Electron Transfer in Electrophilic Aromatic Nitration and Nitrosation: Computational Evidence for the Marcus Inverted Region.

Abstract

Electrophilic aromatic nitrosation and nitration are among the most important electron transfer (ET) reactions. According to the electron transfer theory, an ET process can be described with two electron-localized diabatic states, and the electronic coupling between these two states, together with the organization energy and reaction energy, determine the ET efficiency. A proper definition of the strictly electron-localized states thus is the key. Here we used the valence bond theory to derive the diabatic states and probe the interactions of NO(+) and NO2(+) with benzene and identify the origin of their significant difference in reactivity. Results show that the high deformation cost for NO2(+) overshadows the fact that it has much high charge transfer interaction in [C6H6,NO2](+). While NO(+) uses π orbitals to bind benzene and the orbital switch results in a high barrier from π- to σ-complex, NO2(+) uses a vacant σ(Nsp(2)) orbital, making the transition nearly barrierless. Significantly, we found that the post-ET state [C6H6(+)-NO] is more stable than the prior-ET state [C6H6,NO(+)]. Energy profiles with respect to the distance between the electrophile and the benzene confirm that the ET reaction of benzene and NO(+) falls in the Marcus inverted region, and the outer-sphere ET occurs at ∼2.6 Å with the electronic coupling energy of 1.06 eV, compared with the experimental estimate 1.4 ± 0.5 eV.