Aromaticity Of Para-substituted Benzene In T₁ State And Transition Structure Of DGT Reaction

At present,most researches on aromaticity of transition states stays in the stage of calculation static structure conjecture.There are few reports on the theoretical observation of aromaticity changes.Although there are research reports on the aromaticity of the transition states transition structure（TS）of the double group transfer reaction（DGTR）between ethane and ethene,the NICS scans method adopted is only a theoretical calculation value on a one-dimensional level,and its research has certain limitations and cannot provide a reliable theoretical reference.In addition,researches on the aromaticity of the excited state of benzene,especially the first triplet excited state（T₁）of substituted benzene,were even rarer.This master dissertation presents study on the aromaticity of the transition state structure of the uncommon ethane and ethylene double group transfer reaction and the influence of the aromaticity of the first triplet excited state of the para-substituted benzene by use of quantum chemical procedure,to provide theoretical reference for experimental,to provide theoretical reference for experimental explorations of mechanism of DGTR and characteristics of the first triplet excited state（T₁）of substituted benzene.Geometries of the transition states TS of the DGTR between ethane and ethane,and of the ground state（S₀）,the first excited triplet state（T₁）of benzene and 17 kinds of para-substituted benzenes were optimized by density functional theory（DFT）at high-precision quantum chemistry level of B3LYP/6-311+G（d,p）and B3LYP/6-311+G（d）with Gaussian 09 program.The NICS of each optimized structure was calculated by using the gauge-independent atomic orbital（GIAO）method.Chemical bonds of the transition state structure of the migration reaction of ethane and ethylene were analyzed by use of the highly universal Mayer bond order.The localized orbital positioning function（LOL）was used to reveal the dynamic changes of the atoms and bonding regions,and the current density was calculated using NICS scans,iso-chemical shielding surface（ICSS）iso-surfaces,and the continuous set of gauge transformations（CSGT）method,and anisotropy of the induced current density（AICD）maps were drawn to verify size and degree of aromaticity.The electron localization function（ELF）iso-surfaces were drawn,and the aromaticity types of benzene and each para-substituted benzene were further analyzed.The Mayer bond order data was analyzed and processed with Multiwfn（v 3.8 dev）,and the real spatial function was drawn,and the geometric structure diagram,LOL color-filled map of the studied molecules and ICSS(ICSS_ZZ)iso-surface map and ELF iso-surface map were drawn and rendered with the help of the visualization program VMD（v 1.9.3）.The current density anisotropy was achieved through the AICD code based on output of Gaussian 09,and finally the graphics of current density was rendered and generated by POV-Ray.Structures of some molecules were drawn by Gauss View（v6.0）program,and relevant curves were drawn by Origin（v 9.6）.The following results were obtained.（1）There is a transition state in thereaction of ethane and ethylene,and its structure is a planar six-membered heteroaromatic ring composed of two H and four C atoms,and this transition state structure only hasσaromaticity but noπaromaticity,and that（2）benzene and 17 kinds of para-substituted benzene,in which the substituted groups areσ-electron-donating groups（CH₃ and Si H₃）,π-electron-donating groups（NH₂,N（CH₃）₂,OH,OCH₃,F,PH₂,SH,Cl,Br）,π-electron-accepting groups（COOH,COCH₃,CHO,NO,NO₂）,σ-electron-accepting（NH₃⁺）group,maintain planar structures in the S₀ state and are aromatic,and in the T₁ state,benzene retains a planar structure and has strong antiaromaticity,but the symmetry is reduced to D_2h.In the T₁ state,the ring of those 6 para-substituted benzenes whose substituted groups are Si H₃,PH₂,SH,Cl,Br,NH₃⁺,no longer retains the planar structure,and the benzene ring of the other 11 kinds of para-substituted benzene still retains the planar structure.The C-X bond length in benzene increases and the bond length between the benzene ring and the substituents in the other para-substituted benzenes is shortened.σ-electron-donating groups para-substituted benzenes have strong anti-aromaticity.As for the para-substituted benzene withπ-electron-donating groups,Br-substituted benzene has weak aromaticity,and SH-substituted benzene and Cl-substituted benzene have weak anti-aromaticity.The other 6 para-substituted benzenes all have strong antiaromaticity.Concerningπ-electron-accepting groups para-substituted benzene,except for COOH-substituted benzene,which has strong antiaromaticity,the others all have strong aromaticity.Forπ-electron-accepting group although the para-substituted benzene is antiaromatic in the T₁ state,it has a similar degree of aromaticity in the S₀ state.