Theoretical Study On The Aromaticity Of Heterobenzenes And Intramolecular Proton Transfer Of2-(6’-hydroxy-heterophenyl) Benzomidazoles

As one of the basic reaction of chemical and biochemical systems, existing in avariety of chemical and biochemical processes, proton transfer reactions have beendrawn considerable attention of both experimentalists and theoreticians. The benzeneand heterobenzenes were constructed in the class of benzoimidazole, and theinvestigation on some important features of the electron feature, the binding energies,aromaticity, reaction mechanism in intermolecular proton transfer, and so on, werepresented for these compounds based on the quantum chemical computation in thispaper. The main results are as follows:1、 The molecular geometries were fully optimized without imposing anysymmetry constraints, although in some instances the resulting structure show variouselements of symmetry. The geometries and energetics of the stationary points on thepotential energy surface were calculated using the DFT（X3LYP） method inconjunction with the6-311++G（d, p） and LANL2DZ（d, p）（Los Alamos NationalLaboratory2double） basis set. Employing an ECP basis set such as LANL2DZ（d, p）for X atom,and adopting CCSD/[6-311++G（d, p）+LANL2DZ（d, p）] calculated thevalues of isomerization stabilization energies（ISE）. Based on the analyzing maximumvalue of NICS above the cycle planar about0.8～0.9, some important features ofbenzene and heterobenzenes C₅H₅X （X＝CH, N, P, As, Sb, Bi） are present. Comparedwith density functional theory （DFT）, the calculated results by ab initio （HF） are moreaccurate and acceptable for the NICS of heterobenzenes. According to the calculationof natural localized molecular orbitals （NLMO）, we can get a conclusion that the bonds are main contributions to the zztensor of NICS（max）, and the order is C₆H₆>C₅H₅N> C₅H₅P> C₅H₅As> C₅H₅Sb> C₅H₅Bi.2、Based on the structures of above mentioned methods and basis set, and thenadopted HF/[6-311++G（d, p）+LANL2DZ（d, p）] calculated the values of ELF andNICS. The global aromaticity on the five group heterobenzenes C₅H₅X （X=N, P, As,Sb and Bi） have been researched with regard to the ELF, the NICSs and the aromaticstabilization energies （ASEs） methods. Using ELF and NICS methods to research theand with the global aromaticity of the nonmetal and semi-metal aromaticcompound allow us to evaluate the distributions of aromaticity in magnitudes. Thegroup5heterobenzenes were well measured through the better linear relationships andthe values of bonds showed aromaticity is usually less than that of bonds, so ELF and NICS（max） zzare easily and efficiently applied to judge overall aromatic character. The global aromaticity order of the group five hetrobenzenes （C₆H₆>C₅H₅N> C₅H₅P> C₅H₅As> C₅H₅Sb> C₅H₅Bi） is established firmly.3、Ab initio （MP2） and density functional theory （X3LYP） calculations havebeen carried out. Fully optimizations have been performed on different molecules（The HPyBI、HPpBI、HApBI、HSpBI and HBpBI that C₅H₅X is constructed from thebenzoimidazole, respectively） to understand the electronic structure of bond angels,lengths and so on, using MP2, X3LYP methods at the6-311++G（d, p） basis set on themetalloid. For metals, adopted GEN of the keyword calculates and the structures wereoptimized by using LanL2DZ basis set. The zero-point vibrational energy corrections（EZEP） have been obtained within the harmonic approximations. These computingresults introduce PCET mechanism by Charge Transfer （CT） and NICS （1）distributions in benzoimidazole molecules. When the atom types have been increasingfrom nonmetal to metal, the energies barriers transition state that the difference valueis less than2kcal mol-1have changed little and the order is HPyBI （9.66kcal mol-1）>HBpBI （8.72kcal mol-1）> HSpBI （8.36kcal mol-1）> HApBI （8.31kcal mol-1）>HPpBI （8.17kcal mol-1）. The charge of oxygen atom and dual descriptors map areaccord with transition-state barriers in PT process. These same charge trends of Natoms in different molecules reveal that PCET mechanism wildly exists in HXBIsmolecules. Based on PCET mechanism of2-（6’-hydroxy-2’-X-heterocycle）benzoimidazole,"anti-ring affecting" and charge distribution are two complementphenomena each other. Again, we note the way is thermodynamic in nature and doesnot consider kinetics or dynamics related information, but this process explainsspecifically a broad PT phenomenon bringing about the charge distribution ofchemistry and biology.4、All geometries of2-（6’-hydroxy heterocycle） benzoimidazole in differentpositions were fully optimized without imposing any symmetry constraints by usingthe X3LYP method in conjunction with the6-311++G（d, p） and LANL2DZ（d, p） basisset, although in some instances the resulting structure showed various elements ofsymmetry. These energy of the stationary points、 transition state （TS） and hydrogenbond energy （EHB）on the potential energy surface were calculated. The results showedthat the special geometries and energies emerged from the type4’ about HSbBI andHBiBI in isomers. The90°changes of the stucture leaded to increase the energies oftransition state of the former and there are no transition state and the keto form, sothat it exists only in one enol structures. Compared with the different geometries, abetter phenomenon that is the energy barriers increased by hetero-atom number will be to explain the energies of transition state from2’ to5’ process. There are strongintramolecular hydrogen bond formation in enol and keto structures. The results ofbinding energies showed that the more energies in transition state, the more gettingrelative energies of hydrogen bond. Especially, the hydrogen bond of enol structuresdon’t depend on their2’、3’、4’ or5’ position.