Theoretical Study of the Peroxidation of Chlorophenols in the Dye Sensitized Photodegradation Condition
|Keywords||Chlorinated phenols Singlet oxygen Chemiluminescence Density functional theory Solvent effect|
Chlorinated phenols are an important class of chemical raw material, widely used, and because of improper discharge, has also become an important class of environmental pollutants. Remove from the water chlorinated phenols environmental chemistry research in recent years has become an important subject areas, including sensitized photodegradation of chlorophenols due to high efficiency, low cost, and direct use of solar energy and other advantages become a research hotspot. In previous studies in our laboratory found that chlorinated phenols sensitized photodegradation in case the primary product of the oxidant will produce chemiluminescence phenomenon, and accordingly for the determination of chlorinated phenols flow injection chemiluminescence method. Research chlorinated phenols sensitized photodegradation mechanism not only helps photodegradation Optimization and analytical methods to establish, but also from the degradation of these substances theoretically explain the process and the changes in the environment law. Large number of experimental studies have demonstrated that singlet oxygen participates in chlorinated phenols sensitized photodegradation, its primary product is presumably within peroxide or peroxide. But because of their high activity, determined by experimental means to the structure of this product is difficult. Research in recent years, the use of computational chemistry reaction mechanism has attracted people's attention. The laboratory has a preliminary study in the gas phase 2,4 - dichlorophenol and singlet oxygen reaction mechanism, but other common chlorinated phenols sensitized photodegradation whether to follow the same reaction path is still unknown. At the same time, such reactions generally occurs in the aqueous phase, in the discussions solvent effect on the reaction is necessary. It was also found that as the increase in the number chloride, chlorinated phenol degradation increasingly difficult, there is a trend of decrease degradation rate, which is also required to provide a theoretical basis. In this paper, the six selected chlorophenols in substance as a model molecule represents, according to singlet oxygen and aromatic compounds and other unsaturated addition reaction when the four types inferred singlet oxygen reactions with chlorinated phenols four types (twelve specific reaction pathways). Gas and aqueous phases, respectively, using density functional theory B3LYP/6-31 G (d, p) level on the possible path of the reactants, the reaction complexes, transition states and products were optimized geometries and frequency calculation. Using the intrinsic reaction coordinate (IRC) method validated reaction path, and in the B3LYP/6-311 (2df, p) level using a single-point energy calculations. Using polarization unified model (PCM) to consider solvent effects. Computational chemistry methods to help achieve the singlet oxygen and chlorinated phenols peroxidation theoretical simulation, and reached the following conclusions: 1. By calculating the Gibbs free energy change results, both in the gas phase or aqueous phase reaction type A (1,3 allylic hydrogen peroxide to generate a bonus) and response type D (1,4 addition of hydrogen peroxide to generate a ketone) are thermodynamically feasible. Gibbs free energy change in the aqueous phase of the more negative, the reaction is more thermodynamically feasible. (2) for these two types of reaction were optimized with the transition state energy barrier calculation results show that both in the gas phase or aqueous phase, the reaction Type A Type D than the reaction has lower activation energy barrier dynamics have a competitive advantage, namely chlorine on behalf of phenol by singlet oxygen generated allyl hydrogen peroxide attack is the best reaction paths. After considering the solvent effect reduces the material energy. Solvent effects on the different type of reaction calculated activation barrier effects vary. Type A water phase reaction energy barrier with different degrees, indicating a more supportive dynamics calculations in the gas phase reaction. 3 With the increasing number of chlorine substituted, the reaction barrier tendency to increase, from the perspective of the reaction energy barrier explanation of the various chlorinated phenol degradation rate different phenomena.