Exploring Unusual Effects of the Ring Substituents in the Type II Reaction with TD-DFT and DFT

The type II photoelimination reactions are important in biological systems. The different roles played by ring substituents are often not obvious from quantum yields for product formation and the rate constant for hydrogen abstraction is quite low (<10⁷  s^-1 ), but Hautala (Tetrahedron Lett. 1977, 18, 2499-2502) reported unusual effects of the nitro substituent in aryl ketones. They reported the first examples of such and discuss for another role played by the nitro group. They determined an upper limit of less than 2 × 10^-5 to the quantum yield for formation of nitro substituent. They hypothesized the carbonyl localized state may be populated with high quantum efficiency, rapid deactivation occurs to populate the lower lying nitro localized state. We focus on the case where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of hydrogen abstraction. Our proposed new mechanisms for 2-ethoxy-1-phenylethanone are based on the energy, HOMO-LUMO, dipole moments, etc. We provide modifications to experimentally proposed mechanisms or completely new mechanism with Hammett, synchronicity, singlet fission and relative rate equations to explain successfully the previous result. At the end, we show why an unusual effect of the nitro substituent exists in it.