Electrophilic Aromatic Substitution: The Role of Electronically Excited States

Electrophilic aromatic substitutions (EAS), one of the most extensively studied organic reactions, can be considered under certain circumstances as a photochemical reaction without light. Thermochemical considerations show that in the gas phase, the reaction system (electrophile plus aromatic neutral) is often found initially in an electronically excited state, whereas the reaction products are formed on the ground state potential energy surface (PES). The crossing to the ground state is usually very rapid, so that the rate-determining steps take place on the ground state surface. It is shown that after the crossing (through a conical intersection (CI)), the system can be found on different parts of the ground state potential surface. In particular, the CI is connected without a barrier to all moieties assumed to be important in the reaction (pi complex, radical pair, and sigma complex). In some cases, due to a relatively low electron affinity of the electrophile and bond reorganization, the reaction starts on the ground state PES; a conical intersection exists in these cases, but is not accessed by the reactants. The topology of the reaction surface due to the avoided crossing is reminiscent of that in which an actual crossing takes place. The paper provides a comprehensive model for several EAS reactions. The CIs are located computationally, and an energy level diagram is proposed for some representative EAS reactions.