| The structures, stabilities, and bonding properties of the four types of novel compounds containing the planar tetracoordinate carbon (ptC) have been explored by the density functional theory (DFT). The four types of novel compounds are C9B2H6 with a ptC, the belt-like compounds (C3B2)nH4 (n=2-6) with multi-ptC and the tubular compounds (C3B2)n (n=4-8) with multi-quasi-ptC, as well as the zigzag boron-carbon nanotubes (m, 0, i) (m=6, 8, 10 and 12; i=1-4) with multi-quasi-ptC. The main contents and conclusions are listed as follows:1. At the B3LYP/6-311+G** level, the structures, stabilities and vibrational frequencies of a kind of novel diboron organic compound C9B2H6 containing a ptC were investigated. The results showed that there was a significant relationship between the stability and the positions of the two boron atoms, and boron atoms played aσ-donating andπ-accepting role. The most stable structure was isomer (1,5) with C2v symmetry, in which the two boron atoms connected with the ptC atom and located on a same six-membered ring. Analysis of occupied n orbitals of isomer (1,5) indicated that it satisfied 4n+2 rule (10πe). And the calculated NICS (nucleus-independent chemical shift) values of isomer (1,5) show that the high aromaticity located at the two three-membered rings, rather than the six-membered rings.2. Two types of compounds containing ptC or quasi-ptC atoms, belt-like compounds (C3B2)nH4 (n=2-6) and tubular compounds (C3B2)n (n=4-8), have been constructed with the most stable structure of H2C2CB2H2 as a basic building block. At the B3LYP/6-311+G** level, structures, stabilities, clustering rules, and vibrational frequencies of belt-like compounds (C3B2)nH4 (n=2-6) have been investigated. The results indicate that the most stable structures of (C3B2)nH4 (n=2-6) have atomic arrangement H2(C2CB2)nH2, in which the C2CB2 units are assembled in the same direction. The calculated NICS values show that the three-membered rings have aromatic character whereas the six-membered rings have antiaromaticity. The electronic spectra from the first excitation in the most stable isomers H2(C2CB2)nH2 (n=2-6) have been calculated and predicted first excitation energies linearly depend upon n. Structures and stabilities of the tubular compounds of (C3B2)n (n=4-8) constructed by (C2CB2)n units have been discussed. The results indicated that the low-energy stable isomers of tubular structures (C2CB2)n (n=4-8) contain four, five, six, seven and eight quasi-ptC, respectively, and their HOMO-LUMO energy gaps have odd-even alternation character.3. By use of four kinds of basic units (N1, N2, N3, and N4) with different numbers of the ptC layers, we have constructed four kinds of novel zigzag boron-carbon nanotubes (m, 0, i) (m=6, 8, 10 and 12; i=1-4) with multi-quasi-ptC atoms. Their structures, stabilities, and bonding properties have been carried out by the DFT calculations. The results show that the structure with the C2CB2 units in the same direction is the most stable, in which the structures have Cnv symmetry. The HOM-LUMO gap of (m, 0, i) decreases monotonously with the increase of diameter and length of the boron-carbon nanotube, and the novel nanotubes (m, 0, i) behave as metals for enough long tubes. The different series of boron-carbon nanotubes have different window sizes. There are one, two, three, and four kinds of ten-membered ring windows for the nanotubes (m, 0, i) (i=1-4) as the index i increases from i=1 to 4, respectively. And their sizes are about between 2.9×5.3 (?)2 and 3.8×6.0 (?)2. Such large windows allow relatively large atoms and small molecules to enter into the nanotubes, and thus such boron-carbon nanotubes are promising in design for novel functional materials. |
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