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Abstract
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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.
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