The DFT Study On Formation Mechanism Of Dioxins And The Direct Dynamic Calculation Of The Reaction Of HN₃ With OH Radical

Known as the typical persistent organic pollutants, dioxin is lised in the Stockholm convention. And as the high toxic man-made compounds undesignedly, dioxin is paid more and more attention to by environmental chemist nowadays. Researches on the cause, path and mechanism of formation are increasing throughout the world. Chlorophenols are major precursors of PCDD/Fs' formation. In pyrogenation reactors, incinerators and photolysis process, to ascertain the role which chlorophenols act is the key to find out the formation mechanism of PCDD/Fs. The reasonable mechanism is important to cut off emission of PCDD/Fs. A series quantum chemistical calculations based on density functional theory have been carried out in the study. All results were obtained at the UB3LYP/6-311+g(3df,2p)//UB3LYP/6-31+g** level using Gaussian 03 program. Major valuable results are listed as follows:1. The 2-chlorophenol is a suitable kind of model compound to study the dioxin formation reaction. The influence of the number and site of Cl substitution is too weak to change reaction mechanism.2. There are three major reaction channels in OCP radical-radical reaction, relevant products are DD, 1-CDD, 1,6-DCDD and 4,6-DCDF, respectively. There are also three major reaction channels in OCP radical-molecular reaction, relevant products are DD, 1-CDD, DF and 4CDF, respectively.3. The experimental result that 4,6-DCDF is the major product in incinerator flowed by the 20% oxygen in helium is explained reasonable at the first time using computational radical-radical reaction mechanism. Two OCP radicals couple through 6-6 site carbon atoms and form a keto-keto intermediate. The keto-keto intermediate transforms into hydroxyl biphenyl radical after an interior H transferring or radical H abstraction process. The hydroxyl biphenyl radical emits OH radical, and then 4,6-DCDF forms subsequently.4. The mechanism explains properly the experimental result that the products differ in kind and ratio via different 2-CP pyrogenation processes. And the mechanism supposed by experimental scientists is corrected. Mechanism shows radical-radical and radical-molecular reactions occur in the same time during pyrogenation and incineration processes. Radical-molecular reaction is not minor path of forming PCDD/Fs at all reaction conditions, when formation reaction takes place in lower temperature or lower oxygen concentration conditions Radical-molecular reaction acts an irreplaceable role. In PCDD/Fs formation process, hydrogen atom transference occurs in allophenyl more than homophenyl. Allophenyl hydrogen atom transference has a five-member-ring transition state, and homophenyl hydrogen atom transference has a four-member-ring transition state. Four-member-ring transition state has high energy, so allophenyl H transference is preponderant process. The reaction in which H₂O, HCl or H₂ is emited from intermediate has an energy barrier higher than 50 kcal/mol, and they are prove to be inferior position in mechanism.Furthermore, another research is listed in the thesis: mchanism and knetics for the raction of HN₃ with OH rdicals. Reaction rate of HN3 and OH radical between 200～3000K has being studied using the canonical variational transtition state theory with the small-curvature tunneling correction method. The result shows that the abstraction reaction is the major tunnel, and the calculated CVT/SCT rate constants exhibit typical non-Arrhenius behavior. The relativity of rate-temperature can be described as follows:...