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.