Photochemical hydrogen abstractions as radiationless transitions. Part 4.—Substituent effects on hydrogen photoabstraction reactions by ketones

Abstract

Intermolecular and intramolecular hydrogen abstractions of ketones have been studied by a tunnel-effect theory of radiationless transitions. The effects of ketone substituents on reaction rates can be accounted for the change in the reduction potential of the ketones, by the nature and energy of the excited states and by the CH bond strengths. It is shown that π, π* states have an intrinsic reactivity for hydrogen abstraction ≈ 10^–2-10^–4 times lower than n, π* states, but when both levels are energetically close, the observed reactivity is caused by the thermal equilibrium population of the two states. Substrates with n electron orbitals react generally with ketones via a charge-transfer mechanism, which is controlled by the reduction potential of the ketones, the ionization energy of the substrates, the electronic energy of excitation and the CH bond strengths. Such a mechanism should be viewed as a mechanistic continuum between a radical-like reaction and an electron transfer process and can lead to an increase or to a decrease in the reaction rates, when compared with the radical-like mechanism. Reactions of ¹(n, π*), ³(n, π*) and ³(π, π*) states are found to be adiabatic, but photoabstractions by ¹(π, π*) levels have an electronic forbidden factor ⩽ 10^–2. Such findings are rationalized in terms of Salem's state correlation diagrams.