| Chlorophenols are important chemical materials and are used in broad ranges. As frequently encountered organic pollutants in the environment, the study of their degradation in the water has become a hot issue in recent years. Various methods have been studied, in which the dye-sensitized photodegradation become a favored process because it is highly efficient, low cost, as well as the direct use of solar energy. In previous studies, a unique chemiluminescence phenomenon was observed when the primary photodegradation products meet with an oxidant. Based on this reaction, a flow injection chemiluminescence (FIA-CL) method was established for chlorophenol detection. A further study on the sensitized photodegradation mechanism can contribute to a better understanding of the degradation process and the transformation of chlorophenols in environment.Many experiment evidences have confirmed that the participation of singlet oxygen led to the photodegradation of chlorophenols and the primary product of this reaction was presumably peroxide or endoperoxide. At present, however, to determine the structure of the initial product by experiments means is difficult due to their high activities. Many results have been made on the reaction mechanism in the use of computational chemistry method. In recent work, our laboratory has studied the reaction between singlet oxygen with 2,4-dichlorophenol in gas phase. It is still not known, however, if the reactions of other chlorophenols would follow the same route when reacting with singlet oxygen. Furthermore, these reactions often take place in the aqueous solutions, thus the influence of solvent on the reaction process should be considered. At the same time, experiment shows that the degradation rate has decreased with the increase of the number of halogen substitutes. This phenomenon needs explanation theoretically.In this paper, the peroxidation reactions of six chlorophenols were investigated using computational chemistry method. Four reaction types (twelve reaction routes) for the reaction between singlet oxygen and six chlorophenols were induced by the four principal types of oxygen-addition reactions to aromatic and unsaturated compounds. The geometric parameters and thermodynamic properties of the reactant complexes, transition states, and the possible intermediate products for all possible reaction routes were calculated both in the gas and aqueous phases using B3LYP at 6-31+G(d,p). Single point energy was calculated at B3LYP/6-311+(2df,p) level. The reaction paths were verified by the intrinsic reaction coordinate (IRC) method. Polarizable continuum model (PCM) method was performed on the optimized structures to estimate the effects of bulk salvation. The main results were given as follows:1. Theâ–³G298 of all the reactions studied in gas phase and aqueous solution were studied and the results suggest that reaction type A(1,3-addition resulting in the formation of allylic hydroperoxides) and reaction type D(1,4-addition to form of hydroperoxide ketones) are thermodynamically more likely to take place. The reactions in aqueous phase are thermodynamically more likely to take place when comparing to that in gas phase. 2. Comparing the lowest energy barrier for each reaction both in gas phase and aqueous solutions, it is observed that the barriers in the reaction type D especially in second step, are higher than those of reactions in the type A, which indicates that the reaction of 1,3-addition resulting in the formation of allylic hydroperoxides would be the more likely reaction path. The presence of water molecules considerably reduces the total energies, whereas the changes in energy barriers are different for different reactions or steps. When the solvent effect is considered, the lowest energy barriers are increased.3. It is observed that with the increase of chlorine substitutions, the reactions become dynamically less favorable due to the increase in reaction barriers. This explains the photodegradation different rates. |
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