Boron monoxide (BO) can be formed via the condensation of tetrahydroxydiboron at elevated temperatures. However, the exact structure of the resulting material has not determined. Different molecular structures and 1D, 2D, or 3D polymeric structures have been suggested. Proposed layered 2D structures are composed of B₄O₂ or B₃O₃ units. The material’s reactivity indicates that B–B bonds are retained in boron monoxide.

Frédéric A. Perras, Wenyu Huang, Ames National Laboratory and Iowa State University, Ames, USA, and colleagues have used advanced ¹¹B NMR experiments as well as powder diffraction data to investigate the structure of boron monoxide. The team first synthesized boron monoxide by exposing tetrahydroxydiboron to elevated temperatures of 200, 300, 400, or 700 °C. The resulting samples were investigated using solid-state NMR spectroscopy methods such as magic angle spinning (MAS), multiple-quantum (MQ)MAS, dipolar heteronuclear multiple-quantum coherence (D-HMQC), double-quantum-filtered (DQF) J-resolved NMR, symmetry-based resonance-echo double-resonance (S-REDOR), DQ/SQ (double quantum/single quantum) correlation, and triple-quantum single-quantum (TQ/SQ) correlation.

The team found that there were two resonances in ¹¹B MAS solid-state NMR spectra, one of which they assigned to BO. The other appeared at synthesis temperatures of 300 °C and above. It was assigned to B₂O₃, which forms a separate phase from BO.

NMR spectra of the BO sample prepared at 200 °C, in combination with density functional theory (DFT) calculations, showed that the BO structure is probably two-dimensional, with planar, locally D_2h-symmetrical O₂B–BO₂ units. This allowed the team to narrow down the structure to 2D sheets composed of either B₃O₃ or B₄O₂ rings. The TQ/SQ correlation spectra then agreed better with simulated spectra for a B₄O₂-based structure (pictured). Using X-ray powder diffraction, the researchers found evidence that these layers formed from connected B₄O₂ rings stack in a random pattern.

• The Structure of Boron Monoxide,
  Frédéric A. Perras, Henry Thomas, Patrick Heintz, Ranjan Behera, Jiaqi Yu, Gayatri Viswanathan, Dapeng Jing, Scott A. Southern, Kirill Kovnir, Levi Stanley, Wenyu Huang,
  J. Am. Chem. Soc. 2023.
  https://doi.org/10.1021/jacs.3c02070