Theoretical Studies On The Molecular Structures And Properties Of Porphyrin

Porphyrins,as a class of the rigid ring conjugate molecule,exist in the way of metal porphyrin complexes. They are very important in the life science for oxygen and electronic transmission and so on. So they have a special physiological activity. Studying on structure of porphyrin molecules of porphyrin electrochemical nature by the experiment chemistry and theoretical chemistry is one of the hot topics.Density Functional Theory (DFT), by which the quantum chemistry calculation is applied effectively in study on the issuers of chemistry, is one of the theoretical calculation method. The molecular characters including configuration, energy, atomic charge, electron density and molecular orbital can be obtained by DET. From these charactors the chemical phenomena of moleculer can be explained. The results of DET study can guaid theratically the chemical experiment.This paper analyses the electrochemical nature of mental meso-tetraphenylporphyrin porphyrin (MTPP) with the different substitutions with the quantum chemistry calculation method, forecasts the electrochemical properties of similar substitution complexes,and the DFT calculations provide support for the electrochemical experimemns theoretically.The paper is divided into four parts outlined as follows:1. In this section the theoretical foundation of DFT is simply introduced firstly in aspects of the foundation of quantum chemical and it's calculations on the basic of the frequency analysis, charge density distribution and Mulliken charge. Then the application of DFT in solving chemical problems and in study of porphyrin moleculars are described, followed by the forecasting aspects of the field.2. The B3LYP calculation study on the difference of substitutions in ZnTPP: -OH, -OCH₃, -NH₂ are selected as electron-donating groups, and -COOH, -NO₂ are selected as electron-withdrawing groups, all them are connected to the site of 4-benzene. In the calculation, 6-31G basis set is selected for C, N, O and H atom, LanL2DZ and SDD pseudopotential basis sets are selected for Zn atoms. The computational calculations are performed with the Gaussian 03 software package. The effects on bond lengths, bond angles and the dihedral angle parameters of the molecular structure are analyzed.The results indicate the effect of substitutions on the shape of ZnTPP is -NO₂>-NH₂>- COOH>-OCH₃>-OH. The better optimized effect is obtained when using LanL2DZ pseudopotential basis set than SDD one. The research of Mulliken charge indicates that the electron flow out the molecular from N or Cβatom when oxidizing. The electron-donating groups enhance the LUMO+1,LUMO,HOMO,HOMO-1 energy, and the stronger the ability of electron-donating, the more of the energy enhancement, while the electron-withdrawing groups decrease the LUMO+1,LUMO,HOMO,HOMO-1 energy, and the stronger the ability of electron- withdrawing, the more of the energy decrease. The oxidation potential of ZnTPP-Rs are sequence as ZnTPPNO₂>ZnTPPCOOH>ZnTPP>ZnTPPOH>ZnTPPOCH₃>ZnTPPNH₂, the reduction potential of ZnTPP-Rs are sequence as ZnTPPNO₂32,and their catalytic activity are sequence as ZnTPPNO₂>ZnTPPNH₂>ZnTPPOCH₃>ZnTPPOH>ZnTPP。3. The calculation study based on the different mental, different site and number of substitutions: B3LYP/6-31G-basis set is used in the TPP(OH)1-4, and B3LYP/6-31 G-LanL2DZ-basis set for ZnTPP (NH₂) 1-4 and ZnTPP (NO₂) 1-4. The effects of the number and the positions of substituents on the molecular structure of porphyrin are researched.The results indicate that the effect trend of -OH, -NO₂ and M on the structure of TPP is M>-NO₂>-OH, and a same trend on Mulliken charge is obtained too. But the trend of enhancement for oxidation activity of TPP is different, that is -OH>M>-NO₂. The negative shift of radox potential increases with the increase of the number of the electron-donating groups, and its positive shift increases with the increase of the number of the electron-withdrawing groups. The HOMO,LUMO energy changing trend of ZnTPP-Rn obtained by the calculation is agree with the results of electrochemical experiments. In ZnTPP-Rn(R=electron-donating groups), the catalytic activity increases with the increase of n, and in ZnTPP-Rn(R=electron-withdrwing groups), the catalysis decreases with the decrease of n。4. The calculation study basices on MTPP (M=Fe, Co, Ni, Cu, Zn), MTPPCOOH(M=Fe, Co, Ni, Cu, Zn) and MTPPNH₂ (M=Fe, Co, Ni, Cu, Zn). The results indicate the effect of different metals on the shape of TPP is Ni>Co>Fe>Cu>Zn. The research of the Mulliken charge indicates that the effect of different metals on the electron transfer activity on surface of electrode and liquid/liquid interface is NiTPP-R>CoTPP-R>FeTPP-R>CuTPP-R>ZnTPP-R. The research of the energy of HOMO-1, HOMO and LUMO, LUMO +1 for fifteen MTPPs indicates that in MTPP, MTPPCOOH and MTPPNH₂ , the energy of HOMO is ZnTPP-R>NiTPP-R>CuTPP-R>FeTPP-R>CoTPP-R.Electrochemical activity of MTPP-R is forecasted by analyzing the Electron-density distributions of MTPP-R. The metal of Fe, Co, Ni, Cu and Zn has the lower effect on redox potential of MTPP-R。...