Double σ-Aromaticity in a Planar Zinc-Doped Gold Cluster: Au9Zn. J Phys Chem A 2021;125:4606-4613. [PMID: 34014680 DOI: 10.1021/acs.jpca.1c02954]

Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAun clusters (n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UFn species, but significant deviations are seen starting with UAu5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

Planar pentacoordinate s-block metals

The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level ab initio computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa5, Li5Mg+, Na5Mg+, K5Ca+, CaRb5+, Rb5Sr+, and SrCs5+) adopts a singlet D5h structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity.

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.

Density Functional Study to Investigate the Ability of (ZnS)n (n = 1-12) Clusters Removing Hg0, HgCl, and HgCl2 via Electron Localization Function and Non-Covalent Interactions Analyses

In this study, the density functional theory is used to study the ability of (ZnS)_n clusters to remove Hg⁰, HgCl, and HgCl₂ and reveals that they can be absorbed on (ZnS)_n clusters. According to electron localization function (ELF) and non-covalent interactions (NCI) analyses, the adsorption of Hg⁰ on (ZnS)_n is physical adsorption and the adsorption ability of (ZnS)_n for removing Hg⁰ is weak. When (ZnS)_n adsorbs HgCl and HgCl₂, two new Hg-S and Zn-Cl bonds form in the resultant clusters. An ELF analysis identifies the formation of Hg-S and Zn-Cl bonds in (ZnS)_nHgCl and (ZnS)_nHgCl₂. A partial density of states and charge analysis confirm that as Hg⁰, HgCl, and HgCl₂ approach (ZnS)_n clusters, atomic orbitals in Hg and Zn, Hg and S, as well as Zn and Cl overlap and hybridize. Adsorption energies of HgCl and HgCl₂ on (ZnS)_n clusters are obviously bigger than those of Hg⁰, indicating that HgCl and HgCl₂ adsorption on (ZnS)_n clusters is much stronger than that of Hg⁰. By combining ELF analysis, NCI analysis, and adsorption energies, the adsorption of HgCl, and HgCl₂ on (ZnS)_n clusters can be classified as chemical adsorption. The adsorption ability of (ZnS)_n clusters for removing HgCl and HgCl₂ is higher than that of Hg⁰.

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.

Planar Pentacoordinate Zinc Group Elements Stabilized by Multicentric Bonds

Planar pentacoordinate zinc group elements, (M = Zn, Cd, Hg) were computationally found to be at a global minimum in Li₅M⁺ clusters. The stability of these clusters is due to the presence of multicentric bonds. The central element (Zn, Cd, Hg) in each cluster features a negative oxidation state owing to the in-plane electron donation by the Li₅⁺ framework. A similar global minimum planar pentacoordinate structure is found in Na₅Zn⁺ and Na₅Cd⁺ clusters.

σ-Aromaticity Planar Pentacoordinate Beryllium Atoms

Six-valence-electron planar pentacoordinate beryllium (ppBe) is explored herein as a global minimum, which is only constructed by s-block metals in BeM₅⁺ (M = Cu, Ag, Au). The bonding in ppBe can be regarded as the excited-stated Be with a 2p_x¹2p_y¹ electronic configuration, forming electron sharing with doublet M₅⁺ motifs followed by two sets of Be(p_∥) → [M₅⁺] σ donations and one Be(s) ← [M₅⁺] σ back-donation. Thus, the σ aromaticity originating from three delocalized σ orbitals gives rise to the whole stability of the high D_5h-symmetry ppBe and strongly enriches s-block planar hypercoordinate bonding.

Photoelectron Spectroscopy of Size-Selected Bismuth-Boron Clusters: BiBn- (n = 6-8)

Because of its low toxicity, bismuth is considered to be a "green metal" and has received increasing attention in chemistry and materials science. To understand the chemical bonding of bismuth, here we report a joint experimental and theoretical study on a series of bismuth-doped boron clusters, BiB_n^- (n = 6-8). Well-resolved photoelectron spectra are obtained and are used to understand the structures and bonding of BiB_n^- in conjunction with theoretical calculations. Global minimum searches find that all three BiB_n^- clusters have planar structures with the Bi atom bonded to the edge of the planar B_n moiety via two Bi-B σ bonds as well as π bonding by the 6p_z orbital. BiB₆^- is found to consist of a double-chain B₆ with a terminal Bi atom. Both BiB₇^- and BiB₈^- are composed of a Bi atom bonded to the planar global minima of the B₇^- and B₈^- clusters. Chemical bonding analyses reveal that BiB₆^- is doubly antiaromatic, whereas BiB₇^- and BiB₈^- are doubly aromatic. In the neutral BiB_n (n = 6-8) clusters, except BiB₆ which has a planar structure similar to the anion, the global minima of both BiB₇ and BiB₈ are found to be half-sandwich-type structures due to the high stability of the doubly aromatic B₇^3- and B₈^2- molecular wheel ligands.