Density functional theory study on the mechanism of OH-initiated atmospheric photooxidation of ethylbenzene

The reaction mechanism for ethylbenzene with OH radical and O2 was investigated by density functional theory (DFT) method. The structures, energetics, and relative stability of OH-ethylbenzene reaction intermediate radicals have been determined, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the ethylbenzene oxidation. The results of the theoretical study indicate that OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.53, 0.31, 0.10, and 0.06 for ortho, para, meta, and ipso additions, respectively, and the calculated overall rate constant is 1.24 × 10−11 cm3 molecule−1 s−1, showing a very good agreement with available experimental data. Under atmospheric conditions, oxygen is expected to add to the ethylbenzene-OH adducts forming ethylbenzene peroxy radicals. And subsequent ring closure of the peroxyl radicals to form bicyclic radicals. With relatively low barriers, isomerization of the ethylbenzene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O2 addition to form bicyclic peroxy radicals. The study provides thermochemical data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from ethylbenzene photooxidation.