Theoretical Study Of The Reaction Of ¹O₂ With Chlorophenols

Chlorophenols are frequently encountered organic pollutants in the environment. The study of its degradation has become a hot issue. In previous study, it was found in our laboratory that luminescence phenomenon can be produced when the photosensitized degradation products (light emitting precursors) of chlorophenols are treated with NBS. And there is a close correlation between chemiluminescence intensities and the concentrations of chlorophenols. Based on this principle, a series of flow injection chemiluminescence (FIA-CL) analytical methods for the determination of chlorophenols were established. It was generally recognized that the reaction between singlet oxygen and chlophenols is responsible to cause the photosensitized degradation of chlorophenols directly. But due to the complexity of the reaction matrix, the determination of the structures of the light emitting precursors by experimental means is often difficult. Hence it is also difficult to explain the reaction mechanism between singlet oxygen and chlorophenols. Computational chemistry methods are increasingly used for resolving chemistry problems by calculating the molecular properties with computer program. To better understand the chemical structures of the intermediate products formed in the photodegradation process, the reaction between singlet oxygen and chlorophenols is investigated with computational methods in this paper. 2, 4-dichlorophenol and o-chlorophenol were chosen as the model molecules for chlorophenols. Twelve reaction routes for the reaction between singlet oxygen and 2,4-dichlorophenol (o-chlorophenol) were induced by the four principal types of oxygen-addition reactions to aromatic and unsaturated compounds proposed by Bobrowski. The geometries of reactants (reactant complexes), intermediates, transition states and products were optimized at B3LYP/6-31+G (d, p) level. The corresponding vibration frequencies and zero-point energies were calculated at the same level. The single-point energies for all the stationary points were obtained at B3LYP/6-311+ (2df, p) level. We find that the reaction types of"1,3-addition to a double bond connected to a hydrogen-carrying group, resulting in the formation of allylic hydroperoxides"and"1,4-addition to chlorophenols with the formation of hydroperoxide ketones"have much probability than the other two reaction types. But the type of"1,3-addition to a double bond connected to a hydrogen-carrying group, resulting in the formation of allylic hydroperoxides"has the lowest reaction barriers, therefore is predicted to be the preferred route. We also analysis the distances between the correlation atoms and their NBO charges in order to study the reaction mechanism between 2, 4-dichlorophenol (o-chlorophenol) and singlet oxygen.