Anion Photoelectron Spectroscopy and Theoretical Studies of Al4C6-/0: Global Minimum Triangle-Shaped Structures and Hexacoordinated Aluminum

Planar Hexacoordinate Beryllium: Covalent Bonding Between s-block Metals

Achieving a planar hypercoordinate arrangement of s-block metals through covalent bonding with ligands is challenging due to the strong ionicity involved. Herein, we report the first case of a neutral binary global minimum containing a planar hexacoordinate beryllium atom. The central Be atom is coordinated by six active Be atoms, the latter in turn are enclosed by an equal number of more electronegative chlorine atoms in the periphery, forming a star-like phBe cluster (Be©Be6 Cl6 ). Importantly, the cluster exhibits dynamically stabilized stemming geometrically from the appropriate matching of metal-ligand size and electronically from adherence to the octet rule as well as possessing a 6σ/2π double aromaticity. Remarkably, energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) analysis reveals a significant covalent interaction between the ligand and the central metal beryllium atoms, a fact further supported by a large Wiberg bond index. This cluster is a promising synthetic as its excellent electronic, dynamic and thermodynamic stability.

Reconsidering the Structures of C2 Al4 - and C2 Al5. Chemistry 2023;29:e202301338. [PMID: 37498677 DOI: 10.1002/chem.202301338]

The study of C2 Al4 -/0 and C2 Al5 -/0 was conducted using anion photoelectron spectroscopy and quantum chemical computations. The present findings reveal that C2 Al4 - has a boat-like structure, with a single C2 unit surrounded by four aluminum atoms. In contrast, the neutral C2 Al4 species adopts a D2h planar structure with two planar tetracoordinate carbon (ptC) units, consistent with previous reports. Furthermore, the global minimum isomer of C2 Al5 - adopts a D3h symmetry, where the C2 unit interacts with five aluminum atoms. It was also found that a lower symmetry structure of C2 Al5 - , where all five aluminum atoms are located on the same side of the C2 unit, albeit slightly higher in energy compared to the D3h structure. These computations show that the D3h structure of C2 Al5 - is highly stable, exhibiting a large HOMO-LUMO gap.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be₂M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be₂Al and Be₂Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (T_c) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their T_c. The high thermochemical and kinetic stability of Be₂M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be₂M on the electride Ca₂N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be₂M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations

Carbon-doped aluminum cluster anions, Al_nC^- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of Al_nC^- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al₆C^- is a carbon endohedral triangular prism. The Al₇C^- anion is a magic cluster with high stability. The structures of Al_7-9C^- can be viewed as the additional aluminum atoms attached around the triangular prism Al₆C^-. Two isomers of Al₁₀C^- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al₉C^- with the carbon atom located in a pentagonal bipyramid. The Al₁₁C^- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al_12-14C^- can be viewed as evolving from Al₁₁C^- by adding aluminum atoms to interact with the carbon atom. In Al₁₅C^-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al₁₃ unit can be identified as a part of the geometric structure of Al₁₅C^-.

σ-Aromaticity in planar pentacoordinate aluminium and gallium clusters

Planar hypercoordinate structures are gaining immense attention due to the shift from common paradigm. Herein, our high level ab initio calculations predict that planar pentacoordinate aluminium and gallium centres in Cu5Al2+ and Cu5Ga2+ clusters are global minima in their singlet ground states. These clusters are thermodynamically and kinetically very stable. Detailed electronic structure analyses reveal the presence of σ-aromaticity which is the driving force for the stability of the planar form.

Planar hexacoordinate gallium

We report the first planar hexacoordinate gallium (phGa) center in the global minimum of the GaBe6Au6 + cluster which has a star-like D 6h geometry with 1A1g electronic state, possessing a central gallium atom encompassed by a Be6 hexagon and each Be-Be edge is further capped by an Au atom. The electronic delocalization resulting in double aromaticity (both σ and π) provides electronic stability in the planar form of the GaBe6Au6 + cluster. The high kinetic stability of the title cluster is also understood by Born-Oppenheimer molecular dynamics simulations. The energy decomposition analysis in combination with the 'natural orbitals for chemical valence' theory reveals that the bonding in the GaBe6Au6 + cluster is best expressed as the doublet Ga atom with 4s24p⊥ 1 electronic configuration forming an electron-sharing π bond with the doublet Be6Au6 + moiety followed by Ga(s)→[Be6Au6 +] σ-backdonation and two sets of Ga(p‖)←[Be6Au6 +] σ-donations.

Shape Symmetrization and IR-Spectral Enhancement of Aluminum Clusters via Doping with a Carbon Core

Composite nanosystems are a class of objects with interesting and potentially useful properties. Here we study mixed-composition species representing interfaces at the molecular level between such technologically relevant materials as carbon and aluminum. Specifically, core-shell C₈@Al_n (n = 16, 18) species and their isomers with the core and relaxed-shell attached outside are investigated at a DFT level in terms of structures and stabilities, charge distributions and polarities, and IR spectra and electron affinities. Among the interesting findings is the possibility of bringing the aluminum cluster into a more symmetric shape (thus making a convenient building block) via insertion of a suitable molecular-carbon skeleton. Another notable feature is the system-selective dependence of polarity on spin multiplicity, suggesting possible molecular-electronic applications. The IR spectra of the composite species are much brighter compared to those of the separated components and are highly focused for the core-shell isomers. A related aspect of interest is the apparent reflections of the system structural details in the IR spectra features (line intensities and separations) via related vibrations, facilitating an experimental analysis of the structure and detection of the species formation and transformation as well as potentially enabling the means of achieving desirable optical characteristics via a geometric design.