Theoretical Study On The Structure And Aromaticity Of Acenes And Hanging Ring Substances

The aromaticity index of acenes and Hanging benzene was evaluated by theoretical factors of considering mono-dimensional extensive quantity.Up to now,there is no experimental index and considering multi-dimensional intensive quantity including two or more factors to judge the aromaticity of acenes and Hanging benzene.Finding a new criterion for aromaticity of acenes and Hanging benzene,including geometric and energetic factors—Degree of aromaticity（DOA）,has certain theoretical research value.It is also of great practical significance to explore a method for quantitative measurement of aromaticity of acenes and Hanging benzene by using Ring-Stretching Vibration Raman Spectroscopy frequency（RSVRSF）.In this study,based on the study of the aromaticity of DOA applied to single-ring conjugated molecules,the feasibility of DOA applied to aromaticity of acenes and Hanging ring compounds was further explored.The geometric optimization of linear acenes C_4n+2H_2n+4（n≤10）,graphenenanoflakes（GNFs）,graphenenanoflakeswithhollow sites（GNFHs）,hanging benzene C_24+18nH_18+12n（0≤n≤4）was studied by using density functional theory（DFT）B3LYP/6-311G（d,p）calculation method.The RSVRSF of A1g/A1 symmetrical types of optimized molecular structures,aromatic stabilization energy（ASE）,bond equalization energy and DOA were calculated.The nucleus independent chemical shifts（NICS（0））of the optimized structures of each molecule were calculated by B3LYP-GIAO method.The Characteristic molecular orbital maps of optimized structures were plotted by GaussView 5.0.8.The Pearson correlation coefficients between NICS,RSVRSF and DOA were calculated by Origin 8.0 program,and the corresponding correlation curve were plotted.It was found that the point group of linear acenes is the stable structure of D_2h(the point group of benzene removal is D_6h),graphene nanoflakes（GNFs）and graphene nanoflakes with hollow sites（GNFHs）belong to D_6hh point group,hanging benzene C_24+18nH_18+12n（0≤n≤4）belongs to D_3hh point group.The RSVRSF with A_1gg or A₁ symmetric exists in the optimized structures of the four types of molecules studied.As the increasing of the number of benzene ring,The larger the negative NICS of odd and even linear acenes is,the larger the RSVRSF of odd and even linear acenes central ring.The correlation coefficients between NICS and RSVRSF values of even and odd linear acenes central rings are more than0.9.The NICS values for GNFs（1,3）can be roughly estimated by the NICS values of the odd fragments of GNFHs 1′,3′minus the NICS value of benzene,respectively.The NICS values for GNFs（2,4）can be roughly estimated by the NICS value of benzene minus the NICS values of the even fragments for GNFHs 2′,4′,respectively.The RSVRSF values for GNFs（1-4）can be roughly estimated by the RSVRSF value of benzene plus the RSVRSF values of GNFHs1′-4′,respectively.As the increasing of the n of number,The NICS（0）value and RSVRSF value of the central ring of hanging benzene were basically unchanged.It is theoretically predicted that the aromaticity of the target molecule can be measured experimentally by measuring the RSVRSF of the molecule.The calculation results of DOA of linear acenes central ring show that the DOA value of odd and even linear acenes central ring increases gradually with the increase of the number of benzene rings,and the DOA value of linear acenes central ring has a good quadratic correlation with NICS value.The DOA values for GNFs（1-4）can be roughly estimated by the DOA value of benzene plus the DOA values of GNFHs1′-4′,respectively.There are significant correlations between DOA and NICS（0）,DOA and RSVRSF of GNFs（1-4）,GNFHs1′-4′and benzene.The DOA value of hanging benzene central ring is basically unchanged and have certain correlation between the three aromaticity criteria.The RSVRSF(A_1g/A₁),DOA and NICS（0）are significantly correlate with the optimized structures of the four types of molecules studied.The DOA and RSVRSF can be used to determine the aromaticity of highly symmetric acenes and hanging ring compounds.