Lin-Yan Feng

Coaxial Triple-Layered versus Helical Be6B11- Cluster: Dual Structural Fluxionality and Multifold Aromaticity

Two low-lying structures are unveiled for the Be6 B11- nanocluster system that are virtually isoenergetic. The first, triple-layered cluster has a peripheral B11 ring as central layer, being sandwiched by two Be3 rings in a coaxial fashion, albeit with no discernible interlayer Be-Be bonding. The B11 ring revolves like a flexible chain even at room temperature, gliding freely around the Be6 prism. At elevated temperatures (1000 K), the Be6 core itself also rotates; that is, two Be3 rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four-fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix-type cluster contains a B11 helical skeleton encompassing a distorted Be6 prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple-layered one.

Star-Like CBe5Au5+ Cluster: Planar Pentacoordinate Carbon, Superalkali Cation, and Multifold (π and σ) Aromaticity

We report on the computational design of star-like CBe5Au5+ cluster with planar pentacoordinate carbon (ppC), which is also classified as a superalkali cation. Relevant isovalent CBe5Aunn-4 (n = 2-4), BBe5Au5, and NBe5Au52+ clusters with ppC/B/N are studied as well. Global-minimum structures of the clusters are established via computer global searches. The species feature a pentacoordinate pentagonal XBe5 (X = C, B, N) core, with Au occupying outer bridging positions. Molecular dynamics simulations indicate that they are dynamically stable. Bonding analysis reveals 3-fold (π and σ) aromaticity in CBe5Au5+, a key concept that overrides the 18-electron rule and should be applicable for (or help revisit existing models of) other planar hypercoordinate systems. Vertical electron affinities of CBe5Au5+ and its lighter counterparts (CBe5Cu5+ and CBe5Ag5+) are calculated to be unusually low, which are below 3.89 eV, the smallest atomic ionization potential of any element in the periodic table. Thus, these three clusters belong to superalkali cations. The merge of ppC and superalkali characters makes them unique chemical species.

Chemical Bonding and σ-Aromaticity in Charged Molecular Alloys: [Pd 2 As 14 ] 4− and [Au 2 Sb 14 ] 4− Clusters

We report a computational study on the structures and bonding of a charged molecular alloy D2h [Pd2As14]4− (1), as well as a model D2h [Au2Sb14]4− (2) cluster. Our effort makes use of an array of quantum chemistry tools: canonical molecular orbital analysis, adaptive natural density partitioning, natural bond orbital analysis, orbital composition analysis, and nucleus independent chemical shift calculations. Both clusters consist of two X7 (X = As, Sb) cages, which are interconnected via a M2 (M = Pd, Au) dumbbell, featuring two distorted square-planar MX4 units. Excluding the Pd/As or Au/Sb lone-pairs, clusters 1 and 2 are 50- and 44-electron systems, respectively, of which 32 electrons are for two-center two-electron (2c-2e) As-As or Sb-Sb σ bonds and an additional 16 electrons in 1 for 2c-2e Pd-As σ bonds. No covalent Pd-Pd or Au-Au bond is present in the systems. Cluster 1 is shown to possess two globally delocalized σ electrons, whereas 2 has two σ sextets (each associated with an AuSb4 fragment). Thus, 1 and 2 conform to the (4n + 2) Hückel rule, for n = 0 and 1, respectively, rendering them σ-aromaticity.

Dynamic Mg2B8 Cluster: A Nanoscale Compass

Boron-based binary cluster Mg2 B8 is shown to adopt a compass-like structure via computational global searches, featuring an Mg2 dimer as the needle and a disk-shaped B8 molecular wheel as baseplate. The nanocompass has a diameter of 0.35 nm. Born-Oppenheimer molecular dynamics simulations indicate that Mg2 B8 is structurally fluxional with the needle rotating freely on the baseplate, analogous to a functioning compass. The dynamics is readily initiated via a ultrasoft vibrational mode. The rotational barrier is only 0.1 kcal mol-1 at the single-point CCSD(T) level. Chemical bonding analysis suggests that the cluster compass can be formulated as [Mg2 ]2+ [B8 ]2- ; that is, the baseplate and the needle are held together primarily through ionic interactions. The baseplate is doubly aromatic with π and σ sextets. The bonding pattern provides a dilute, continuous, and delocalized electron cloud, which underlies the dynamics of the nanocompass.

Planar Pentacoordinate versus Tetracoordinate Carbons in Ternary CBe4Li4 and CBe4Li42– Clusters

Planar hypercoordinate carbon molecules are exotic species, for which the 18-electron counting has been considered a rule. We report herein computational evidence of perfectly planar C2 v CBe4Li4 (1) and D4 h CBe4Li42- (3) clusters. These ternary species contain 16 and 18 electrons, respectively. The dianion is highly symmetric with a planar tetracoordinate carbon (ptC), whereas the neutral features a planar pentacoordinate carbon (ppC). Thus, charge-state alters the coordination environments of a cluster. Chemical bonding analysis shows that both clusters have 2π and 6σ delocalization around the C center, suggesting that ppC or ptC clusters are governed by double π/σ aromaticity, rather than the 18-electron rule. The outer Be4Li4 ring in 1 and 3 also supports 2σ aromaticity, collectively leading to 3-fold π/σ aromaticity for these ppC/ptC clusters. Structural transformation from ptC (3) to ppC (1) is discussed, in which the 16-electron quasi-ptC CBe4Li4 (2) cluster serves as an intermediate. Cluster 2 as a local minimum has severe out-of-plane distortion. Flattening of 2 leads to reorganization of Be4 ring around the C center, which offers space for the fifth atom to coordinate and facilitates ppC formation. The latter arrangement optimizes π aromaticity and better manages intramolecular Coulomb repulsion. This work highlights the geometric factor (and unconventional electron counting) in the design of planar hypercoordinate carbons.

Planar Tricyclic B8O8 and B8O8– Clusters: Boron Oxide Analogues of s-Indacene C12H8

Boron clusters and their oxides are electron-deficient species with (π and σ) aromaticity and antiaromaticity, enabling a structural and bonding analogy between them and the aromatic hydrocarbons. s-Indacene C12H8 is normally considered as a border system between the classes of aromatic and antiaromatic hydrocarbons. We show herein, via computer global-minimum searches and B3LYP and single-point CCSD(T) calculations, that boron oxide clusters D2h B8O8 (1, 1Ag) and D2h B8O8- (2, 2B2g) adopt planar tricyclic structures, which feature fused heterocyclic B3O2/B4O2/B3O2 rings and two boronyl (BO) terminals, a structural pattern analogous to the C5/C6/C5 rings in s-indacene. Bonding analyses indicate that B8O8 (1) is a formally antiaromatic 12π system, the molecular orbitals of which are largely similar to those of s-indacene. Infrared and ultraviolet-visible spectra of B8O8 (1) neutral, as well as the photoelectron spectrum of B8O8- (2) anion, are predicted computationally. The latter spectrum shows a sizable energy gap of 3.5 eV for 2, demonstrating the electronic robustness of 1. Our bonding analyses also shed critical light on the nature of bonding in s-indacene.