Quantum Chemical Investigation Into The Atmospheric Transformation Mechanisms Of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDEs) are a kind of worldwide used brominated flame retardants. They are added to products (such as plastics, electronic, furniture), lacking of chemical force, so they are likely to be released into the environment. PBDEs have been detected in air, water, sediment and organism. Because of their property of long range transformation, they have been detected even in Arctic where is without use of PBDEs. PBDEs are so stable that they can stay for a quite long time in media such as solid and sediment. Toxicology studies on the animals suggest that PBDEs are toxic to nervous system, immune system, endocrine system, liver and thyroid. Particularly, their derivatives, OH-PBDEs, have been proved to generate halogenated dioxins through photochemical conversions.2009EST articles noted that more attention should be paid to environmental pollution caused by PBDEs. Therefore, microscopic mechanism of transformation of PBDEs in the environment and environmental toxicology are urgent to be solved. Several PBDEs with high frequency detected and high concentration in the atmosphere are selected as model compounds to understand their transformation in environment. Based on quantum chemical calculation and molecular reaction dynamics method, the transformation mechanisms for PBDEs are available. And thus provide original innovations on molecular process of transformation and kinetics of PBDEs in the environment.In this thesis, quantum chemical method, which is of high efficiency and accuracy but low consumption, is chosen to calculate the detail reaction properties of hydroxyl radicals with several high-frequency detected PBDEs (BDE-7, BDE-28and BDE-47). Throughout the research in this thesis, following results are obtained.(1) BDE-47, one of the high-frequency detected PBDEs, exists in atmosphere in large amount, and is toxic to nervous system, endocrine system and liver. We have calculated the micro reaction mechanism of BDE-47with OH radical by DFT/MPWB1K method. The results suggest that BDE-47react with OH through OH addition and hydrogen abstraction reaction. Addition reaction plays a dominant role. OH radicals are likely to add to non substituted C atoms. The total rate constant is8.29×10-13cm-3moleculer-1s-1at room temperature of which the addition reaction contributes to88percent. During200-1000K, both total rate constant and elementary constants show positive dependences on temperature.(2) Based on the above research, the reaction properties of BDE-7and OH have been discussed by DFT/MPWB1K method. The phenyl with Br atoms in BDE-7is defined as ring1while the other is ring2. The elementary reactions of ring2have lower reaction energy barriers than ring1. But addition reactions are still primary reactions for both phenyl rings. Therefore, OH is most likely to react with BDE-7through added to ring2, which is in consistence with the frontier orbital analysis. The frontier orbital analysis shows that the reaction activities of R and OH radicals are predicted to be electron transformation from HOMO, HOMO-1and HOMO-2orbital of R to SOMO orbital of OH radical. The CVT/SCT rate constant of the total reaction is3.76×10-12cm3molecule-1s-1at298K. Thus the atmospheric lifetime of BDE-7is calculated to be3.2days.(3) OH-PBDEs, the derivatives of PBDEs, are more toxic than PBDEs. Their transformation mechanisms to PBDDs are provided.6-OH-BDE47and its chlorinated derivatives (3-C1-6-OH-BDE47,5-C1-6-OH-BDE47and3,5-diCl-6-OH-BDE47) are selected in this thesis. Chlorine substitution varies the reaction energy barriers and reaction heats, but not the reaction mechanisms. After OH abstracted the phenoxyl hydrogen, active intermediates are obtained which instantaneously react through direct ring-closing and hydrogen elimination, ring-closing by bromine elimination and Smiles rearrangement. Finally, four kinds of products can be obtained. But the products of direct ring-closing by bromine elimination (1,3,8-TB(C)DDs) are major products. The kinetic results give supporting of this conclusion.(4) The study of atmospheric reaction of BDE-28with OH radicals shows that OH radicals are likely to add to the C(2’) atom of BDE-28. The intermediate produced through above elementary reaction is supposed to react through unimolecular degradation and bimolecular reaction with the existence of O2/NO. Finally, stable dialdehyde and unsaturated ester products, such as2-bromo-5-(2,4-dibromophenoxy)-4-hydroxycyclohexa-2,5-dien-1-one (P4),2,4-dibromo phenyl-(3E)-4-bromo-6-hydroxy-2,5-dioxohex-3-enoate (P7),2,4-dibromophenyl-(2Z)-4-bromo-4-oxobut-2-enoate (P9) and oxalaldehyde (P10) are obtained.In a word, the energy barriers of elementary reactions referred to hydroxyl radical increase along with the increasing number of bromine atoms. Bromine seems to deactivate the phenyl ring where it locates while activate the other ring slightly. Additionally, bromine atom caused the elementary reactions of same ring more exothermic. Besides, locations of bromine are important for the physical and chemical properties of PBDEs. For example, only ortho-brominated diphenyl ethers can undergo intramolecular cyclization or produce brominated dioxins.