Novel Beltlike and Tubular Structures of Boron and Carbon Clusters Containing the Planar Tetracoordinate Carbon:  A Theoretical Study of (C₃B₂)_nH₄ (n = 2−6) and (C₃B₂)_n (n = 4−8)

By use of the most stable structure of C3B2H4 as a basic block, we construct two types of novel compounds containing the planar tetracoordinate carbon (ptC), beltlike compounds (C3B2)nH4 (n = 2−6) and tubular compounds (C3B2)n (n = 4−8). Equilibrium geometries, vibrational frequencies, and electronic spectra of both series of compounds have been determined by the density functional theory. The results reveal that the same direction arrangement of C2CB2 units with ptC will result in the most stable isomers of (C3B2)nH4 (n = 2−6). Predicted nucleus-independent chemical shift values show that the three-membered rings have aromaticity, whereas the six-membered rings have antiaromaticity. Calculations show that these compounds containing ptC atoms exhibit remarkable size dependence of their highest-occupied molecular orbital−lowest-unoccupied molecular orbital energy gaps and the first excitation energies.