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

In an experiment, cyclo[18]carbon (C-18), 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 C-18 is weak at room temperature in gas. Thus, it is difficult to study the spectrum, orbital, and bonds characters of the C-18 molecule in the experiment. In this paper, we have obtained absorption spectrum, orbital, and bonding characters of the C-18 molecule in theory. Besides, bonds and spectra of C24O6, C22O4, C20O2, B9N9, C-6, C-12, C-16, and C-20 molecules have been investigated to further confirm the structure and the characters of the C-18 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 C-18 has a larger bandgap and shorter wavelength of absorption spectra than those of the C-18 molecule. Moreover, all bonds between boron and nitrogen in B9N9 molecule are single one. Study bonding characters for C-6, C-12, C-16, and C-20 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 C-18 and B9N9 molecules in experiment.