Charge-transfer mechanism for electrophilic aromatic nitration and nitrosation via the convergence of (ab initio) molecular-orbital and Marcus-Hush theories with experiments

The highly disparate rates of aromatic nitrosation and nitration, despite the very similar (electrophilic) properties of the active species: NO+ and NO2+ in Chart 1, are quantitatively reconciled. First, the thorough mappings of the potential-energy surfaces by high level (ab initio) molecular-orbital methodologies involving extensive coupled-cluster CCSD(T)/6-31 G** optimizations establish the intervention of two reactive intermediates in nitration (Figure 8) but only one in nitrosation (Figure 7). Second, the same distinctive topologies involving double and single potential-energy minima (Figures 6 and 5) also emerge from the semiquantitative application of the Marcus-Hush theory to the transient spectral data. Such a striking convergence from quite different theoretical approaches indicates that the molecular-orbital and Marcus-Hush (potential-energy) surfaces are conceptually interchangeable. In the resultant charge-transfer mechanism, the bimolecular interactions of arene donors with both NO+ and NO2+ spontaneously lead (barrierless) to pi-complexes in which electron transfer is concurrent with complexation. Such a pi-complex in nitration is rapidly converted to the sigma-complex, whereas this Wheland adduct in nitrosation merely represents a high energy (transition-state) structure. Marcus-Hush analysis thus demonstrates how the strongly differentiated (arene) reactivities toward NO+ and NO2+ can actually be exploited in the quantitative development of a single coherent (electron-transfer) mechanism for both aromatic nitrosation and nitration.