| Chemical change cannot do without chemical reaction,while the micro-chemical reaction mechanism is very complicated,thus the research in this area has become one of the important topics experimentally and theoretically.The study on micro-chemical reaction process cannot only deepen the understanding of the nature of the chemical, but also directly relate to the environment of human existence and various applications of daily life.Theoretically,therefore,to explore the process,mechanism and nature of chemical reactions is one of the most fundamental tasks in theoretical chemistry.Chemical reactions are involved in a wide range of areas,but the studies in this thesis focus on two types of chemical reactions which are relevant to the pollution control process and the development of new functional materials.The atmospheric pollution problems such as acid rain,photochemical smog and gobal warming have become increasingly prominent,and the environment issue has been one of the important issues of common concern.Control of chemical reactions of atmospheric pollutions is a possible way to solve the problem of atmospheric pollution.In order to effectively control environmental pollution,molecules and free radicals related to atmospheric pollution have been studied as an entry point by researches of reaction mechanism in the atmospheric environment.Since reactions of atmospheric radicals are very quick and their mechanism are very complex, experimental research for mechanism of these reactions has been hampered by many difficulties;so in recent years,detailed theoretical studies on the mechanisms of atmospheric radicals reactions become one of the hot topics in the field of theoretical chemistry.On the other hand,as significant material in both chemical industry and everyday life,the polymerization mechanisms of olefins have also obtained sustained attention in experiment and theory.In recent years,olefin polymerization to synthesize polyolefins with unsaturated double bonds catalyzed by transition metal complexes has aroused great interest,mainly because the unsaturated double bonds in the polymer product are easily transformed into functional group by chemical modifcation,and as a result of developing the excellent performance of functional polymers.With so large and broad application of these kinds of polymers,in-depth understanding the microscopic mechanism of these catalytic reactions becomes one of new hotspots of experimental and theoretical studies.Therefore,in this thesis,we have conducted quantum chemistry calculations to study the mechanism of the two kinds of reactions mentioned above.The main results are summarized as follows:(1).The reactions of CH3SH+Cl and CH3SH+H have been theoretically studied by using dual-level direct dynamics method.Three reaction channels for each reaction, including two H-abstraction(from the -SH and -CH3 groups) and one substitution channels,have been found.The geometries and harmonic vibrational frequencies of all the stationary points(reactants,adduct,transition-states,and products) were calculated at the MP2/6-311+G(d,p) level.The minimum energy path(MEP) was constructed by intrinsic reaction coordinate theory(IRC).Furthermore,the potential energy surface information is further refined at the CCSD(T)/6-311++G(d,p)//MP2, G3(MP2)//MP2 and CCSD(T)/6-311++G(2df,2p)//MP2 levels and rate constants of these two reactions are evaluated by variational transition state theory(VTST).The present work show that the CCSD(T)/6-311++G(d,p) level can obtain more accurate reaction energy barrier information for the CH3SH+Cl reaction;and for the CH3SH+H reaction,G3(MP2) is more accurate.Rate constant calculations indicated that the H-abstraction from the -SH group is the major channel for two reactions,while the H-abstraction from the -CH3 group channel should be taken into account at high temperatures for CH3SH+Cl reaction but could be ignored for CH3SH+H reaction. The substitution channel of the CH3SH+Cl reaction has little contribution to the overall rate constant due to high energy barrier;while for the CH3SH+H reaction,its contribution to the overall reaction increases as temperature increases.Finally,we present the rate-temperature expressions(in units of cm3 molecule-1 s-1) of the CH3SH+Cl(in the 193-1000 K temperature range) and CH3SH+H(in the 220-1000 K temperature range) reactions:kCH3SH+Cl=5.08×1014 T1.12 exp(539.70/T) and kCH3SH+H=5.00×10-18 T2.39 exp(-119.81/T).(2).The optimized geometries and frequencies of all the stationary points involved in each reaction channel of the CF3CH2CH3+OH reaction are calculated at the BB1K/6-31+G(d,p) level,and the potential energy profiles are further refined by single-point energy calculations at the BMC-CCSD,MC-QCISD and G3(MP2) levels. Same order of the barrier heights of the reaction channels is obtained at three high levels.Furthermore,rate constants of all channels are evaluated by variational transition state theory(VTST) over a wide temperature range of 200-1000 K.The present calculations show that the rate constants evaluated at the BMC-CCSD//BB1K level are in excellent agreement with the available experimental values in the measured temperature range.So,the three-parameter rate-temperature expression of the total reaction over the whole temperature range 200-1000 K is given based on the BMC-CCSD//BB1K results.The branching ratios of H-abstraction from the -CH2(R1) and -CH3 groups(R2) obtained at three high levels agree with each other.It is shown that R1 is the major channel and R2 is the competitive one and also has contribution to the total reaction.Finally,due to lack of the experimental data,we calculated the enthalpies of formation of reactant CF3CH2CH3 and the product radicals CF3CHCH3 and CF3CH2CH2 by using isodesmic reactions.(3).Density functional theory(DFT) has been used to study the polymerization of the 1,5-hexadiene(HD) by Cp*TiCl2(OAr) and Cp2ZrCl2 catalysts.All DFT calculations have been performed based on the generalized gradient approximation (GGA) augmented with the Becke-Perdew exchange-correlation corrections.The 1,5-hex polymerization can take place through repeated 1,2 insertion and cyclization, and the former has two possible insertion pathways:frontside and backside insertion. Our research show that the backside insertion takes place starting fromβ-agostic complex,while the frontside insertion and cyclization starts fromα-agostic complex, which is in higher energy thanβ-agostic complex.Due to the lower barrier of cyclization reaction,the rate determining step of the whole cyclization process is the rearrangement process ofβ-agostic toα-agostic complex.While for the repeated insertion process,the case is different;the barrier higher of the insertion reaction is the key factor to determine whether this path occurs or not.Our calculation show that the HD polymerization by Cp*TiCl2(OAr) favored the repeated 1,2-insertion rather than cyclization while the polymerization by Cp2ZrCl2 favored cyclization,in agreement with corresponding experimental results.(4).We have investigated the mechanism of the copolymerization of ethyl(Et) with cyclopentadiene(CPD) catalyzed by[PhNC(CH3)CHC(CF3)O]2TiCl2.All results were obtained from DFT calculations based on the generalized gradient approximation(GGA) augmented with the Becke-Perdew exchange-correlation corrections.Our calculations show that the CPD can incorporate into polymer chain via both 1,2 and 2,1-insertion,and it is predicted that 1,2-insertion might be a preferred pathway due to the lower barrier.And in the copolymerization process, reaction barriers of the ethyl insertion into both metal-Et and metal-CPD bonds are lower than the barriers for the CPD insertion,so the ethyl insertion is the preferred pathway after ethyl and CPD insertion.As a consequence,there might be more Et sequences in the growing polymer chain.The CPD insertion into Metal-Et bond can occur in some probability due to the lower insertion barrier,while for the process of CPD insertion into Metal-CPD chain,its insertion barrier is too high to make this reaction occur.The present conclusion agrees with the experimental fact that no consecutive sequences of CPD have been observed in the polymer product.
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