Photochemical hydrogen abstractions as radiationless transitions. Part 1.—Ketones, aldehydes and acids

Abstract

Hydrogen abstraction reactions of electronically excited carbonyl compounds are treated as radiationless transitions using the tunnel-effect theory. The rates of reaction are controlled by the Franck–Condon factors of the vibrational stretching motions of the carbonyl group in the excited carbonyl compounds, and the CH oscillator of the substrate with the CO and OH vibrations in the ground state of the final products. The rates are dependent on electronic energy, vibrational frequencies, the reduced mass of the oscillators, the CH bond strength of the reactive bonds of the substrates and bond distances. In cases where a charge-transfer mechanism is involved, the rates are also a function of the carbonyl compound reduction potential and substrate ionization energy. The model gives rates in good agreement with experiment and explains singlet and triplet reactivity of n, π* and π, π* states, inter- and intra-molecular processes, normal and inverse deuterium isotope effects and the variation of the rates with temperature. Rates are predicted for several systems which have not yet been studied experimentally. Results from the present model are compared with those from other theories of photochemical reactivity.