Super-valence phenomenon of carbon atoms in C20 molecule

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.

MC19 (M = B, Si, Al and Ga) fullerenes: Adsorption mechanisms of 1,4-diformylpiperazine

Fullerenes and piperazines have been investigated, particularly, in the field of nanoscience and medicinal chemistry. In the present research, besides discussing structural and electronic properties, the most probable interaction mechanisms between C20, B-, Si-, Al-, Ga-doped C20 and 1,4-diformylpiperazine (1,4-dfp) were studied by employing density functional theory (DFT) in both the gas phase and water as the solvent. Stabilities of the investigated complexes were discussed based on the binding energy and electronic properties such as band gap energy, chemical hardness and electrophilicity index. It is found that doped complexes are more stabilized in water compared to the gas phase. However, the interaction between C20 and 1,4-dfp weakens upon the introduction of water as the solvent.

Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9'-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.