Theoretical investigation on bond and spectrum of cyclo[18] carbon (C₁₈) with sp-hybridized

In an experiment, cyclo[18]carbon (C₁₈), prepared with low-temperature STM-AFM (scanning tunneling microscopy-atomic force microscopy) from C₂₄O₆, C₂₂O₄, and C₂₀O₂ molecules, have been confirmed being alternating single and triple bonds structure. Nevertheless, the stability of C₁₈ is weak at room temperature in gas. Thus, it is difficult to study the spectrum, orbital, and bonds characters of the C₁₈ molecule in the experiment. In this paper, we have obtained absorption spectrum, orbital, and bonding characters of the C₁₈ molecule in theory. Besides, bonds and spectra of C₂₄O₆, C₂₂O₄, C₂₀O₂, B₉N₉, C₆, C₁₂, C₁₆, and C₂₀ molecules have been investigated to further confirm the structure and the characters of the C₁₈ molecule. The results show that carbon-carbon bonds of C₂₄O₆, C₂₂O₄, and C₂₀O₂ molecules in ring are alternating single and triple bonds except those connected with CO group. B₉N₉ molecule as the isoelectronic structure of C₁₈ has a larger bandgap and shorter wavelength of absorption spectra than those of the C₁₈ molecule. Moreover, all bonds between boron and nitrogen in B₉N₉ molecule are single one. Study bonding characters for C₆, C₁₂, C₁₆, and C₂₀ molecules have confirmed that carbon-carbon bonds of cyclo[n]carbon changed gradually from double bonds to alternating single and triple bonds with increasing n value. The data from theory would give help for future research on C₁₈ and B₉N₉ molecules in experiment. Graphical abstract Theoretical investigation for bond and spectra characters of cyclo[18]carbon (C₁₈), prepared with low-temperature STM-AFM (scanning tunneling microscopy-atomic force microscopy) from C₂₄O₆, C₂₂O₄, and C₂₀O₂ molecules, which have been confirmed being alternating single and triple bonds structure.