Density functional theoretical investigation on structure, optical response and hydrogen adsorption properties of B9/metal–B9 clusters

Abstract

DFT calculations have been carried out to shed light on the electronic structure, optical properties and hydrogen adsorption capability of neutral MB₉ (where M = Li₃, Na₃, K₃, Al, Ga, In, Rh and Co) clusters. Electronic structural studies on the parent B₉³⁻ clusters reveal that a less aromatic hypervalent B-centred B₈ ring geometry is energetically more favoured. However, GM³⁺ (Al, Ga and In) in GM@B₉ prefers a highly aromatic metal-centred 9-membered molecular wheel structure. In addition, the larger size of indium breaks the D_9h symmetry of the molecular wheel by coming slightly out of the ring plane unlike Al@B₉ and Ga@B₉. B₉ⁿ⁻ (n = 1–3) is also neutralized with ‘n’ number of Li, Na and K which revealed that AM₂B₉ and AM₂B₉⁻ result in pyramidal geometries while AM₃B₉ results in an irregular shape by retaining the B₉ framework similar to the most stable conformer of B₉³⁻. Metal@B₉ molecular wheels show optical absorption in a broad range of the spectrum (from UV to NIR: 260–1000 nm), which is attributed to the metal's ability to perturb the ring centred excited state. Ga@B₉ and Co@B₉ bind with hydrogen molecules in a dissociative manner, forming two covalent bonds with peripheral B atoms of the B₉ ring.