Photoexcited quinones (Q*) are efficiently quenched by polymethylbenzenes (ArH) via electron transfer (ET). However, the second-order rate constants (k 2) exhibit Rehm−Weller (outer-sphere) dependence on the free energy (ΔG ET), despite our new findings that the quenching occurs via a series of rather strong encounter complexes [Q*, ArH] with substantial (charge-transfer) bonding. The relatively high formation constants (K EC) of the encounter complexes indicate that any mechanistic interpretation of the driving-force dependence of the observed rate constants is highly ambiguous since k 2 must be a composite of K EC and the intrinsic rate constant (k ET) for electron transfer within the intermediate (inner-sphere) complex. As such, the reorganization energies extracted from Rehm−Weller plots lack thermodynamic significance. On the other hand, the unambiguous driving-force dependence of k ET represents a unique example for the “normal” Marcus behavior of the endergonic electron transfer between the donor/acceptor pair in van der Waals contact as extant in the encounter complex.

The Wheland intermediate in electrophilic aromatic nitrosation, viz. the nitrosoarenium σ-complex, is directly observed by transient absorption spectroscopy. Femtosecond time-resolved laser experiments based on charge-transfer photoexcitation of electron donor/acceptor (EDA) complexes of nitrosonium cation with various arenes reveal the ultrafast formation of nitrosobenzenium to occur in less than 10 ps via the radical/radical coupling of arene cation radicals and nitric oxide. The lifetimes of the σ-complexes in dichloromethane solution are strongly temperature dependentvarying from nanoseconds (T = 298 K) to microseconds (T = 195 K). Steady-state photolysis of arene/NO+ complexes in n-BuCl glasses at T = 77 K leads to nitrosoarenium σ-complexes which persist for several hours. Based on a reaction scheme that includes an ultrafast equilibrium between the [ArH+•,NO•] radical pair and the nitrosoarenium σ-complex, energy diagrams are constructed which establish the highly endergonic reaction profile of electrophilic aromatic nitrosations with the arene/nitrosonium π-complex as the thermodynamic sink.