From Quasi-Planar B₅₆ to Penta-Ring Tubular Ca©B₅₆: Prediction of Metal-Stabilized Ca©B₅₆ as the Embryo of Metal-Doped Boron α-Nanotubes

Motifs of planar metalloborophenes, cage-like metalloborospherenes, and metal-centered double-ring tubular boron species have been reported. Based on extensive first-principles theory calculations, we present herein the possibility of doping the quasi-planar C_2v B₅₆ (A-1) with an alkaline-earth metal to produce the penta-ring tubular Ca©B₅₆ (B-1) which is the most stable isomer of the system obtained and can be viewed as the embryo of metal-doped (4,0) boron α-nanotube Ca©BNT_(4,0) (C-1). Ca©BNT_(4,0) (C-1) can be constructed by rolling up the most stable boron α-sheet and is predicted to be metallic in nature. Detailed bonding analyses show that the highly stable planar C_2v B₅₆ (A-1) is the boron analog of circumbiphenyl (C₃₈H₁₆) in π-bonding, while the 3D aromatic C_4v Ca©B₅₆ (B-1) possesses a perfect delocalized π system over the σ-skeleton on the tube surface. The IR and Raman spectra of C_4v Ca©B₅₆ (B-1) and photoelectron spectrum of its monoanion C_4v Ca