Ribbon aromaticity in double-chain planar B(n)H2(2-) and Li2B(n)H2 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.