Double-chain planar D2h B4H2, C2h B8H2, and C2h B12H2: conjugated aromatic borenes.

Based upon comprehensive theoretical investigations and known experimental observations, we predict the existence of the double-chain planar D(2h) B(4)H(2)(1), C(2h) B(8)H(2)(3), and C(2h) B(12)H(2)(5) which appear to be the lowest-lying isomers of the systems at the density functional theory level. These conjugated aromatic borenes turn out to be the boron hydride analogues of the conjugated ethylene D(2h) C(2)H(4)(2), 1,3-butadiene C(2h) C(4)H(6)(4), and 1,3,5-hexatriene C(2h) C(6)H(8)(6), respectively, indicating that a B(4) rhombus in B(2n)H(2) borenes (n = 2, 4, 6) is equivalent to a C=C double bond unit in the corresponding C(n)H(n+2) hydrocarbons. Detailed canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), and electron localization function (ELF) analyses unravel the bonding patterns of these novel borene clusters and indicate that they are all overall aromatic in nature with the formation of islands of both σ- and π- aromaticity. The double-chain planar or quasi-planar C(2v) B(3)H(2)(-)(7), C(2) B(5)H(2)(-)(8), and C(2h) B(6)H(2)(9) with one delocalized π orbital, C(2v) B(7)H(2)(-)(10), C(2) B(9)H(2)(-)(11), and C(2h) B(10)H(2)(12) with two delocalized π orbitals, and C(2v) B(11)H(2)(-)(13) with three delocalized π orbitals are found to be analogous in π-bonding to D(2h) B(4)H(2)(1), C(2h) B(8)H(2)(3), and C(2h) B(12)H(2)(5), respectively. We also calculated the electron affinities and ionization potentials of the neutrals and simulated the photoelectron spectroscopic spectra of the monoanions to facilitate their future experimental characterization. The results obtained in this work enrich the analogous relationship between hydroborons and their hydrocarbon counterparts and help to understand the high stability of the theoretically predicted all-boron nanostructures which favor the formation of double-chain substructures.