Theoretical Study Of The Aromaticity And Optical Property Of Cyclo[18]carbon Precursors

Cyclo[18]carbon is a new carbon allotrope discovered after diamond,graphite,fullerene,carbon nanotubes,and graphene,namely C₁₈ ring.Since cyclo[18]carbon was prepared and imaged in the laboratory,chemists have studied it extensively.However,due to the high coupling reactivity of cyclo[18]carbon,it can hardly exist stably under natural conditions,which makes it difficult to be fully characterized and applied in practice.However,when synthesizing cyclo[18]carbon in experiment,the researchers found that its precursors C₁₈（CO）_n（n=6,4,and 2）and C₁₈Br₆ are more stable than it,and even their crystal structures can be obtained.Therefore,the research on the precursors of cyclo[18]carbon is of more practical significance.This paper focuses on the electronic structure,aromaticity,and optical properties of cyclo[18]carbon precursors.The details are as follows:（1）The bonding character,electron delocalization,and aromaticity of the cyclo[18]carbon precursors,C₁₈（CO）_n（n=6,4,and 2）,have been studied by combining quantum chemical calculations and various electronic wavefunction analyses.It was found that C₁₈（CO）_n（n=6,4,and 2）molecules exhibit alternating long and short C-C bonds,and have out-of-plane and in-plane dualπsystems(π^out andπⁱⁿ)perpendicular to each other,which are consistent with the relevant characteristics of cyclo[18]carbon.However,the presence of carbonyl（-CO）groups significantly reduced the global electron conjugation of C₁₈（CO）_n（n=6,4,and 2）compared to cyclo[18]carbon.Specifically,the-CO group largely breaks the extensive delocalization ofπⁱⁿ system,and theπ^out systemis also affected by it but to a much lesser extent;as aconsequence,C₁₈（CO）_n（n=6,4,and 2）with larger n shows weaker overall aromaticity.Mostly because of the decreased but still apparentπ^outelectron delocalization in the C₁₈（CO）_n（n=6,4,and 2）,a notable diatropic induced ring current under the action of external magnetic field is observed,demonstrating the clear aromatic characteristic in the molecules.The correlation between C₁₈（CO）_n（n=6,4,and 2）and C₁₈ in terms of the gradual elimination of-CO from the precursors showed that the direct elimination of two CO molecules in C₁₈（CO）_n（n=6,4,and 2）has a synergistic mechanism,but it is kinetically infeasible under normal conditions due to the high energy barrier.（2）The electronic spectrum and（hyper）polarizability of C₁₈（CO）_n（n=2,4,and 6）are studied by theoretical calculations for revealing the utility of introducing carbonyl（-CO）groups on molecular optical properties.Successive introduction of-CO groups is observed to cause a red-shift in the absorption spectrum,but maximum absorption of all molecules is mainly due to the charge redistribution within C₁₈ moiety.The（hyper）polarizabilities of the cyclocarbon oxides present an ascending trend with the-CO groups in the molecule,and the higher-order response properties are more sensitive.With（hyper）polarizability density analysis and（hyper）polarizability contribution decomposition,the fundamental reasons for the difference of（hyper）polarizability of different molecules are systematically discussed from the perspectives of physical and structural origins,respectively.The significant optical resonances under the frequency-dependent fields are found on the hyperpolarizabilities of the cyclocarbon oxides,which is in contrast to the insignificant influences on their polarizability.（3）The electronic structure and bonding character of the stable cyclo[18]carbon precursor,C₁₈Br₆,are thoroughly characterized by molecular orbital（MO）,density of states（DOS）,bond order（BO）,and interaction region indicator（IRI）analyses.The delocalization characters of out-of-plane and in-planeπ-electrons(labeled asπ^out-andπⁱⁿ-electrons,respectively)in bonding regions are examined using localized orbital locator（LOL）and electron localization function（ELF）.Then,the aromaticity is addressed via the molecular magnetic response to external magnetic field by computing the magnetically induced current density(J^ind),iso-chemical shielding surface（ICSS）,anisotropy of the induced current density（AICD）,and the induced magnetic field(B^ind).All these analyses indicate that C₁₈Br₆ is a globally aromatic species with lower aromaticity than cyclo[18]carbon,and the blocking of in-planeπ-conjugation(labeled asπⁱⁿ-conjugation)by the presence of-Br substituents in it is the underlying cause for the weakening of molecular aromaticity.