Carbonyl group excited electronic states

Transient spectroscopy and emission spectroscopy have clarified the spectra and chemistry of the excited electronic states of lignin. Lignin aromatic carbonyl groups show reactivity from their first excited singlet states, an unusual behavior for aromatic... 

Alkenes and acetylenes will cycloadd photochemically to other alkene molecules, especially those conjugated to carbonyl groups, to give cyclobutanes or cyclobutenes [29]. The molecules are raised to an excited electronic state, sometimes via a radiation-absorbing sensitizer compound, add to form the ring, and descend to the electronic ground state. In doubly unsym-metric cases, the regio- and stereochemistry can be complex and dependent on conditions. Nevertheless, many are synthetically useful. Two examples are shown in Equations 8.19 [30] and 8.20 [31]. 

Thermal decomposition of cyclic peroxides producing an electronically excited state of a carbonyl group... 

A final example of an n-n reaction will be cited to illustrate still another type of reactivity of the n—r excited state. This is the photolysis of frans-dypnorie oxide (XXXII) to afford as the major product the unsaturated alcohol XXXIV (22). In this case the mechanism, as outlined in Chart VI, involves hydrogen abstraction from the methyl group by the m-situated excited carbonyl group the odd electron containing... 

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