DFT studies on the reaction mechanism and kinetics of dibutyl phthalate initiated by hydroxyl and sulfate radicals: Prediction of the most reactive sites

In this study, the reaction mechanism and kinetics of dibutyl phthalate (DBP) initiated by hydroxyl ((OH)-O-center dot) and sulfate radicals (SO4 center dot-) were investigated at the CAM-B3LYP/6-311+G(d,p) level through Density Functional Theory (DFT), where the preferred reaction sites of DBP were determined. The calculation results revealed that the reactions of SO4 center dot- with DBP exhibited considerably higher energy barriers than that with (OH)-O-center dot because of the steric hindrance. For (OH)-O-center dot, the addition of (OH)-O-center dot to unsaturated carbons of phenyl ring was kinetically favored with respect to the direct H abstraction from the phenyl ring. In contrast, the reactions of SO4(center dot)- triggering formal hydrogen atom transfer from the phenyl ring of DBP were more likely to occur. Interestingly, all SO4 center dot-initiated reactions were not thermodynamically favorable in the gas phase, but the additions of SO4 center dot- to C2, C3, and C4 of the phenyl ring preferred to take place in water. The C2 and C3 were the most reactive sites by SO4 center dot- and (OH)-O-center dot attacks with energy barriers of 86.5 and 20.1 kJ mol(-1), respectively in the gas phase, and the corresponding rate constants were 4.33x10(-3) and 1.86x10(9) cm(3) molecule(-1) s(-1). In addition, obtained results indicated that both free radicals preferred to attack the butyl chains rather than phenyl ring, which was in good agreement with our previous experimental data. The present work could provide supplementary information on the dual-radicals dependent PAEs degradation.