Theoretical Study on the Origin of the Behavior of Ketones and Thiones in Hydrogen Photoabstraction Reactions

Natural correlation diagrams have been established to describe hydrogen photoabstraction in ketones and thiones for three different reaction paths (nx, πx and πc approaches). Their conclusions have been confirmed by ab initio SCF‐CI calculations. This procedure leads to the following conclusions: Energy barriers exist on the potential energy surfaces of all the potentially reactive excited states. The height of these barriers is strongly dependent on the distance separating the hydrogen acceptor and hydrogen donor atoms in the transition state. For the n approach, the reactive channel only involves nπ* excited states while, for the π approach, nπ* (hydrogen capture by the carbon atom) and ππ* (hydrogen capture by the oxygen or the carbon atom) excited states must also be taken into consideration. A proposal for resolving the ketone‐thione dichotomy is presented on the basis of these results.