Hückel rule. The tetrahedral Na₄ ligand in the sandwich has a [Na₄]²⁺ charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2σ electron counting renders σ aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6π/6σ/2σ aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature, with a negligible energy barrier for intramolecular twisting between the B₇ wheel and the Na₄ ligand. The Na₅B₇ cluster offers a new example for dynamic structural fluxionality in molecular systems. Full article

A chemical bonding of several metallabenzenes and metallabenzynes was studied via an adaptive natural density partitioning (AdNDP) algorithm and the induced magnetic field analysis. A unique chemical bonding pattern was discovered where the M=C (M: Os, Re) double bond coexists with the delocalized 6c-2e π-bonding elements responsible for aromatic properties of the investigated complexes. In opposition to the previous description where 8 delocalized π-electrons were reported in metallabenzenes and metallabenzynes, we showed that only six delocalized π-electrons are present in those molecules. Thus, there is no deviation from Hückel’s aromaticity rule for metallabenzynes/metallabenzenes complexes. Based on the discovered bonding pattern, we propose two thermodynamically stable novel molecules that possess not only π-delocalization but also retain six σ-delocalized electrons, rendering them as doubly aromatic species. As a result, our investigation gives a new direction for the search for carbon-metal doubly aromatic molecules. Full article

We have explored the chemical space of BAl${}_4$Mg${}^{-/0/+}$ for the first time and theoretically characterized several isomers with interesting bonding patterns. We have used chemical intuition and a cluster building method based on the tabu-search algorithm implemented in the Python program for aggregation and reaction (PyAR) to obtain the maximum number of possible stationary points. The global minimum geometries for the anion (1a) and cation (1c) contain a planar tetracoordinate boron (ptB) atom, whereas the global minimum geometry for the neutral (1n) exhibits a planar pentacoordinate boron (ppB) atom. The low-lying isomers of the anion (2a) and cation (3c) also contain a ppB atom. The low-lying isomer of the neutral (2n) exhibits a ptB atom. Ab initio molecular dynamics simulations carried out at 298 K for 2000 fs suggest that all isomers are kinetically stable, except the cation 3c. Simulations carried out at low temperatures (100 and 200 K) for 2000 fs predict that even 3c is kinetically stable, which contains a ppB atom. Various bonding analyses (NBO, AdNDP, AIM, etc.) are carried out for these six different geometries of BAl${}_4$Mg${}^{-/0/+}$ to understand the bonding patterns. Based on these results, we conclude that ptB/ppB scenarios are prevalent in these systems. Compared to the carbon counter-part, CAl${}_4$Mg${}^{-}$, here the anion (BAl${}_4$Mg${}^{-}$) obeys the 18 valence electron rule, as B has one electron fewer than C. However, the neutral and cation species break the rule with 17 and 16 valence electrons, respectively. The electron affinity (EA) of BAl${}_4$Mg is slightly higher (2.15 eV) than the electron affinity of CAl${}_4$Mg (2.05 eV). Based on the EA value, it is believed that these molecules can be identified in the gas phase. All the ptB/ppB isomers exhibit $\pi$/$\sigma$ double aromaticity. Energy decomposition analysis predicts that the interaction between BAl${}_4^{-/0/+}$ and Mg is ionic in all these six systems. Full article

Elements from groups 14–18 and periods 3–6 commonly behave as Lewis acids, which are involved in directional noncovalent interactions (NCI) with electron-rich species (lone pair donors), π systems (aromatic rings, triple and double bonds) as well as nonnucleophilic anions (BF₄⁻, PF₆⁻, ClO₄⁻, etc.). Moreover, elements of groups 15 to 17 are also able to act as Lewis bases (from one to three available lone pairs, respectively), thus presenting a dual character. These emerging NCIs where the main group element behaves as Lewis base, belong to the σ–hole family of interactions. Particularly (i) tetrel bonding for elements belonging to group 14, (ii) pnictogen bonding for group 15, (iii) chalcogen bonding for group 16, (iv) halogen bonding for group 17, and (v) noble gas bondings for group 18. In general, σ–hole interactions exhibit different features when moving along the same group (offering larger and more positive σ–holes) or the same row (presenting a different number of available σ–holes and directionality) of the periodic table. This is illustrated in this review by using several examples retrieved from the Cambridge Structural Database (CSD), especially focused on σ–hole interactions, complemented with molecular electrostatic potential surfaces of model systems. Full article

The minimum energy structures of the Si₃C₅ and Si₄C₈ clusters are planar and contain planar tetracoordinate carbons (ptCs). These species have been classified, qualitatively, as global (π) and local (σ) aromatics according to the adaptive natural density partitioning (AdNDP) method, which is an orbital localization method. This work evaluates these species’ aromaticity, focusing on confirming and quantifying their global and local aromatic character. For this purpose, we use an orbital localization method based on the partitioning of the molecular space according to the topology of the electronic localization function (LOC-ELF). In addition, the magnetically induced current density is analyzed. The LOC-ELF-based analysis coincides with the AdNDP study (double aromaticity, global, and local). Moreover, the current density analysis detects global and local ring currents. The strength of the global and local current circuit is significant, involving 4n + 2 π- and σ-electrons, respectively. The latter implicates the Si-ptC-Si fragment, which would be related to the 3c-2e σ-bond detected by the orbital localization methods in this fragment. Full article

Isomers of CAl₄Mg and CAl₄Mg⁻ have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl₄Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl₄Mg^0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl₄Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability. Full article

The NICS_zz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICS_zz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 Å with the induced diatropic ring current arising from T_x,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICS_zz-scan curves (half-band width approximately higher than 3 Å) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from T_x,y-allowed transitions involving both σ- and π-type molecular orbitals. NICS_zz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICS_zz(0) value is close to zero and the molecule exhibits pure π-aromaticity. Full article