Hydrogen abstraction from ethylbenzene by imide-N-oxyl radicals with and without O2: a DFT theoretical study

In this paper, we report a theoretical study of the hydrogen abstraction reactions from ethylbenzene by a series of imide-N-oxyl radicals. Geometry optimizations and vibrational frequencies were performed using density functional theory at the B3LYP/6-31G(d,p) level. Single-point energy calculations were carried out at the PMP2/6-31G(d,p) and B3LYP/6-311+G(2df,2p) levels. Calculations reproduce experimental trends. In the absence of dioxygen, calculated barriers are not too high to prevent H-abstraction but the process is endothermic. The factors governing the reactivity of nitroxide radicals have been discussed in the scope of the state correlation diagram approach. Moreover, the influence of dioxygen on the mechanism of these reactions has also been studied. Thus, the addition of dioxygen occurs after the H-abstraction by nitroxide radicals and no clear evidence for an energetic barrier to O2 addition was found. However, in the presence of dioxygen the whole process is exothermic and thus H-abstraction becomes irreversible.