Bond Length Equalization,Ring Stretching Vibration Raman Spectroscopy Frequency With Molecular Aromaticity

The conventional Benzene Kekuléformula fails to reflect single and double Bond-Length Equalization?BLE?.Up to today,aromaticity hasn't been viewed as a multi-dimensional intensive quantity.Different types of molecules were done to study its aromaticity from the new angle of BLE.Degree Of Aromaticity?DOA?,a two-dimensional intensive quantity including geometric and energetic factors,as a new measurement of aromaticity is proposed.Few literatures can be our reference about hanging ring aromatic compounds,no matter in the aspect of experiment or in the aspect of theoretical calculation.This article studied the relationship between Ring Stretching Vibration Raman Spectroscopy Frequency?RSVRSF?and Aromaticity;the correlation between DOA and Nucleus Independent Chemical Shifts?NICS?in the process of BLE.This research studied aromaticity of Single Ring Aromatic Molecule?SRAM?G_nH_n^m?G=C,Si,Ge,n=3,5-8,m=+1,-1,0,+1,+2?,G_nH_n^m?G=B,Al,Ga,n=3,5-8,m=-2,0?,Double Hanging Ring Molecule?DHRM?[(X_nH^m_n-1)(X_nH^m_n-1)]^2m?X=C,Si,Ge,n=3,5-8,m=+1,-1,0,+1,+2?and Triple Hanging Ring Molecule?THRM?[?X₃H₂⁺??Y₃H₂⁺?₃]⁴⁺?X=C,Si,Ge,Y=C,Si,Ge?in the process of BLE with the method of hybrid density functional B3LYP?of basis set 6-311G+?d,p??,using the quantum chemistry program Gaussian09 to optimize molecular structure,calculate single point energy,NICS and RSVRSF.--decreasing energy during the process of BLE,—all bonds energy of ring atoms,and ring radius r will be calculated to form the formula of DOA which was in direct proportion towhile inversely proportional toand r².?1?The results of the study of C₆H₆ in the process of Bond Length Equalization?BLE?revealed that as the decreasing of₁?single bond radius angle?,electrons delocalizing gradually,bonds of adjacent ring atoms changed its type from localized to delocalized bond,molecular energy decreasing gradually.The correlation between₁ and molecular energy is absolute quadratic.As the decreasing of₁,the value of NICS become more negative.In the process of BLE,diatropic ring currents increase along with the decreasing of?₁,meanwhile localized electrons become more delocalizing.So the process of BLE is the process of aromaticity increasing.The results of DOA of series aromatic ring molecules of group?A G_nH_n^m?G=C,Si,Ge,n=3,5-8,m=+1,-1,0,+1,+2?and group?A G_nH_n^m?G=B,Al,Ga,n=3,5-8,m=-2,0?show that,for a certain kind of ring atoms,as the increasing number of ring atoms,the value of DOA decreasing gradually,aromaticity decreasing gradually.The correlation between DOA and NICS is quadratic.The correlation between DOA and RSVRSF is also quadratic,which on some extent also reflects the validity of DOA.?2?Geometrical optimization results show that dihedral angle is an important factor for the stable structure of DHRM[(X_nH^m_n-1)(X_nH^m_n-1)]^2m?X=C,Si,Ge,n=3,5-8,m=+1,-1,0,+1,+2?;Dihedral=90.0°is the stable structure when n=3,7,8;while n=5 corresponds to Dihedral=30.0°,n=6 corresponds to Dihedral=50.0°.For a series of DHRMs of same element?C or Si or Ge?,the value of DOA decreased with increasing number of cyclization atoms,aromaticity decreased gradually.The property of DOA keep its continuity from single ring molecule to its corresponding DHRM.Absolute value of NICS decreased with the increasing number of cyclization atoms,aromaticity decreased gradually.The value of RSVRSF decreased gradually with the increasing number of cyclization atoms,which shows a same tendency as its Corresponding Single Ring Molecules?CSRM?.The value of RSVRSF of DHRM is very close to the value of RSVRSF of its CSRM,which could be acted as characteristic frequency of ring molecules to identify its aromaticity and vibration mode.The correlation between DOA and NICS of DHRM is quadratic,which reflects the validity of DOA.Correlation between RSVRSF and DOA of DHRM is quadratic;correlation between its RSVRSF and NICS is linear,which shows a same tendency as its CSRM.?3?The results show that the value of RSVRSF?center?and RSVRSF?ligand?of THRM[?X₃H₂⁺??Y₃H₂⁺?₃]⁴⁺?X=C,Si,Ge,Y=C,Si,Ge?keep its characteristic frequency as its CSRM.Atomic mass of ligand ring atoms affect RSVRSF?center?more than the atomic mass of center ring atoms affect RSVRSF?ligand?.The correlation between the value of RSVRSF(A_1g)of[?X₃Z₂⁺??X₃Z₂⁺?₃]⁴⁺?X=C,Si,Ge,Z=H,F,Cl,Br?and the reciprocal of relative atomic mass?of H,F,Cl,Br?is linear.NICS?ligand?barely change when center ring atoms changed as the order of C,Si,Ge;while the absolute value of NICS?center?decreased gradually when ligand ring atoms changed as the order of C,Si,Ge,its aromaticity also decreased gradually.The correlation between RSVRSF?center?and DOA?center?of THRM[?X₃H₂⁺??Y₃H₂⁺?₃]⁴⁺?X=C,Si,Ge,Y=C,Si,Ge?was linear when center ring atoms changed as the order of C,Si,Ge.Thus,DOA could be applied in many types of molecules to compare their aromaticity.Characteristic RSVRSFs among SRAM,DHRM and THRM could be used to identify molecular aromaticity and vibration mode,which prove again that RSVRSF is an effective aromatic experimental index.