Charge distributions in BN nanocones: electric field and tip termination effects

STRUCTURE AND STABILITY OF $CLOSO-B_{n}H_{n-1}N_{2}^{-} (n=5-12)$

[Formula: see text], framework-isoelectronic to [Formula: see text], have been investigated at the B3LYP/6-311+G** level of theory. The most stable positional isomers of individual [Formula: see text] are similar to those of [Formula: see text] and [Formula: see text] in framework configuration. The energy analysis reveals that the cage [Formula: see text] with the icosahedron framework, [Formula: see text] with the octahedron framework and [Formula: see text] with the bicapped square antiprism framework are, in order, the most stable in the [Formula: see text] system. Furthermore, three-dimensional aromaticity of all [Formula: see text] is characterized. The most stable cages, [Formula: see text], [Formula: see text] and [Formula: see text], also have relatively large aromaticity. All the predicted N–N stretching frequencies (2127–2303 cm−1) of the most stable positional isomers in the [Formula: see text] system are lower than the computational N–N stretching frequency (2445 cm−1) of free N2 at the same level, which results from the σ donation of N2 to B frameworks. The stretching frequencies of N–N in the [Formula: see text] cages increase with the decrease of N–N distances or the increase of B–N distance. In addition, B n−1 H n−1 fragment has larger affinity for group CH than that for group BNN, namely, it favors to form the [Formula: see text] rather than the [Formula: see text].

Density Functional Study of the Influence of Carbon Doping on the Aluminum-27 and Nitrogen-14 Electric Field Gradient Tensors in (10, 0) Single-Walled Aluminum Nitride Nanotube

Density functional theory (DFT) study was performed to investigate the influence of carbon doping (C-doping) on the electric field gradient (EFG) tensors at the sites of aluminum-27 (27Al) and nitrogen-14 (14N) nuclei in (10, 0) single-walled aluminum nitride nanotube (AlNNT). Two models of original and C-doped AlNNTs were optimized at first and then the EFG tensors were calculated in the optimized structures. The calculated EFG tensors were converted to measurable nuclear quadrupole resonance (NQR) spectroscopy parameters, quadrupole coupling constant (C Q ) and asymmetry parameter (η Q ). The calculated parameters in the original model reveal that the Al and N atoms located at the mouths of nanotube have the largest C Q values revealing their remarkable role in AlNNTs. When C-doping occurs, the C Q values of those nuclei directly bonding to C are significantly increased meaning that the active sites in the C-doped model are increased. The results also reveal that the Al and N play the basic and acidic agents roles, respectively, in AlNNT which is better observed in the C-doped model.

A quantum chemistry study: A new kind of boron nitrides

At the B3LYP/6-311+G** and the BP86/6-311+G** levels of theory, BNN, H(3)BNN, NNBH(2)-BH(2)NN, (BNN)(2)H(2), NNBBNN, (BNN)(3) (+), (BNN)(4), (BNN)(4) (2+), (BNN)(4) (2-), B(4)(NN)(2), (BNN)(5) (-), (BNN)(6), (BNN)(7) (+), and (BNN)(8) (2+) are investigated. Neutral (BNN)(4) is aromatic with its triplet state but antiaromatic with its singlet state. (BNN)(4) dication favors D(2d) structure, while (BNN)(4) dianion favors a planar D(4h) structure. (BNN)(3) (+), (BNN)(4) (2-), (BNN)(5) (-), (BNN)(6), (BNN)(7) (+), (BNN)(7) (3-), (BNN)(8) (2+), and (BNN)(8) (2-) are all aromatic with planar monocyclic conformation, following the 4n + 2 rule. Moreover, according to the CASSCF and MRCI calculations, the planar B(4)(NN)(2) of D(2h) symmetry prefers to be a sigma-pi diradical in spite of open-shell singlet or triplet and is also aromatic. Akin to the sigma-pi back interaction in compounds containing transition metal, there exists the sigma-pi back interaction between boron and N(2) ligand among some species reported herein, which strengthens B(-)N bond but activates N-N bond, especially in (4)Sigma(-) BNN. The T-shaped structure lies lowest in energy among seven isomers of the (BNN)(2)H(2) dimer, and the parallel-displaced structure is favored between two isomers of the (BNN)(6) dimer.

Metallopyrrole-Capped Carbon Nanocones

We design nickel-doped and nitrogen-doped carbon nanocones with various amounts of buckling that feature square-planar, (approximate) tetrahedral, and octahedral coordination. The optimized geometries and electronic structures of these novel metallocarbon complexes are calculated by using the B3LYP (Gaussian03) and GGA-BLYP (ADF) exchange-correlation functionals. We analyze buckling and stability of the nanocones, and discuss their potential for additional functionalization at the metallic site.

The influence of the tip structure and the electric field on BN nanocones

Different configurations for BN nanocones with 240° disclination are analysed through first-principles calculations based on the density-functional theory. The studied cone tips involve either four pentagons with two homonuclear bonds or two squares and two hexagons with no homonuclear bonds. The structures are both terminated by two three-coordinated atoms. Cohesive energy results show that the cone terminated in pentagons is slightly energetically preferred. The influence of a 1.7 V Å(-1) external electric field, applied along the axis, on the structural changes and charge distributions is analysed. The responses of the charge distributions to the external electric field demonstrate the main role played by the B atoms for the applications of BN cones as probes in electronic microscopy as well as field emitters.