Alexander I. Boldyrev

A concentric planar doubly π-aromatic B19− cluster

Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B₁₉⁻ cluster, which shows chemical bonding reminiscent of that in [10]annulene (C₁₀H₁₀) and [6]circulene (C₂₄H₁₂). Photoelectron spectroscopy reveals a relatively simple spectrum for B₁₉⁻, with a high electron-binding energy. Theoretical calculations show that the global minimum of B₁₉⁻ is a nearly circular planar structure with a central B₆ pentagonal unit bonded to an outer B₁₃ ring. Chemical bonding analyses reveal that the B₁₉⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.

Structure of the NaxClx+1− (x=1–4) clusters via ab initio genetic algorithm and photoelectron spectroscopy

The application of the ab initio genetic algorithm with an embedded gradient has been carried out for the elucidation of global minimum structures of a series of anionic sodium chloride clusters, Na(x)Cl(x+1) (-) (x=1-4), produced in the gas phase using electrospray ionization and studied by photoelectron spectroscopy. These are all superhalogen species with extremely high electron binding energies. The vertical electron detachment energies for Na(x)Cl(x+1) (-) were measured to be 5.6, 6.46, 6.3, and 7.0 eV, for x=1-4, respectively. Our ab initio gradient embedded genetic algorithm program detected the linear global minima for NaCl(2) (-) and Na(2)Cl(3) (-) and three-dimensional structures for the larger species. Na(3)Cl(4) (-) was found to have C(3v) symmetry, which can be viewed as a Na(4)Cl(4) cube missing a corner Na(+) cation, whereas Na(4)Cl(5) (-) was found to have C(4v) symmetry, close to a 3x3 planar structure. Excellent agreement between the theoretically calculated and the experimental spectra was observed, confirming the obtained structures and demonstrating the power of the developed genetic algorithm technique.

First experimental photoelectron spectra of superhalogens and their theoretical interpretations

Photoelectron spectra of the MX2− (M=Li, Na; X=Cl, Br, I) superhalogen anions have been obtained for the first time. The first vertical detachment energies (VDEs) were measured to be 5.92±0.04 (LiCl2−), 5.86±0.06 (NaCl2−), 5.42±0.03 (LiBr2−), 5.36±0.06 (NaBr2−), 4.88±0.03 (LiI2−), and 4.84±0.06 eV (NaI2−), which are all well above the 3.61 eV electron detachment energy of Cl−, the highest among atomic anions. Experimental photoelectron spectra have been assigned on the basis of ab initio outer valence Green function (OVGF) calculations. The corresponding theoretical first VDEs were found to be 5.90 (LiCl2−), 5.81 (NaCl2−), 5.48 (LiBr2−), 5.43 (NaBr2−), 4.57 (LiI2−), and 4.50 eV (NaI2−), in excellent agreement with the experimental values. Photodetachment from the top four valence molecular orbitals (2σg22σu21πu41πg4) of MX2− was observed. Analysis of the polestrength showed that all electron detachment channels in this study can be described as primarily one-electron processes.

Search for the Lin0/+1/-1 (n = 5−7) Lowest-Energy Structures Using the ab Initio Gradient Embedded Genetic Algorithm (GEGA). Elucidation of the Chemical Bonding in the Lithium Clusters

We report the study of small lithium clusters Lin(0/+1/)(-)(1) (n = 5-7), performed via the novel Gradient Embedded Genetic Algorithm (GEGA) technique and molecular orbital analysis. GEGA was developed for searching of the lowest-energy structures of clusters. Results of our search, obtained using this program, have been compared with the previous ab initio calculations, and the efficiency of the developed GEGA method has thus been confirmed. The molecular orbital analysis of the found Lin(0/+1/)(-)(1) (n = 5-7) clusters showed the presence of multiple (σ and π) aromatic character in their chemical bonding, which governs their preferable shapes and special stability.

Comprehensive analysis of chemical bonding in boron clusters.

We present a comprehensive analysis of chemical bonding in pure boron clusters. It is now established in joint experimental and theoretical studies that pure boron clusters are planar or quasi‐planar at least up to twenty atoms. Their planarity or quasi‐planarity was usually discussed in terms of π‐delocalization or π‐aromaticity. In the current article, we demonstrated that one cannot ignore σ‐electrons and that the presence of two‐center two‐electron (2c2e) peripheral BB bonds together with the globally delocalized σ‐electrons must be taken into consideration when the shape of pure boron cluster is discussed. The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n + 2 (or 4n) electron counting rule for either π‐ or σ‐electrons in the planar structures. We showed that pure boron clusters could have double (σ‐ and π‐) aromaticity (B  3− , B4, B  5+ , B  62+ , B  7+ , B  7− , B8, B  82− , B  9− , B10, B  11+ , B12, and B  13+ ), double (σ‐ and π‐) antiaromaticity (B  62− , B15), or conflicting aromaticity (B5−,σ‐antiaromatic and π‐aromatic and B14, σ‐aromatic and π‐antiaromatic). Appropriate geometric fit is also an essential factor, which determines the shape of the most stable structures. In all the boron clusters considered here, the peripheral atoms form planar cycles. Peripheral 2c2e BB bonds are built up from s to p hybrid atomic orbitals and this enforces the planarity of the cycle. If the given number of central atoms (1, 2, 3, or 4) can perfectly fit the central cavity then the overall structure is planar. Otherwise, central atoms come out of the plane of the cycle and the overall structure is quasi‐planar.

Electronic structure and chemical bonding of B5− and B5 by photoelectron spectroscopy and ab initio calculations

The electronic structure and chemical bonding of B5− and B5 were investigated using anion photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were obtained for B5− and were compared to theoretical calculations performed at various levels of theory. Extensive searches were carried out for the global minimum of B5−, which was found to have a planar C2v structure with a closed-shell ground state (1A1). Excellent agreement was observed between ab initio detachment energies and the experimental spectra, firmly establishing the ground-state structures for both B5− and B5. The chemical bonding in B5− was investigated and compared to that in Al5−. While both B5− and Al5− have a similar C2v planar structure, their π-bonding orbitals are different. In Al5−, a π-bonding orbital was previously observed to delocalize over only the three central atoms in the C2v ground-state structure, whereas a similar π orbital (1b1) was found to completely delocalize over all five atom...

Revealing Intuitively Assessable Chemical Bonding Patterns in Organic Aromatic Molecules via Adaptive Natural Density Partitioning

The newly developed adaptive natural density partitioning (AdNDP) method has been applied to a series of organic aromatic mono- and polycyclic molecules, including cyclopropenyl cation, cyclopentadienyl anion, benzene, naphthalene, anthracene, phenanthrene, triphenylene, and coronene. The patterns of chemical bonding obtained by AdNDP are consistent with chemical intuition and lead to unique, compact, graphic formulas. The resulting bonding patterns avoid resonant description and are always consistent with the point symmetry of the molecule. The AdNDP representation of aromatic systems seamlessly incorporates localized and delocalized bonding elements.

B22- and B23-: all-boron analogues of anthracene and phenanthrene.

Clusters of boron atoms exhibit intriguing size-dependent structures and chemical bonding that are different from bulk boron and may lead to new boron-based nanostructures. We report a combined photoelectron spectroscopic and ab initio study of the 22- and 23-atom boron clusters. The joint experimental and theoretical investigation shows that B(22)(-) and B(23)(-) possess quasi-planar and planar structures, respectively. The quasi-planar B(22)(-) consists of fourteen peripheral atoms and eight interior atoms in a slightly buckled triangular lattice. Chemical bonding analyses of the closed-shell B(22)(2-) species reveal seven delocalized π orbitals, which are similar to those in anthracene. B(23)(-) is a perfectly planar and heart-shaped cluster with a pentagonal cavity and a π-bonding pattern similar to that in phenanthrene. Thus, B(22)(-) and B(23)(-), the largest negatively charged boron clusters that have been characterized experimentally to date, can be viewed as all-boron analogues of anthracene and phenanthrene, respectively. The current work shows not only that boron clusters are planar at very large sizes but also that they continue to yield surprises and novel chemical bonding analogous to specific polycyclic aromatic hydrocarbons.

Deciphering Chemical Bonding in Golden Cages

The recently developed adaptive natural density partitioning (AdNDP) method has been applied to a series of golden clusters. The pattern of chemical bonding in Au(20) revealed by AdNDP shows that 20 electrons form a four-center-two-electron (4c-2e) bond in each of 10 tetrahedral cavities of the Au(20) cluster. This chemical bonding picture can readily explain the tetrahedral structure of the Au(20) cluster. Furthermore, we demonstrate that the recovered 4c-2e bonds corresponding to independent structural fragments of the cluster provide important information about chemically relevant fragmentation of Au(20). In fact, some of these bonds can be removed from the initial tetrahedral structure together with the associated atomic fragments, leading to the family of smaller gold clusters. Chemical bonding in the systems formed in such a manner is yet closely related to the bonding in the parental systems showing persistence of the 4c-2e bonding motif. Thus, the multicenter bonds in golden cages recovered by the AdNDP analysis correspond to the fragments that should be seen as building blocks of these chemical systems.

Carbon Avoids Hypercoordination in CB6-, CB62-, and C2B5-Planar Carbon-Boron Clusters

The structures and bonding of CB6-, C2B5-, and CB62- are investigated by photoelectron spectroscopy and ab initio calculations. It is shown that the global minimum structures for these systems are distorted heptacyclic structures. The previously reported hexacyclic structures with a hypercoordinate central carbon atom are found to be significantly higher in energy and were not populated under current experimental conditions. The reasons why carbon avoids hypercoordination in these planar carbon-boron clusters are explained through detailed chemical-bonding analyses.