Yan-Bo Wu

Observation of an all-boron fullerene

After the discovery of fullerene-C60, it took almost two decades for the possibility of boron-based fullerene structures to be considered. So far, there has been no experimental evidence for these nanostructures, in spite of the progress made in theoretical investigations of their structure and bonding. Here we report the observation, by photoelectron spectroscopy, of an all-boron fullerene-like cage cluster at B40(-) with an extremely low electron-binding energy. Theoretical calculations show that this arises from a cage structure with a large energy gap, but that a quasi-planar isomer of B40(-) with two adjacent hexagonal holes is slightly more stable than the fullerene structure. In contrast, for neutral B40 the fullerene-like cage is calculated to be the most stable structure. The surface of the all-boron fullerene, bonded uniformly via delocalized σ and π bonds, is not perfectly smooth and exhibits unusual heptagonal faces, in contrast to C60 fullerene.

Simplest Neutral Singlet C2E4 (E = Al, Ga, In, and Tl) Global Minima with Double Planar Tetracoordinate Carbons: Equivalence of C2 Moieties in C2E4 to Carbon Centers in CAl42− and CAl5+

Ab initio and DFT calculations have been carried out to search for the simplest neutral singlet species with double planar tetracoordinate carbons (dptCs) [the simplest means the species containing the least number (six) and types (two) of atoms]. Under the restrictions to the possible models (M1-M4) with dptCs and to the singlet electronic states, the B3LYP/6-31+G* scanning on the candidates, C(2)E(4) (E = the second- and third-row main group elements), only led to two minima (D(2h) C(2)Al(4) and C(2h) C(2)Be(4)) with stable DFT wave functions. The extensions to the heavier elements after the fourth row in the IIA and IIIA groups revealed that the D(2h) C(2)E(4) (E = Ga, In, and Tl) are also minima with dptCs but C(2)Ca(4) (C(2h)) is a first-order saddle point. Extensive explorations at the DFT level on their potential energy surfaces (PESs) further confirmed that the D(2h) C(2)E(4) (E = Al, Ga, In, and Tl) are the global minima, but the C(2h) C(2)Be(4) is a local minimum. The optimizations at the MP2 level distorted the D(2h) C(2)E(4) (E = Ga, In, and Tl) slightly and the distortion energies are less than 0.02 kcal/mol. The C(2)E(4) (E = Al, Ga, In, and Tl) with dptCs are 18.0, 18.3, 13.4, and 12.2 kcal/mol energetically more favorable than their nearest isomers, respectively, at the CCSD(T)//MP2 level with aug-cc-pVTZ for C and Al and aug-cc-pVTZ-PP for Ga, In, and Tl basis set. The substantial energy differences suggest their promise to be experimentally realized. The strong peak on the C(2)Al(4)(-) component in the time-of-flight mass spectrum from laser vaporization of a mixed graphite/aluminum may relate to the D(2h) C(2)Al(4) global minimum. The analyses of the electronic structures of C(2)Al(4) (D(2h)), CAl(4)(2-) (D(4h)) and CAl(5)(+)(D(5h)) indicates that the C(2) moiety in C(2)Al(4) is the equivalence of carbon centers in CAl(4)(2-) and CAl(5)(+) and unveils the reasons for their stability. The electronic structures of C(2)Al(4) and ethene are compared. On the one hand, an Al atom functions like an H atom because the eight more valence electrons of C(2)Al(4) than C(2)H(4) occupy four nonbonding orbitals and are not effectively utilized for bonding. On the other hand, an Al atom is different from an H atom because an Al atom has p electrons available for peripheral bonding around the C(2) moieties in C(2)Al(4), which further rationalize the origins for C(2)E(4) to achieve double ptCs.

CAl2Be32– and Its Salt Complex LiCAl2Be3–: Anionic Global Minima with Planar Pentacoordinate Carbon

Following the isoelectronic relationship in global minima planar pentacoordinate carbon (ppC) species (cationic CAl(5)(+), neutral CAl(4)Be, and monoanionic CAl(3)Be(2)(-)), we designed a dianionic ppC species C(2v) CAl(2)Be(3)(2-) (1a) and its salt complex C(2v) LiCAl(2)Be(3)(-) (2a) in this work. In combination with DFT and high-level ab initio calculations (CCSD(T)), the extensive exploration on their potential energy surfaces indicates that they are the global minima. Their kinetic stability was proved by two sets of 100 ps ab initio Born-Oppenheimer molecular dynamic simulations at the B3LYP/6-31+G(d) level. The detailed analyses indicate that the introduction of Li(+) into 1a only influences the electrovalent bonding (through changing of the charge distribution) and the σ aromaticity (through changing of the in-plane ring current), while the structures, the bonding properties, the π aromaticity, and so forth are almost unchanged. Nevertheless, the MO energy levels, the HOMO-LUMO gaps, and the values of vertical detachment energies (VDEs) all verify that the lithiation significantly improves the stability. We think the ppC dianion 1a is possible to detect directly in the gas-phase experiments, but it can be detected as its salt complex 2a more easily.

CBe5E− (E = Al, Ga, In, Tl): planar pentacoordinate carbon in heptaatomic clusters

A series of clusters with the general formula CBe(5)E(-) (E = Al, Ga, In, Tl) are theoretically shown to have a planar pentacoordinate carbon atom. The structures show a simple and rigid topological framework-a planar EBe(4) ring surrounding a C center, with one of the ring Be-Be bonds capped in-plane by a fifth Be atom. The system is stabilized by a network of multicenter σ bonds in which the central C atom is the acceptor, and π systems as well by which the C atom donates charge to the Be and E atoms that encircle it.

Hydration and coordination of K+ solvation in water from ab initio molecular-dynamics simulation

Potassium ion in water plays a very important role in chemistry and biology. In this paper, we investigated the hydration structure and coordination of K(+) solvation in water at 300 and 450 K using ab initio Car-Parrinello molecular dynamics. The K(+)-oxygen radial distribution function indicated that the perturbation of K(+) on the water structure is strong in the first hydration shells, while it is mild outside of this region in normal liquid. According to our natural geometric criterion for the coordinated oxygen atom, the average coordination number of K(+) is 6.24 and 6.53 at 300 and 450 K, respectively, which agrees with the experimental value (6.1). This geometric criterion can also be used to define strong, moderate and weak hydrogen bonds in liquid.

Ribbon aromaticity in double-chain planar BnH22− and Li2BnH2 nanoribbon clusters up to n = 22: lithiated boron dihydride analogues of polyenes

We report an extensive density-functional theory and coupled-cluster CCSD(T) study on boron dihydride dianion clusters BnH2(2-) (n = 6-22) and their dilithiated Li2BnH2(0/-) salt complexes. Double-chain (DC) planar nanoribbon structures are confirmed as the global minima for the BnH2(2-) (n = 6-22) clusters. Charging proves to be an effective mechanism to stabilize and extend the DC planar nanostructures, capable of producing elongated boron nanoribbons with variable lengths between 4.3-17.0 Å. For the dilithiated salts, the DC planar nanoribbons are lowest in energy up to Li2B14H2 and represent true minima for all Li2BnH2(0/-) (n = 6-22) species. These boron nanostructures may be viewed as molecular zippers, in which two atomically-thin molecular wires are zipped together via delocalized bonds. Bonding analysis reveals the nature of π plus σ double conjugation in the lithiated DC nanoribbon Li2BnH2(0/-) (n up to 22) model clusters, which exhibit a 4n pattern in adiabatic detachment energies, ionization potentials, and second-order differences in total energies. Band structure analysis of the infinite DC boron nanoribbon structure also reveals that both π and σ electrons participate in electric conduction, much different from the monolayer boron α-sheet in which only π electrons act as carriers. A concept of ribbon aromaticity is proposed for this quasi-one-dimensional system, where regular π versus σ alternation of the delocalized electron clouds along the nanoribbons results in enhanced stability for a series of magic nanoribbon clusters. The total number of delocalized π and σ electrons for ribbon aromaticity collectively conforms to the (4n + 2) Hückel rule. Ribbon aromaticity appears to be a general concept in other nanoribbon systems as well.

CBe5Hnn–4 (n = 2–5): Hydrogen-Stabilized CBe5 Pentagons Containing Planar or Quasi-Planar Pentacoordinate Carbons

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design a new series ppC- or quasi-ppC-containing species C5v CBe5H5(+), Cs CBe5H4, C2v CBe5H3(-), and C2v CBe5H2(2-) by replacing the Al atoms in previously reported global minima planar pentacoordinate carbon (ppC) species D5h CAl5(+), C2v CAl4Be, C2v CAl3Be2(-), and C2v CAl2Be3(2-) with BeH units. The three-center two-electron (3c-2e) bonds formed between Be and bridging H atoms were crucial for the stabilization of these ppC species. The natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses revealed that the central ppCs or quasi-ppCs possess the stable eight electron-shell structures. The AdNDP analyses also disclosed that these species are all 6σ+2π double-aromatic in nature. The aromaticity was proved by the calculated negative nucleus-independent chemical shifts (NICS) values. DFT and high-level CCSD(T) calculations revealed that these ppC- or quasi-ppC species are the global minimum or competitive low-lying local minimum (Cs CBe5H4) on their potential energy surfaces. The Born-Oppenheimer molecular dynamic (BOMD) simulations revealed that the H atoms in C2v CBe5H3(-) and C2v CBe5H2(2-) can easily rotate around the CBe5 cores and the structure of quasi-planar C5v CBe5H5(+) will become the planar structure at room temperature; however, these interesting dynamic behaviors did not indicate the kinetic instability as the basic ppC structures were maintained during the simulations. Therefore, it would be potentially possible to realize these interesting ppC- or quasi-ppc-species in future experiments.

Hydrogen-bond network and local structure of liquid water: An atoms-in-molecules perspective.

The nearly linear relationship between hydrogen-bond strength at the CCSD(T)/Aug-cc-pVTZ level and the electron density at the bond critical point in the atoms-in-molecules theory provides a practical means of calculating the hydrogen-bond strength in liquid water. A statistical analysis of the hydrogen-bonds obtained from Car-Parrinello molecular dynamics simulations shows that the strengths of hydrogen bonds in liquid water conform to a Gaussian distribution. Considering supercooled (250 K) water to have a fully coordinated (icelike) local tetrahedral configuration, we show that the local structure of liquid water is partly distorted tetrahedral in normal liquid water and even in superheated water.

Star-like superalkali cations featuring planar pentacoordinate carbon

Superalkali cations, known to possess low vertical electron affinities (VEAs), high vertical detachment energies, and large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps, are intriguing chemical species. Thermodynamically, such species need to be the global minima in order to serve as the promising targets for experimental realization. In this work, we propose the strategies of polyhalogenation and polyalkalination for designing the superalkali cations. By applying these strategies, the local-minimum planar pentacoordinate carbon (ppC) cluster CBe5 can be modified to form a series of star-like superalkali ppC or quasi-ppC CBe5X5 (+) (X = F, Cl, Br, Li, Na, K) cations containing a CBe5 moiety. Polyhalogenation and polyalkalination on the CBe5 unit may help eliminate the high reactivity of bare CBe5 molecule by covering the reactive Be atoms with noble halogen anions and alkali cations. Computational exploration of the potential energy surfaces reveals that the star-like ppC or quasi-ppC CBe5X5 (+) (X = F, Cl, Br, Li, Na, K) clusters are the true global minima of the systems. The predicted VEAs for CBe5X5 (+) range from 3.01 to 3.71 eV for X = F, Cl, Br and 2.12-2.51 eV for X = Li, Na, K, being below the lower bound of the atomic ionization potential of 3.89 eV in the periodic table. Large HOMO-LUMO energy gaps are also revealed for the species: 10.76-11.07 eV for X = F, Cl, Br and 4.99-6.91 eV for X = Li, Na, K. These designer clusters represent the first series of superalkali cations with a ppC center. Bonding analyses show five Be-X-Be three-center two-electron (3c-2e) σ bonds for the peripheral bonding, whereas the central C atom is associated with one 6c-2e π bond and three 6c-2e σ bonds, rendering (π and σ) double aromaticity. Born-Oppenheimer molecular dynamics simulations indicate that the CBe5 motif is robust in the clusters. As planar hypercoordination carbon species are often thermodynamically unstable and highly reactive, the superalkali cation characters of these ppC species should be highlighted, which may be suitable for experimental realization.

B30H8, B39H9 2−, B42H10, B48H10, and B72H12: polycyclic aromatic snub hydroboron clusters analogous to polycyclic aromatic hydrocarbons

AbstractCalculations performed at the ab initio level using the recently reported planar concentric π-aromatic B18H62+(1) [Chen Q et al. (2011) Phys Chem Chem Phys 13:20620] as a building block suggest the possible existence of a new class of B3nHm polycyclic aromatic hydroboron (PAHB) clusters—B30H8(2), B39H92−(3), B42H10(4/5), B48H10(6), and B72H12(7)—which appear to be the inorganic analogs of the corresponding CnHm polycyclic aromatic hydrocarbon (PAHC) molecules naphthalene C10H8, phenalenyl anion C13H9−, phenanthrene/anthracene C14H10, pyrene C16H10, and coronene C24H12, respectively, in a universal atomic ratio of B:Cu2009=u20093:1. Detailed canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), and electron localization function (ELF) analyses indicate that, as they are hydrogenated fragments of a boron snub sheet [Zope RR, Baruah T (2010) Chem Phys Lett 501:193], these PAHB clusters are aromatic in nature, and exhibit the formation of islands of both σ- and π-aromaticity. The predicted ionization potentials of PAHB neutrals and electron detachment energies of small PAHB monoanions should permit them to be characterized experimentally in the future. The results obtained in this work expand the domain of planar boron-based clusters to a region well beyond B20, and experimental syntheses of these snub B3nHm clusters through partial hydrogenation of the corresponding bare B3n may open up a new area of boron chemistry parallel to that of PAHCs in carbon chemistry.n FigureAb initio calculations predict the existence of polycyclic aromatic hydroboron clusters as fragments of a boron snub sheet; these clusters are analogs of polycyclic aromatic hydrocarbons