A Theoretical Study On The Aromaticity Of Half-sandwich Complexes And Their Correlation With Roman Spectroscopy Frequency

Many half-sandwich complexes have been synthesized,but their properties such as structure,bond energy and stability were seldom systematically studied theoretically,in particularly the aromaticity was rarely paid attention to.Aromaticity is an important property that directly influences the structure and other characteristics,but it can not be measured quantitatively experimentally so far.So it has become scientists' another new goal to find a method for quantitatively determining the aromaticity of aromatic half-sandwich complexes in experimently.From a theoretical point of view,it was found that aromatic molecules exhibit aromaticity because of the presence of cyclic conjugated bonds inside the molecules,which induced the generation of electronic circulation??,??.That is,there are direct relationships between the aromatic nature of the molecule and its internal conjugate forces.The oscillating force is a kind of ring stretching vibration,so there is a certain correlation between aromaticity and the ring stretching vibration of the molecule.Using the group theory to analyze the six-membered ring ligands,five-membered ring ligands,and four-membered ring ligands and their formed half-sandwiched complexes,it was found that all kinds of ligands existed high symmetry structures andbelong to the Dnh point groups.There exist symmetric type of normal vibration modes of A_1g/A₁'.While the half-sandwich complexes belong to the Cnv point groups,there exist the A₁ symmetric type normal vibration modes.According to the spectral law,the ring stretching vibration modes of the A_1g/A₁'/A₁ symmetrical type of the ligands and their formed half sandwich complexes with Raman activity.So their aromaticity were related to the ring stretching vibration Raman spectrum frequency?RSVRSF?of the A_1g/A₁?/A₁ symmetric type.The calculation method was the density functional theory?DFT?and B3 LYP calculation methods in the Gaussian09 quantum chemistry calculation program.The 6-311++G?d,p?basis set was applied to a system with H atoms,and the H atom free systems were applied to 6-311+G?d?basis set.We conducted the calculations for the geometric structure optimization of the ligands and their formed half-sandwich complexes,and based on the highest symmetry,calculated the A_1g/A₁?/A₁ symmetric type ring stretching vibration Raman spectrum frequency?RSVRSF?.Meanwhile,the Nuclear Independent Chemical Shift?NICS?of the optimized structure were calculated at the B3LYP-GIAO method.The NBO3.1 program was used to optimize the structure for charge,orbits,and bond formation analysis.The Gauss View 5.0was used to draw the molecular structure of the optimized structure,the vibration patterns,and the characteristic molecular orbitals.We used the social science software package SPSS17.0 to calculate the correlationcoefficients and linear regression equations between the NICS of the target molecules and their A_1g/A₁?/A₁ symmetric type RSVRSF.Finally,we used Origin 8.0 to draw the correlation curve between the Raman spectra frequencies and the aromaticity of different systems.These ligands [?⁶-C₆X₆],[?⁵-C₅X₅]-,[?⁴-C₄X₄]2-?X=F,Cl,Br?belong to the Dnh?n=4,5,6?point groups,and they have A_1g/A₁' symmetric RSVRSF.All the ligands have negative value for NICS and are aromatic,except for[?4-C4F4]2-which has a positive value for NICS?1.0?and it is anti-aromatic.The absolute values of NICS and A_1g/A₁' symmetrical RSVRSF of [?⁶-C₆X₆]and [?⁵-C₅X₅]-gradually decrease,with the order of substitution atoms F,Cl,Br.That is,the larger the A_1g/A₁' symmetrical RSVRSF,the greater aromaticity of the molecules are,and all the correlation coefficients are more than 0.9.It is theoretically predicted that the determination of the aromatic degree is possible by the experiment determination of the A_1g/A₁' symmetric RSVRSF values for the ligands.The half-sandwiched complexes formed by ligands and metal ions all have A₁ symmetrical RSVRSF,with high intensity.In addition,all designed,optimized and calculated half-sandwich complexes are aromatic,except for[??⁵-C₅X₅?Zn]+ is anti-aromatic.Compared the half-sandwiched complexes formed by the same substituted ligands and different metal ions,the half-sandwiched complexes formed by metals Ti,V,Ni have greater aromaticity than the ligands,While others formed by Cr,Mn,Cu,and Zn havelower aromaticity than the ligands.The aromaticity of half-sandwich complexes formed by alkali metals and alkaline earth metal ions varies with the order of metal change order : Li>K>Na,Be>Ca>Mg.Compared the frequency of half-sandwich complexes,the A₁ symmetric type RSVRSF of the F atom-substituted ligands formed half-sandwich complexes increased,while the A₁ symmetric type RSVRSF of the Cl and Br atom-substituted ligands formed half-sandwich complexes decreased,but the changes are slightly about 20cm-1.While for the half-sandwich complexes formed by the same metal ions and different substitutions ligands,the absolute values of NICS and the RSVRSF value of A₁ symmetric type gradually decreased with the order of F,Cl,Br substitutions,and there is a highly positive correlation between them.The greater the RSVRSF value of A₁ symmetric type,the greater the aromaticity of the half-sandwich complex,and consistent with the law of ligands,the correlation coefficients reached above 0.9.It is theoretically predicted that the determination of the aromatic degree is possible by the experiment determination of the values of A₁ symmetric RSVRSF for the half-sandwich complexes.