Study On The Application Of Quantum Chemistry Theory In Modern Chemistry

Chemistry is originally an experimental science built on atomic and molecular theory.The primary task of chemistry is to study the behavior of molecules,the reactions between them,and based on these understandings to rebuild molecules.Although brilliant achievements in the 20th century were made,chemistry and theoretical chemistry did not gain enough attention as they deserved,especially for theoretical chemistry.Quantum chemistry is a science that studies the structures and reactions of chemical system using quantum mechanics theories and methods.With the development of quantum theory,nowadays quantum chemistry has formed the fundamentals of chemical structure theory and is widely applied in all branches of chemistry and molecular physics.The combination of theory and experimentation today makes it possible for chemical sciences to probe the nature of molecular system and brings remarkable innovations and achievements for the whole chemical field. The modern chemistry is not an experimental science anymore.In this paper,three selected typical issues in the modern chemistry are studied using the combination of quantum chemistry and experimental investigations with the hope that this attempt could make contributions for the application of quantum chemistry in modern chemistry.Porphyrins are special kind of compounds with conjugated aromatic rings.Hemoglobin,Chlorophyll are typical compounds belong to those materials.These compounds find extensive applications,particularly as bionic catalyts.The porphin is the basic structure of this kind of compounds.Therefore,correct description of its electronic structure is the foundation for the research concerning the porphyrin materials.The molecular structure of Free-Base Porphyrin(H₂P) has been fully optimized using the Density Functional Theory(DFT) methods,as well as ab initio Hartree-Fock method at various basis sets levels.The results obtained at various levels are discussed and compared with each other and with the available experimental data.Nuclear Magnetic Resonance (NMR) have also been calculations.It is found that DFT method has better performance in geometry caculating.That indicating electronic correlation must be included in the calculations concerning the structure of this kind of materials.There are still some differences in the optimized structures obtained using different basis in DFT method. Results obtained using B3LYP approach are closer to the experimental data.The optimized structure obtained using STO-3G basis sets is only a saddle point structure in tautomerism.For NMR calculation,DFT is also more appropriate method than HF. Silica is an active and superfine material.It finds wide applications as a reinforcement fill in the rubber industry.But the poor processing properties of silica is a obstacle that restricts its application.As a consequence,surface modification of silica is necessary to improve its processing performance.In this study,the effect of the modification on the silica and its influence on the reinforced SSBR are studied using the combination of quantum chemistry and experimental investigations. The surface of the nano-silica was modified by different silane coupling agents,such as KH-792,KH-590 and KH-570.Then the modified silica was filled into the styrene-butadiene rubber(SSBR).Different instruments were employed to characterize the properties of modified silica and reinforced SSBR.The optimized geometries of molecular modified silica reinforced SBR were obtained by using B3LYP calculation of density functional theory(DFT).The natural bond orbital analyses were carried out.The experimental results showed that:silica surface modification with silane couplings significantly improves the processing properties of silica.The mechanical properties of reinforced rubber were dependent on the type and structure of the silane coupling used.The results of quantum chemistry calculations indicated that the Si-O bond length of the silica modified by with KH-792 is the shortest one with the oxygen atom bearing more negative charge on it.This implies that the silica and KH-792 is reinforced.SSBR reinforced with KH-792 shows a higher tensile strength and a higher elongation.This can be attributed to the effect of intensified delocalization of the lone electron pairs of the two N atoms in KH-792.Modification with KH-590 results in a change in the length of the S-C bond located at the other end of KH-590 molecule.The length of the S-C bond becomes longer as compared to that of the original one.This makes the scission of the S-C bond easy and more sulfur free radical(·S_y·) can be produced.Consequently,the degree of crosslink of the rubber reinforced with the silica modified by KH-590 was increased, resulting in an increase in the stress and tear strength of reinforced SSBR. The differences in the mechanical properties of the reinforced rubber were resulted from the differences in the properties of the bonds formed between silica,silane coupling agents and SSBR.Nitrogen oxides(NO_X) are pollutants that are harm to the ecological environment,industrial and agricultural production as well as human health.The main technology used for the removal of NO_X is selective catalytic reduction(SCR) method.In this study,Ag/Al₂O₃ and Ag/USY catalysts for the SCR of NO_X.were prepared,tested and characterized.It was found that to obtain a higher conversion of NO_X SCR with propane over Ag/Al₂O₃ needs a temperature higher then 350℃.The more appropriate reaction temperature range is 400～450℃.NCO and CN play crucial roles in the SCR reaction over Ag/Al₂O₃.So SCR activity can be greatly promoted with the N₂ and C₃H₆ being activated.The adsorption of NO,O₂ and reducing agents on Ag/Al₂O3.has been studied using DFT. The frequencies of the.optimized geometries of the adsorbed species and NBO were calculated,According to the results of thermodynamic calculations,the adsorption of NO and O₂ on the catalysts containing Ag is an spontaneous process.The bonds of NO and O₂ are observably weakened by the absorption.Adsorption of hydrocarbons with longer and saturated carbon chains is not spontaneous.However,decreasing the length of the carbon chain or saturation of the reducing molecules can enhance their absorption and subsequently increase their reactivity.The geometries of NO₃^-,the important intermediate species, adsorbed on Ag and Ag⁺ were calculated.The results indicate that the unidentated adsorbed nitrate has the highest activity among the three absorption geometries.The adsorption of NO₃^- on Ag⁺ shows stronger impact no the N-O bond.Therefore,catalyst surface acidity can be adjusted to generate more unidentated nitrate,and the load of Ag can be controlled to obtain a more active surface in the future catalyst design.From the study,we can draw a conclusion that the combination of quantum chemistry research and experimental study is the best way of modern chemistry research.